Author Archives: Greening Me

Too smart by half

One of the challenges in the smart home world is to distinguish between a group of functions associated with remote control of devices in the home for aesthetic or convenience reasons versus automation associated with the management of electricity costs, carbon management, or smart grid integration. I thus choose to differentiate using the terms Smart Home and Energy Smart.

Smart Home and Energy Smart

Smart Home

Functions within my Smart Home space include:

Space heating. In many homes space heating is controlled by a central thermostat and timer, possibly in combination with Thermostatic Radiator Valves (TRVs). In my home thermostats and timers are commonly pushed down to room level with individual rooms set points and schedules. General advice to reduce heating costs is to reduce heat loss through insulation and lower temperature set points, which I have but also add only heating rooms in which heat is required.

Window management. On occasions household members were known to go out leaving windows open. We now monitor the most commonly left open windows (plus the garage door) and use their status to illuminate and colour a smart bulb in the hall near the burglar alarm panel. The same window sensors can also be used to disable heating in rooms while the window is open.

Lighting control. We automate dusk-to-dawn external and internal lighting by the front door and in the downstairs hall. There’s overlap in bulbs between Window Management and Lighting Control.

Watchdog. To improve the robustness of all the rules operating the above functions, I have a smart plug that cycles on and off automatically at regular intervals and is used to trigger re-evaluation of the rules.

Energy Smart

Energy Smart functions include:

Battery Storage. Storing surplus output from my solar panels for later use, or buying energy from the grid when the price is low to avoid buying later when the price is higher.

Car charging. Managing my car charger to absorb surplus solar energy or buy energy from the grid when most cost-effective.

Water heating. Managing my immersion heater similarly. My immersun manages self-consumption of the surplus from my solar panels by diverting a proportional amount of power to the immersion heater, while the HEMS can boost the immersun at full power when the bought electricity price is suitably low.

Both Smart Home and Energy Smart

Wet goods. Controlling dishwasher and washing machine for lowest energy cost at the boundary of Smart Home and Energy Smart in both the Apple HomeKit ecosystem and with API integration for HEMS.

The Smart Home group of devices is managed via various user-friendly interfaces within apps like Apple’s Home or the Eve app where rules can be created of the form if {any trigger(s)} and {all conditions} then set {scene(s)}. These are processed by a hub which in my case is an Apple TV (or two).

On the other hand the Energy Smart devices are managed via the HEMS and are controlled by more fundamental programs (strictly in my case scripts) which are executed on my HEMS (which is based on a Raspberry Pi with HEMS-specific programming of my own creation.

In summary then, the Apple HomeKit ecosystem provides a smart home environment with a comparatively wide variety of supported devices managed from Apple’s own Home app or companion apps from the device manufacturers; while the Energy Smart side is in its relative infancy and (at least as far as my integration goes) quite a lot of bespoke software.

For me the need for much bespoke software is because I had the majority of the devices first with no thought when acquired of doing a HEMS-like project. They were originally bought or developed to maximise self-consumption of the output of my solar panels, so I had to develop the software to interface to what equipment I had. However for the Apple HomeKit, having settled on Apple HomeKit largely because we were iPad users, it becomes relatively easy to add additional devices that are sold as compatible with the HomeKit ecosystem.

Works with Apple HomeKit

Getting heated

Regular readers may recall that our hot water can be generated in 3 different ways: (i) conventional gas boiler, (ii) from grid electricity and (iii) from the surplus on my own solar panels. Attractions of these options are that gas is always available and stable in price, but my grid electricity is lower carbon and may at times be cheaper than gas, and my solar electricity is lowest in both carbon and cost but is subject to significant daily and seasonal variation.

The logic to sort out which source to use is managed by my HEMS. Gas is the baseline and the gas boiler is set to heat water for an hour a day in the early evening to ensure that baths etc are possible. The heating is thermostatically controlled so it doesn’t heat if the water is already hot, and that thermostat is set slightly lower than the immersion thermostat too.

