Yesterday provided a good example of my HEMS in action as the electricity price dropped quite low due to stormy weather conditions. Normally at this time of year the HEMS isn’t doing much with the storage battery as daytime solar output is enough to fully charge the battery, but yesterday low pricing was enough to automatically enable both battery charging and water heating overnight. Car charging was due to run anyway driven by the demand for an hour of charging, but battery charging and water heating was triggered by the low price rather than a needed to take power for a pre-defined period of time.
The screenshot above from my phone shows the HEMS’ plan for the the early hours of the 9th. The first price column shows one hour of car charging at the cheapest price. The second column shows half an hour of water heating as the electricity price has fallen below 3.5 p/kWh when it is assumed to be cheaper than gas. The third column shows four hours of battery charging when the electricity price is below 5 p/kWh.
The above image from the HAN side of my smart meter shows the energy consumption of the house varying through the night in response to these requests from the HEMS – battery charging at the widest point, car charging above that for an hour, and water heating above that for 30 minutes.
Finally this image shows the energy consumption versus price data for the same period shows how the action of the HEMS increases electricity demand as the price drops. Indeed on this day there was virtually no consumption at any other time.
For August 9th as a whole I paid 52 pence for 7.547 kWh of electricity. Taking off the 21 pence for the standing charge leaves 31 pence for the electricity kWhs alone, an average of 4.11 p/kWh.
After a series of quite detailed posts, I think that the time has come for an updated high level overview of what we have.
We moved to our early 1970s house almost 4 years ago bringing with us our electric vehicle. The house had already been refurbished with new double-glazed windows, had cavity insulation (although that wasn’t recorded on EPC so must have predated the prior owners), and a token level of loft insulation. The existing gas boiler was arthritic, couldn’t heat the whole house, but was quite good at heating the header tanks in the loft! We had gravity-fed gas hot water (i.e. no thermostat or pump on the cylinder) which was completely obsolete, the cylinder dated back to the building of the house and had no immersion heater (although we had the wiring for one). So what did we do?
We substantially increased the loft insulation to reduce heat loss.
We had a modern condensing gas boiler installed to improve efficiency.
We updated to smart controls using eTRVs to set both temperature set points and schedules at room level. I built a smart interface to the boiler so that heating can be enabled remotely. I programmed a series of rules into Apple Home allowing the smart thermostats to enable the boiler when any thermostat wants heat and disable it when no thermostat wants heat. Some rooms also have additional rules linking heating to open windows or movement sensors. All of this reduces heat losses by only heating rooms that are (or will be shortly be) in use.
We installed our own solar panels given 4 kWp generation. (I also own a small share of a solar farm although there’s no contract that I’m aware of between that farm and my home energy supplier)
I invested in an immerSUN to maximise self-use of our own solar by enabling loads when surplus solar is available.
We switched to a green electricity supplier so when we need to buy electricity it comes from renewable sources.
We bought a small storage battery 4 kWh to store some of our solar production for use later in the day. Subsequently I can also use it in winter to buy when the electricity price is relatively low to avoid buying when the price is relatively high.
We chose a dynamic smart tariff to buy electricity at the lowest price based on market prices established the day before. The prices change each half hour and are established in the late afternoon on the day before.
We replaced the old hot water cylinder with a modern insulated one (to reduce heat loss) with a low immersion heater (to allow more of the water volume to be heated).
Our principal water heating is now by diverting surplus solar electricity proportionately to the immersion heater, that’s backed up by the gas boiler which is enabled briefly in the evening for water heating in case the water isn’t yet up to temperature, and when the electricity price falls below the gas price I can enable the immersion heater on full power.
All accessible hot water pipes are insulated.
Electric car charger:
I built my own electric car charger that takes an external radio signal to switch between four settings 0, 6, 10 and 16 Amps to help me adjust consumption to match to availability of output from my solar panels. (Subsequently such products were developed commercially with continuously variable current limits, but the limitations of my immersun and on/off radio signal don’t allow me to go quite that far. Having said that my car only does 0, 6, 10 and 14 Amps so I would gain no benefit from a continuously-variable charger paired with a 4-level car).
Smart electricity controls:
We have two systems for smart control of electricity:
The immersun to maximise self-use of our solar electricity by proportional control of loads.
A HEMS to manage the purchase of electricity (when necessary) at the lowest price by maximising consumption when the price is lowest.
When both systems want to enable loads (because the bought price is low and we have a surplus from our own panels) then cost is prioritised, so we’ll buy from the grid any demand not being met from our own panels.