The ImmerSUN normally operates automatically to divert surplus solar electricity proportionately to the immersion heater after the needs of general house load, battery charging and car charging have been taken. However if the electricity price is negative (yes, really) then the HEMS may override the ImmerSUN so that water heating is not done by free solar but instead may be delayed to allow use of paid-to-use electricity.

The final part of this triumvirate is buying electricity from the grid to heat water. Here the price of bought electricity is compared either to the price of gas and a decision made to use electricity when it is cheaper (it’s always lower CO2), or compared to the price of surplus solar (effectively zero) to buy from the grid. Both of these are obviously comparisons with a price threshold but until now the choice of threshold has been made manually – typically against gas in winter when solar output is limited and against solar in summer when more readily available. However the reality of UK weather is that this is a compromise as it may be very sunny one day but very dull the next.

Solar forecasting

The new refinement therefore is to use the existing solar forecasting integration. Solar forecasting already informs HEMS decisions about when to charge the storage battery from the grid and when to operate the wet goods (dishwasher and washing machine). The latest change is that the solar forecasting is now also use to choose whether to base a decision to buy electricity for water heating against a threshold related to the gas price or against the price of surplus solar PV.

HEMS schedule for July 4th.

The above schedule shows that, as a result of no significant solar production anticipated on the 4th, the HEMS has compared electricity price to gas price and thus elected to buy electricity from the grid to make hot water overnight since at 1.7640 to 2.4675 p/kWh electricity is cheaper than gas.

You learn something new every day.. WIFIPLUG

Increasingly I’m starting to use Siri when loading the dishwasher, as in (i) load dishwasher, (ii) start dishwasher, (iii) “Hey Siri, turn the dishwasher off”, and (iv) allow the HEMS to resume the cycle when then cycle cost is most attractive. The dishwasher (and washing machine) are operated by WIFIPLUG smart plugs to achieve this.

WIFIPLUGs v1 and v2.

However on occasions I used WIFIPLUG’s own app to control the plugs rather than Apple’s Home app, the Eve app, or Siri.

The WIFIPLUG app used for dishwasher and washing machine control

However I have a frustration with the WIFIPLUG app that it often takes three presses to turn the dishwasher off after starting the cycle:

  1. Initially the app shows that the plug is off, although the plug is physically on – the first press reports a communications issue.
  2. The second press reports that it’s turned the plug on, although the plug was physically already on.
  3. Only the third press turns the plug off as was the original intent.

However today I discovered that the WIFIPLUG app is not supposed to reflect the live status of the plugs, instead one swipes to refresh and then presses to toggle status – so two actions in my case rather than three listed above.

I find the need to swipe to refresh completely non-intuitive as both Apple’s Home app and the Eve app show the live status of devices and there is no refresh concept, but I suppose it’s reassuring to know that the app is designed to work this way rather than being broken.

In the meantime, to the best of my knowledge, WIFIPLUG is the only smart plug not only supporting Apple’s HomeKit ecosystem, but also having an exposed API for smart home integration, making it uniquely suited to my application.

It’s official – I’m a smart home / energy pioneer :-)

It can’t be very often that an energy company blogs about its customers’ achievements. Last week it happened. Octopus Energy wrote a blog entitled “How to hack your home for cheaper, greener, energy with our open API” which featured the achievements of its customers, and Greening Me got two honourable mentions.

For those not familiar with geek-speak, API is Application Programming Interface which is a mechanism by which an app, webpage or computer program may give commands to, or receive data from, another program – often a web server. Such APIs are often closed (that is that they are only available for use by the creator’s own app or webpage etc), but in this case the Octopus APIs are open so that they can be used by others (including me) to create our own apps, webpages, or other integrations to get data from Octopus. That data may be future price information for a UK electricity region or the actual consumption from a specified electricity meter for example. Octopus document their APIs and encourage others to find innovative uses for them.

Other APIs that I use were either documented privately by the manufacturers of the equipment concerned, although the manufacturer has not put the API in the public domain, or were reverse-engineered by myself or others by looking at how the manufacturer used it or at the internet traffic that it generated and working out how we could use it ourselves for a slightly different purpose. Such purposes would include controlling equipment other than by the manufacturer’s own app, or collecting data into some non-supported form.

Diversity in third party solutions using the Octopus API.

Greening Me’s first mention in the blog came under the Smart Electric Vehicle (EV) Charging section where Octopus wrote..

One of our own smart energy pioneers, Greening Me, has used a Raspberry Pi and an add-on circuit board with our API to switch his electric car charger on/off and set the best time for his hot water immersion heater to run. He also has solar generation and so he can direct his solar power to either his smart car charger or hot water.

The first reference

Later in the “I’ll do it myself (tech level 🌶🌶🌶)” section after describing a group of “smart home pioneers” Octopus wrote..

In the home-brew category, users like GreeningMe have created their own Home Energy Management Systems (HEMS), using the ubiquitous Raspberry Pi to manage a large part of their energy consumption.

Together with Western Power Distribution, Passiv Systems have also created something similar to Greening Me’s HEMS, which is currently being trialled and evaluated as another BEIS funded research project called MADE.

The second reference

So it’s official – I’m a “smart energy pioneer” and a “smart home pioneer”. I also quite like the idea of being a “home hacker” in the positive sense of someone who makes their own home conform to their wishes. If you’d like to read the full blog post from Octopus Energy then you can do so here https://octopus.energy/blog/agile-smart-home-diy/.

Overall I’m proper chuffed.

Opportunities in the import / export business

Most of us are used to a simple world of electricity where we pay for what we consume. For most folks like myself based in the UK that’s typically a fixed price per kWh/unit consumed regardless of time of day, even through dual-rate tariffs have been around for decades – the best known being “Economy 7” tariffs. However as the grid gets smarter then there are increasing opportunities to save on, or make money from, electricity.

Electricity opportunities for import / export and positive /negative cost.

Conventional – pay for power.

This is the area with which most of us are most familiar. We all get the idea of paying for the power we consume. Most UK households pay a fixed price per kWh/unit regardless of the time of day. We have a competitive electricity market, so there are the choice of 70 to 80 different providers who will make different offers regarding standing charge (sometimes marketed as a subscription) and unit cost.

There’s also the opportunity to choose between a flat rate tariff or Economy 7 even on conventional meters that provide a discounted night rate for 7 hours.. These typically provide a discounted night rate, but may charge a little more during the day. They used to advertise these as ‘less than half-price electricity’ but that’s often not the case now.

Stepping up in complexity (and opportunity) smart meters provide the opportunity for a more diverse range of tariffs including different cheap night time periods, more than two rates at different times of day (in extreme 48 half-hourly rates), and a free day at the weekend (i.e. a zero rate of a weekend day) etc.

Beyond that my own tariff (Octopus Agile) not only has up to 48 different half-hourly prices/day that change daily based on that day’s market prices. That might sounds a bit scary but it can yield very cheap electricity prices – 4.48 p/kWh for me in April/May 2020 (for example) which is a third of what most people pay.

My electricity costs April/May 2020

(The original version of this post wrongly had the table from my gas bill above and mistakenly claimed that I had paid “a quarter of what most people pay” rather than a third. Total consumption is untypically low at the present time due to limited miles driven.)

Agile – paid to consume

Top left on my initial diagram is Agile – paid to consume.

One of the features of the wholesale electricity market is that at times the market price for electricity goes negative. At such times the a significant excess of supply (typically because of high output from wind turbines) over demand (often but not always at night) yields a negative price so electricity companies looking to buy electricity are being paid to take it. Most electricity companies will continue to charge their customers the standard price in these circumstances but, with the octopus Agile tariff, the negative pricing is passed to the consumer so that you are paid to consume electricity. This is one of the reasons that my electricity costs are so low.

My electricity costs – Saturday 23rd May 2020

The above chart shows my electricity costs for Saturday 23rd May 2020. The blue line shows the half-hourly electricity price varying between minus 10 p/kWh and plus 15 p/kWh. The red bars show my electricity consumption in each half hour. You can see how consumption tends to be highest when the price is lowest leading to an average price paid of minus 6.22 p.kWh (i.e. they paid me to use electricity) – indeed they paid me 82.4 p to buy electricity that day.