Both systems are linked to 3 devices:
Battery storage. The immersun is configured to work alongside the battery storage with the battery storage as the top priority to receive surplus solar PV. The HEMS can switch the status of the battery as required to charge from the grid when the price is lowest, or to discharge when the price is highest, or indeed to revert to default behaviour.
Car charger. Second priority for the immersun after battery storage.
Immersion heater. Third priority for the immersun after car charging.
I have no firm plans for the future. I’m toying with adding to the HEMS various features including:
Making the display switch between GMS and BST as appropriate (it’s all UTC at the moment).
Edit configuration via the web interface rather than a virtual terminal.
Control a domestic appliance. Our washing machine was replaced relatively recently, but the dishwasher is playing up a little and may be a candiadte for HEMS integration where the optimum start time is selected to deliver lowest energy price.
The above images show four different perspectives on the same day of data (April 24th) from different sources within the home.
Firstly, the Smart Meter HAN image shows bought electricity to the home. Each smart meter sits on a Home Area Network (HAN) which is how the In-home display provided with the meter gets its data. The in-home display is an example of a Consumer Access Device (CAD). In my case I also have a Hildebrand Glow Stick as a CAD. The Glow Stick, which looks something like an oversized USB stick, also connects as a CAD to the smart meter allowing the meter to be read. An associated app, Hildebrand’s Bright, allows the Glow Stick to be read via the cloud. In principle the Bright app can display either energy in kWh or cost, but in my case can only display energy in kWh as Octopus don’t push the price data into the smart meter so energy cost always reports as zero. The data is presented by the minute.
Secondly, the Smart Meter WAN image shows the same data but from the perspective of the Wide Area Network (WAN) whch connects the smart meter to the energy retailer (Octopus for me). This half-hourly data is reported via the Octo Watchdog app. The data reported is cost per kWh (the blue line) and energy consumer / kWh (the red columns). The energy data in the red columns follows that of the red line in the prior illustration but in lower resolution (half-hourly versus minute-by-minute). You can clearly see most energy being bought when the price is lowest.
Thirdly, the Powervault image shows grid in/out and battery in/out. The green grid-in line mimics the red data from the above images. The battery in/out data is solely visible in this image. The resolution is good enough to see shorter events like kettle boil cycles.
The final image, from the Immerun, is probably the most useful although it lacks energy price and hides battery in/out within the House data (hence ‘House’ being zero at times). The immersun alone reports output data from the solar panels and diversion to the immersion heater. It also lumps the car charger energy within ‘House’, indeed none of these views can directly report the car charger behaviour although its the dominant energy consumer here.
I’m planning to construct my own view showing all the different prices of data together in one place. I already have access to:
The Immersun data via the same API called by their app. I came across a blog post that described how to do this.
The Powervault data API (I only have a control API at the moment) which should give me battery in/out (at least I’m on a promise of the API at the moment).
The Hilbebrand data which duplicates the Powervault Import/Export at the moment, but has the potential to provide independent monitoring of my car charger.
In principle then that would leave me able to report 3 x energy sources (grid, panels and battery; of which grid and battery would be bi-directional) and report 3 x energy consumers (car, water heating and home).
A recent discussion centred on whether, as someone in receipt of UK feed-in tariff (FiT) and having a smart meter, I should be on metered export or deemed export. I had previously been advised that my smart meter did not have an export register, but playing with the buttons revealed an active export register.
There could be an opportunity to use the export meter as part of an export tariff instead of the FiT export component, which wouldn’t have a restriction on buying and exporting grid power, but my high levels of self-use make this unattractive compared to deemed export at 50% of generation.
Today I’ve been thinking about configuring the Powevault storage system around my smart electricity tariff in which costs can vary every half hour.
My battery has an ability to be configured around a tariff called TIDE. I don’t have TIDE, but the ability to configure around TIDE can be reused for my tariff. The unit can be configured using a table or the clock as illustrated. There are several sections:
Force charge – the battery storage charges at its maximum power (only 800 Watts) for 3.5 hours while electricity is cheapest.
Charge only – a period while electricity is not quite so cheap when the battery will charge proportionally to any solar surplus, but will not discharge. It wouldn’t make sense to start discharging the battery when electricity is only marginally more expensive than when the battery was charged. This also prevents the battery discharging into my electric car if the car charges for a longer period than the battery.
Normal – storage system will either charge proportionally to solar surplus, or discharge to minimise input.
Force charge – as previously charges at maximum power in this case to ensure that some power is stored prior to the most expensive period.
Normal – as previously but intended to cover the peak rate 4:00 to 7:00 PM period and beyond if there’s still stored energy.
Now that this pattern has been created, it can be adjusted by dragging the tabs around clock to adjust for when the cheapest power is at a different time day-to-day.