Conventional export – paid to export

The next opportunity to make money from electricity is to sell it to the grid. Obviously that depends on having a source for the electricity typically a generating asset like solar panels or a wind turbine, possibly coupled with a storage device like a battery. It’s also possible with a battery alone, but I know no-one who does that as the economics are more challenging.

The UK currently has a scheme called Smart Export Guarantee (SEG) where you can sell your export to an electricity company. Prices vary enormously so it’s worth shopping around and not just assuming that your electricity company will give you a good offer.

SEG rates from the Solar Trade Association

There is also a smarter SEG option where Octopus offer a dynamic SEG based on market rates (Octopus Agile Export) which may at times offer a high rate, but also offers a lower rate at times, and is thus perhaps better suited to those with storage.

I myself am NOT on such a tariff as I’m on an older legacy Feed-in Tariff (FiT). Despite its name FiT is a generation incentive, not an export incentive. As a generation incentive FiT encourages self-consumption since each kWh that I consume myself does not reduce my income, whereas on SEG each kWh that I use myself (such as making hot water) would reduce export income. So, for example, if I use a kWh of electricity to make hot water that’s saved a kWh (or thereabouts) of gas at around 3 p/kWh, but if I was on SEG then I might have lost 5.5 p/kWh of export revenue to save 3 p/kWh on gas which is clearly an on-cost not a saving. There are other benefits of course because I’ve reduced my carbon footprint by using my own low CO2 electricity to replace a fossil fuel, but it’s not (in this case) improving my financial position.

A further area of research by others is V2X (V2H and V2G) – taking electricity stored in an electric vehicle and using that within the home (V2H) or exporting it to the grid (V2G).

Export penalty – penalised for export

A logical consequence of this smart grid that I’ve outlined is being penalised for export. If there are times when the market price for electricity is negative then if I were part of that market then I might expect to be penalised for export. This doesn’t actually exist in the UK, as the only model that links SEG payments to the market price, Octopus Agile Export, protects its customers from negative pricing.

Should consumers be exposed to this risk then a logical behaviours would be:

  1. To manage self-consumption into the negative export periods, and potentially thus increase export in the positive export periods. For example disable diversion to an immersion heater or car when export price is positive, and then maximise self-consumption when the export price (and presumably the import price also) is negative.
  2. To disable the generating asset to avoid the export penalty.

Conclusions

Some people like myself will find developments in the smart energy sector a fascinating and engaging topic with opportunities both the save money and engage in creating a cleaner and greener electricity system.

However given that many choose not to even participate in the competitive market for electricity supply created when the regional electricity companies were privatised in late 1990 (i.e. 30 years ago) then there will be a significant number who are not so motivated.

This then creates opportunity for a wider variety of smart offers. Some products, at the Agile Octopus end of the spectrum, giving the consumer the opportunity to benefit from their own decision making, while others look more like a traditional dumb tariff with a very simple price structure but potentially making the energy company a more active manager of the home appliances so that the consumer hopefully plays a lower unit rate while the energy company takes responsibility for managing the assets within the home.

Bright revisited

Back in late 2018 I purchased a Hildebrand Glow Stick Consumer Access Device (CAD) to monitor my electricity consumption. A CAD is a consumer device that can be paired with domestic smart meters to provide the consumer with a means of reading the meter. All UK smart meters are supplied with a dedicated in-home display (IHD) to display energy consumption, which is also an example of a CAD. The Glow Stick pairs with the meters like the IHD but shares the data to the cloud from where it can be read either via an app (Bright) or another device using APIs.

Glow Stick CAD

Each smart meter effectively has two interfaces – a Wide Area Network (WAN) connection used for metering and billing and a Home Area Network (HAN) used for connection between meters (electric and gas), hub (embedded within the electric meter) and IHD. The HAN is also available for smart home devices.

“Network hub“ including Glow Stick

“Network hub” including (from top to bottom):

  1. Network switch providing additional hardwired connections to the internet, placed behind..
  2. TalkTalk router providing WiFi and 4 hardwired connections to the external internet, placed above..
  3. Network storage, placed above..
  4. Immersun bridge (left) and Glow Stick (right and forwards)

When I initially installed the Glow Stick it provided a very useful tool to see current and historic energy consumption, but the equivalent cost displays were incorrect (at no fault of Hildebrand) because the CAD correctly read the meter costs, but the meter was not sufficiently sophisticated to store the complex Agile tariff (where unit cost changes every 30 minutes).

I recently learned that Hildebrand now had the ability to take the tariff directly from Octopus Energy via API, bypassing the incorrect tariff data in the meter. A quick support email to Hildebrand confirmed that this was not only possible, but also that the cost data would be corrected back to when I bought the Glow Stick back in 2018. A few days later and the conversion was complete.

These two views show today’s part-complete data:

The screenshot on the left shows today’s part-complete energy data. That on the right shows the equivalent cost data. Had the unit rate been constant throughout the day then the two profiles would have been proportional, but instead the screenshots show the magnifying impact of the higher unit rates in the four to seven PM window with equivalent consumption to the late afternoon resulting in rather higher costs.

I should emphasise however that my average unit rate is very low as I usually have much higher consumption in low cost periods than I do in high cost periods.

My electricity bill to May 2020

One of my recent electricity bills had an average of 3.49 p/kWh ex-VAT. Half-hourly rates varied between around minus 10 p/kWh (I.e. I was paid to use electricity) to plus 25 p/kWh. A low average price was achieved by shifting electricity consumption to when the price was lowest.

My next step is likely to be to use the API to get the real time household load for load management as an increasing number of electrical consumers (potentially a second car charger) risks overloading my supply fuse if all loads were on simultaneously.

Washing away any confusion

I previously described how I had integrated the washing machine into the smart home ecosystem using a smart plug so that it is (re-)started by the HEMS when the cost to complete a washing cycle will be lowest, bearing in mind that my electricity supply is a combination of paid-for electricity where the price varies each half hour and ‘free’ solar. As the means to get the best from the combination of washing machine and smart plug has been the source of some confusion within the household then I thought I would lay out how to get the best from that combination.

Bosch Washing Machine

Starting the cycle:

  1. Normally as found the smart plug should be on having been left on following the end of the prior load, but if not then turn the plug on using either (i) the button on the plug itself, (ii) the WIFIPLUG app or (iii) the Apple Home app.
  2. Load the drum in the normal manner which is optional washing liquid tab first with colour-catcher if required and clothes on top.
  3. Select and start the required cycle.
  4. Almost immediately stop the cycle on the smart plug using the same options as in #1.
  5. Put washing powder, fabric softener and water-treatment tablet (e.g. Calgon) in the drawer as required.
  6. At the optimum time the HEMS will restart the cycle. Do NOT move the cycle selector dial on the washing machine while waiting for the cycle to restart or the washing machine may become confused.
WIFIPLUG smart plug

Adding more clothes while waiting for step #6 above:

  1. Pull the drawer forward so that any water admitted to the washing machine will not take the contents of the drawer with it.
  2. Turn on the smart plug as per step #1 in the ‘Starting the cycle’ list – our washing machine starts to fill with water at this point.
  3. On our washing machine clothes may be added during the early stages of load by pushing the start button. Washing machine briefly displays “No” and then (i) stops the water flow, (ii) unlocks the door and (iii) displays “Yes”.
  4. Extra clothes may then be added.
  5. Once the extra clothes have been added, the door can be closed, the start button is pressed and then the washing cycle will resume.
  6. Stop the cycle almost immediately with the WIFIPLUG using any of the usual options.
  7. Close the drawer.
  8. At the optimum time the HEMS will restart the cycle.
Home

Ending the cycle is absolutely unchanged versus the washing machine without smart controls. The washing cycle finishes. The WIFIPLUG is left on by the HEMS. The door can be opened and the clean clothes dispositioned appropriately in the normal manner.

The dishwasher works similarly but more simply. The door can be opened at any time while awaiting the restart instruction from the HEMS and items removed or added as required.