In the longer term I hope to automate such adjustment, although my priority is getting the car charger to automatically operate when power is cheapest.
In the last few days I’ve reported our status on electricity generation from our solar panels and our gas consumption, so here comes some thoughts on electricity purchase from the grid.
Starting in late 2015 after the meter was changed to Economy 7, there’s a general downward trend from November 2015 to March 2016, before my car charger project kicks in maximising use of my own solar electricity to charge my car (when available) which causes a significant drop in purchased electricity between march and April 2016. That seasonal saving gradually drops through the autumn, although it’s interesting that by November 2016 we’re back on what seems to be a continuation of a downward trend from January to March 2016. Electricity purchased is also significantly lower than 2015 as we enter the second year.
The second significant change is the addition of the storage battery in December 2016. However from January to August 2017 (yellow) electricity purchased is significantly below the prior year (magenta) – potentially showing the benefit of the battery in saving electricity generated during the day to reduce consumption later in the day. This benefit largely disappears from September to December 2017, presumably because my increased vehicle mileage after my daughter started school is offsetting the prior savings.
2018 (orange) generally falls somewhere between 2016 and 2017 as it combines both the storage battery and the higher vehicle mileage throughout the year to date.
Summer is definitely upon us now as we enjoy the glorious summer weather. Disappointing weather earlier in 2018 has given way to two record months in May and June which yielded the highest monthly outputs for their respective months since the system was installed back in 2015. Some days we buy no measurable electricity or gas (given that the electricity meter has a resolution of 1 kWh) depending on what the need to charge the car. If the car is at home then I can fully charge it from the solar panels, whereas if the car is at away from home during the day then I may need to give it some charge overnight. When charging overnight I have been tending to charge for the minimum number of hours up to 7:30 AM when I typically leave home on a weekday – that pattern provides for mostly Economy 7 Energy from the grid less whatever comes off the solar panels from the rising sun less whatever might be left in the PowerVault from the previous day as illustrated below:
The green ‘hill’ from around 3:30 to 7:30 AM is created by car charging. Normally this would be seen as a rectangular block as the car charger effectively runs at a constant 10 Amps (2.3 kW) through the operating period. However in the illustration the charging event (at least in terms of power drawn from the grid) seems rounded at both the beginning and the end. At the beginning of the car charging period the mustard ‘Device Power Out’ curve shows the last remaining stored energy from yesterday being drawn from the PowerVault, while at the end of the charging period the ramp down is a result of increasing output from the solar panels reducing the need for power from the grid. Hence at the moment the car charging ends there’s a sudden switch to charging the PowerVault at full power (the blue line) and some surplus power not used by the PowerVault (the purple line) – suggesting that something around 1 kW is suddenly available. Although the purple line is described as ‘Grid Power Out’ that’s not strictly true here as much of that surplus is being diverted to make hot water (although this is invisible to the PowerVault).
After that digression, my actual purpose in making this post was to reflect upon relative energy costs and the best use of my solar power to reduce energy cost.
n/a - no economic case to charge battery from grid during day
Optional - need to consider value of saved energy versus cost of 1 cycle of battery cycle-life
Self-use Priority #1 via PowerVault (daytime electricity -> solar)
Manual 3rd backup (typically only used for long journeys when charging en-route becomes impractical)
Manual 2nd back-up
Automated 1st backup
Self-use Priority #2 via ImmerSUN (nighttime electricity -> solar)
Manual 3rd backup (never used in 3 years)
Manual 2nd backup (never used in 3 years)
Automated 1st backup for dull days
Self-use Priority #3 via ImmerSUN (gas to solar)
n/a - a summer solar surplus is a poor match to winter heating demand but could be Priority #4
The table above shows columns of energy sources ordered by reverse energy cost versus the major energy consumers in the house: battery storage, car charging, space heating and water heating. Energy consumers are ranked according to the value of displacing the the alternative energy course if not solar:
Battery storage – I currently only charge the battery storage from solar, although there would be a seasonal economic argument to charge from cheap rate electricity if the differential between day and night rates was higher.
Car charging – I generally charge on cheap night-rate electricity when I don’t have enough solar. In summer I program my car charger via the ImmerSUN’s 7-day timer to deliver sufficient charge for the day ahead, but sufficient headroom to make use of any available solar.
Water heating – water heating is my 3rd priority for solar self-use and is automatically based up by the gas boiler which runs for an hour making hot water in the early evening if the tank isn’t already hot from diverted solar power during the day. The gas thermostat is also set slightly colder than the immersion heater – still very usable for a bath or shower from gas but giving some ability to delay water heating from a dull day to a following sunny day.