Thoughts on intensity (of the CO2 variety)

CO2 production is increasingly of interest as the world struggles to limit man-made climate change. As we use different energy sources each represents a certainly amount of CO2 reflecting a combination of the energy invested to create that power source (e.g. the wind turbine may generate wholly renewable power, but its construction created some CO2) and the CO2 created as it generates energy once constructed (nothing for renewables but relatively high for fossil-fuelled generation).

I’ve previously shared this table showing the IPCC’s view of the embedded CO2 in different sources of electricity generation.

IPCC’s view of embedded CO2 in different sources of electricity generation

A recent question and resulting discussion in an on-line forum prompted me to think more about the area of embedded CO2.

My first observation would be that my rooftop solar panels do quite well on this scale with a CO2 figure of 41 gCO2/kWh.

The second observation would be regarding energy storage. My view would be that any energy storage device from a small scale domestic battery like my own to a large pump storage scheme can never deliver better embedded CO2 that the source of its energy. So, for example, if I charge my battery from my own solar at 41 gCO2/kWh with a cycle efficiency of 80% (the maker’s claim) then the embedded CO2 in the energy coming out of the battery cannot be better than 41 gCO2/kWh / 80% = 51 gCO2/kWh. Indeed it would be worse than that as this doesn’t account for the CO2 generated in creating the battery nor its operational life, but I don’t have figures for those.

Example of UK grid CO2 intensity

Thirdly, as my own embedded CO2 is relatively low whether exported directly from my panels or indirectly via the storage battery, then the CO2 intensity of the grid always benefits from my export. The 116 gCO2/kWh illustrated above is pretty low for the UK grid which varies widely but is still more than my solar PV directly or stored solar PV. Indeed had I exported onto the grid at the time illustrated above then my 41 gCO2/kWh versus the grid’s 116 gCO2/kWh would have saved 75 gCO2 for each kWh that I exported.

However if, for example, I export electricity but need to then buy more gas to make hot water then that too has a CO2 impact.

CO2 intensity of different fossil fuels (source: Volker Quaschning)

If I need to buy a kWh of gas to make hot water that’s 0.2 kgCO2/kWh or 200 gCO2/kWh even before I’ve accounted for the relative inefficiency of the gas boiler versus my electric immersion heater. If I assume that the gas boiler is 90% efficient then I will be responsible for 200 gCO2/kWh / 90% = 222 gCO2/kWh for a kWh used to make hot water. Thus, while exporting 1 kWh of solar PV may save the electricity grid 75 gCO2/kWh, it’s added 222 gCO2/kWh to gas consumption – a net deterioration of 147 gCO2/kWh.

Natural gas of course is the lowest CO2 of the fossil fuels listed above – if your home is heated by oil, coal or wood then the analysis is further skewed towards using your own self-generated power rather than exporting electricity and importing another fuel for heating.

The electricity grid’s carbon intensity also varies. In 2019 the UK average was 256 gCO2/kWh (a little higher than my estimate for gas) however this varies considerably through the year with the highest embedded CO2 in early winter evenings when I have little if any solar PV to contribute to the grid, and may well be lowest when I and others have surplus solar PV. My understanding is that the lowest grid CO2 occurs with a combination of high renewables (such as particularly windy weather) coupled with low demand (such as summer nights).

Thus my own strategy is to:

  1. Maximise self-consumption of my own solar PV as my energy source with the lowest embedded CO2 (except in the event of an extreme plunge pricing event when the grid is under highest stress)
  2. Make best use of storage to minimise consumption from the grid in the evening peaks when embedded CO2 is likely to be highest.
  3. When a solar-shortfall is anticipated then buy electricity selectively from the grid at lowest CO2 (using Agile electricity price as a surrogate for CO2).

Taking the plunge

For some time now my Home Energy Management System (HEMS) has been managing many of my domestic electricity consumers including:

  • car charging
  • dishwasher
  • home storage battery
  • washing machine
  • water heating via immersion heater
Domestic solar panels

The overarching strategy has been to:

  • maximise use of my own solar energy (rather than consume from the grid)
  • prioritise consumers for best value within the constraint of available solar generation
  • when power is needed from the grid to optimise the purchase price by shifting consumption to the cheapest periods (my price changes every half an hour)

For some consumers such as car charging and water heating this has resulted in those consumers switching between two modes:

  1. self-consumption when they are enabled to use the ‘free’ electricity from my own solar panels (subject to device prioritisation) with some proportional control
  2. boost when they run at full power drawing some if not all of the required power from the grid at the lowest available price

However the quite exceptional stress being put on the grid in the UK has prompted some expansion of capability. It’s normal once in a while that my electricity prices go negative, commonly caused by the overlap of large amounts of renewable electricity on the grid (e.g. excess solar output on sunny afternoons and/or high windfarm output due to wind conditions) and low demand (summer nights without heating demand, summer afternoons, bank holidays etc) which is exacerbated by the current corona situation. The current corona situation has made this more common and the plunge pricing events more extreme with multiple hours of negative pricing today some of which are into double digits (which I think is unprecedented). For car charging and water heating this has resulted in a new control mode.

The new control mode is a disabled status where the the device neither self-consumes nor is forced on. In the short term this increases export to the grid, but the mode is intended to help balance the grid disabling consumption for now to enable more consumption when the grid is under most extreme stress from an excess of generation over demand. Or to think of it another way, it passes up the opportunity to use free electricity now in order to be paid to use electricity later, responding to the price incentive to support balancing the grid.

Thus on a normal day these devices switch half-hourly between self-consumption (free electricity) and Boost (paid for electricity), but on a price plunge day then they switch half hourly between the new disabled state (no electricity, increased export) and the existing Boost (but now paid-to-consume electricity).

The full availability of modes is thus:

mode / otherabbreviationbattery storagecar chargerwater heating
Make and modelPowervault G200N/AImmersun v2
Control meansAPIRelayMixed API and relay
Mode – Boost++Powervault “Force charge”via HEMS relay (Ch 1)Via HEMs relay (Ch 2) to Immersun “External Boost” input.
Mode – Self-consumption only+Priority #1
Powervault “Charge only”
Priority #2
via Immersun relay output (Ch 3)
Priority #3
Immersun default behaviour
Mode -Disabled0N/A – Available in API but not used by HEMS.via HEMS relay (Ch 4)Immersun “Holiday” mode via API
Mode – Both self-consumption and self-discharge available+/-Powervault “Normal” via APIN/A – No reverse flow from car to home available (not V2X capable)N/A
Major device modes available to HEMS

We should thus be better equipped to support the grid in the current circumstances.

Import / Export at the smart meter for Saturday 23rd May

The chart above shows the resulting behaviour. In particular the large negative currents through the morning to early afternoon show that much of the normal self-consumption has been disabled. Then from mid-afternoon the import shows the effect of enabling multiple consumers simultaneously. Here the behaviour of the car charging and water heating was boosted as this point, while the dishwasher’s and washing machine’s existing behaviour added to load.

Not that it has anything to do with the revised controls, but the spikiness of the export during the day shows the highly variable nature of the export through the day being a function of both variable generation through passing clouds and variable consumption with kettle boils and the like. Thus it’s important that consumers for self-consumption have automated closed loop control since manual control of an immersion heater or car charger to achieve high self-consumption with minimal import would have been almost impossible even with a level of human intervention wholly at odds with the scale of savings achieved – small savings hour-by-hour add up over the day, weeks and months but their value is relatively tiny compared to the labour to attempt equivalent control manually.

Here is a similar import-only half-hourly view from the smart meter WAN side:

Half-hourly consumption and cost from smart meter WAN side.

The screenshot above clearly shows those periods of import being targeted at the periods with the most negative prices. Since different consumers need power for different periods of time (for example it takes about 7 hours to charge the battery, but only 2 or 3 for a tank of hot water) then consumption rises as cost falls. My consumption-weighted average cost was -6.22 p/kWh yesterday. However the same price point during the day or night has delivered different consumption from the grid as the output of the solar panels must be consumed before consumption from the grid starts. We are still some way from the point where it becomes economically attractive to disable the solar panels.