Space heating – my space heating is generally gas. It would be possible to run a heater (or heaters) such as storage radiators via the ImmerSUN’s third output, but I consider that the cost of the heater(s) and installation is unlikely to be recouped given the major mismatch between surplus solar generally being in summer and heat demand being in winter.
In the last few days I’ve been assisting a reader of this blog who also has an ImmerSUN immersion heater controller plus PowerVault storage battery combination. Like me, he had the immerSUN first and later added a PowerVault, but had immediately disabled the ImmerSUN to get the PowerVault to work. Left to their own devices, the ImmerSUN will normally take the surplus power first before the PowerVault has chance to respond since it has a more dynamic control system, however economically it makes more sense to prioritise the PowerVault.
Both ImmerSUN and PowerVault rely on current clamps to get their control signal. Such clamps fit around an electrical cable and measure the flow of electricity through that cable. Normally clamps for both these devices alone would be around the live in the incoming mains cable, but do that with both and the ImmerSUN will always take the available power first which is undesireable.
The illustration shows my solution, as per the prior post, where the PowerVault clamp surrounds both the incoming live and the live output to the immersion heater. If the clamps are correctly orientated, this allows the PowerVault storage battery to be prioritised over the ImmerSUN immersion heater controller. When the PowerVault clamp surrounds the two cables, it is important that outgoing power to the grid and outgoing power to the immersion heater from the consumer unit pass through the clamp in the same direction. This solution should work for clamp-driven solutions too.
The PowerVault clamp is directional – it has an arrow which should point towards the consumer unit. That means, if you pair the two cables as I described, so outgoing power flows in the same direction in both cables through the clamp, then the arrow should point in the opposite direction i.e. towards the consumer unit.
Fundamentally it doesn’t matter which way the ImmerSUN clamp faces, as during commissioning the ImmerSUN will work it out by cycling the power several times, but you shouldn’t change it after commissioning.
On Wednesday an unusual pattern of vehicle use occurred (at least for me) which showed another way the charger could be used. I needed to take my daughter on a round trip in the morning (dad’s taxi), return home, and then repeat the trip later in the afternoon. Each round trip pretty much exhausted the range of my electric car, so I needed to recharge in between. My normal home charge routine would have prioritised the home storage battery, but that would have left me completing the second round trip partially on petrol which isn’t the optimum solution.
To fully recharge the vehicle to complete the second round trip (and thus avoid using petrol) I disabled the programmable logic controller (PLC) on the car charger using a push button which suspends the program causing the charger to operate at full power. It was a sunny day (at least for March) so the solar panels were producing a similar amount of power to that required to charge the car and, with the fixed battery partially charged during the period I was making the first round trip, the fixed battery was able to manage the difference between power generated by the panels and that required by the home (including car charging) for much of the period.
The result was an almost fully charged car for the second round trip (sufficient to avoid using petrol) for only around 1kWh of grid energy.
The end of 2017 sees 2 full calendar years of output completed (plus a few months at the end of the prior year) so it seemed like a good time to assess performance and return. Return comprises two parts – firstly the payments received for electricity generation (and export) the so-called feed-in tariff or FiT and secondly the savings obtained from using that free energy myself instead of buying it from the grid. In my case I can use and store my solar electricity directly, or use it to heat water via the immersion heater thus replacing gas.
I’ve summarised the status in the following table:
Return in calendar year
of PV cost
of PV cost
Output from the panels was slightly reduced in 2017 versus 2016, but still significantly above the performance projected in the quotation. I put the slight reduction down to year-to-year differences in the weather. Over time I would expect panel output to decline, but I think it’s too soon to attribute any decline to this.
Income from FIT was slightly higher as inflation on the price / kWh has overcome the slight output reduction.
Electricity self-use is up considerably from 41% to 64% due to my 4 kWh storage battery. The costs of that battery are not reflected in the table. Return would have been 8.8% in 2017 (rather than 15.1%) if the cost of the battery was included.
Gas replacement is down from 27 to 23% versus 2016 as more of the electricity from the panels is used for high value activities like charging the storage battery or my electric car, and less is left over for water heating. The low price of gas per kWh makes gas replacement my lowest priority for self-use.
Exported electricity (i.e. what’s left-over that I cannot use myself) is considerably reduced from 32 to 12% largely as a result of increased electricity self-use.
Financial return in calendar year 2017 is improved from 13.6 to 15.1% neglecting the costs of the battery, or reduced from 15.1 to 8.8% taking into account capital costs of the battery.
It will take approximately 7 years (i.e. 2 past + 5 future years) for the combination of the solar panels and battery storage to pay for the solar panels (neglecting the battery costs), and a further 2 years of system savings to pay for the storage battery.