Daily electricity cost for 28 days (vertical scale is fractions of a pound)

Finally the above image shows the last 28 days of electricity average cost in p/kWh. Although some other days included some periods of negative pricing, the quite exceptional pricing yesterday is amply illustrated with the combination of extreme prices and the new load management mode delivering revenue (i.e. negative cost) of 82.4 pence through consumption of 13.252 kWh at an average 6.22 p/kWh.

This is something of a zero sum game in terms of consumption as I don’t artificially increase consumption to improve income – such as leaving the oven on with the door open during a summer day – this is all about shifting consumption that would have happened anyway. However we have not only supported the grid when it is most stressed but also reduced our energy costs significantly (to the point of being significantly negative) by moving consumption from being predominately self-consumption (i.e. from our own solar panels) to being predominantly grid consumption.

Monitoring the HEMS

For some time now I’ve been thinking about creating a real time display which pulls together data from a variety of sources around the home to provide an overview of what’s going on without the need to visit multiple web pages or apps. Until the last 10 days or so that involved little more than thoughts of how I might evolve the existing immersun web page with more content (I don’t have the skills to write my own app), but then about 10 days ago I saw an online gauge that someone else had created to show energy price and inspiration struck. Ten days later I have my monitor working, albeit not complete:

HEMS monitor

The monitor pulls together information from:

  • My electricity tariff for p/kWh
  • My immersun for power data (to/from: grid, solar, water, house)
  • My storage battery for power in/out and state of charge
  • My HEMS for electricity cost thresholds between different battery modes.

The gauge consists of two parts: (i) an upper electricity cost part and (ii) a lower power part.

The upper electricity cost part is effectively a big price gauge from 0 p/kWh to 25 p/kWh with a needle that moves each half hour as the price changes. It has various features:

  • The outer semi-circular ring (blue here) shows today’s relationship between battery mode and electricity price. Today is very sunny, so no electricity was bought from the grid to charge the battery, and this part is all blue for normal battery operation. If the days was duller and electricity was to be bought to charge the battery, then two further sectors would appear:
    1. a dark green sector from zero upwards showing the grid prices at which the battery would be force charged from the grid, and
    2. a light green sector showing when the battery is not permitted to discharge but may continue to charge from solar.
  • In inner semi-circular ring (green / yellow / red here) currently just colour-codes increasing electricity price, but will be used to show today’s prices at which car charging and water heating are triggered from the grid.
  • The current price per kWh is taken from Octopus’s price API, while the current cost per hour is derived both from this and the grid power from the immersun.
  • The needle grows from a simple dot indicating the price per kWh only when no power is drawn from the grid to a full needle when the electricity cost is 10 pence per hour or more.

The lower power part is effectively a power meter ranging from 5,000 Watts of export to the left to 5,000 Watts of import to the right. It updates every few seconds. It has various features:

  • The outer semi-circular ring (orange /maroon / green here) shows how power is being consumed:
    • orange – shows consumption by the house less specified loads
    • maroon – shows battery charging
    • blue (not shown) – shows water heating
    • green – shows export to the grid
  • The inner semi-circular ring (yellow here) shows the source of power. The sum of the sources should equal the sum of the consumers. The sources are:
    • maroon (not shown) – shows battery discharge
    • yellow – shows solar power
    • red (not shown) – shows grid power
  • The power value shows the net import or export from / to the grid, while SoC refers to the state of charge of the battery (0-100%). The combination of import power and electricity price gives the cost per hour in the top gauge.
  • The needle position shows net import (to the right) or next export (to the left). The needle should thus be to the left of the green sector, or to the right of the (unseen) red sector. Needle length show the full power being handled and is thus proportionate to the angle of the sector including all the colours in the lower gauge and extends from 0 to 5 kW.
Monitor installed on an old phone in the kitchen.

The gauge scales to fill the smallest of screen height or width and translates to be centrally positioned regardless of screen size. My intention is to display it on an old mobile phone as an energy monitor, but I can also access it on any web browser on any device within the home.