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.
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.
As regular readers will know, I shift my major loads (including battery, car and water heating) around to exploit the cheapest periods on my tariff. Yesterday I had a conversation with someone who was very keen to do the same with a fridge. I’ve never considered doing this a fridge on two grounds:
Is it safe to mess with a fridge or freezer if that risks compromising the temperature and thus the safe storage of food?
Is there enough saving to be worth even considering this?
Although I don’t explicitly measure the power consumption of the fridge, we can make some assumptions from data that I do have.
According to the US Department of Health, a fridge can be safely left for 4 hours during a power cut, but you should avoid opening the door. That period at least corresponds to the duration of the early peak (and thus potentially the highest cost power), but I still don’t think that this would be worth the effort.
The above illustration shows my home drawing 168 Watts overnight. That load is all the standby loads in the house (TV, DVD, oven, microwave etc), assorted phone and iPad chargers, cordless phone power, WiFi router, alarm clock, central heating controls, smart plugs and the fridge-freezer.
For a couple of periods overnight the consumption dips to around 114 Watts, a difference of 54 Watts. I can think of nothing on my list of loads that might cycle on/off except the fridge-freezer so potentially that’s the near-continuous 54 Watt load. It’s conceivable of course that the fridge-freezer load cycling between two levels, but the lower level cannot be more than 114 Watts. 114 Watts is 4 p/hour at my highest electricity cost of 35 p/kWh or 1 p/kWh at my average rate of 9 p/kWh. This doesn’t seem a worthwhile saving to me.
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).
My current energy management arrangements are designed to maximise use of the output of my solar panels for lowest energy cost by diverting any excess to PowerVault storage system, car charger or immersion heater. I can also manually configure the PowerVault and ImmerSUN to minimise costs of bought energy from the grid (I get 7 hours of cheaper night time electricity) by setting time periods for charging.
However as I move to a smart meter and smart tariff then I’m looking to start automating the selection of when to draw power from the grid based on costs that change half-hour-to-half-hour and day-to-day. The hardware to achieve this is illustrated here. To the right is a Raspberry Pi – a small computer with a wide range of connectivity – and to the left is a module that sets on top and has four relays able to switch mains loads, although at the moment I only anticipate needing 2 of them.
One of the relays will switch the boost input to the ImmerSUN to enable water heating, potentially when electricity is cheaper than gas, and a second relay will operate the transmitter that turns the car charger on alongside the ImmerSUN’s relay output during the cheapest available energy times.
The image to the right shows the timers that can be used to enable the ImmerSUN outputs to draw power from the grid. I never use this for water heating as currently gas is always cheaper than bought electricity, but do use it to more or less effect seasonally to charge the car from cheap night rate power when there isn’t enough daytime solar. For the new HEMS I plan a table of 7 days specifying the number of hours required for each output and let the HEMS find the cheapest half hours to deliver the total hours required and enable the charger or water heating as required.
Earlier in the week I received notification from my electricity and gas supplier that my 12 month contract was coming to an end. I did my usual search for the best value Economy 7 tariff but drew a blank – everything including renewal with my existing supplier was rather more expensive than I’m paying now – so I decided to be rather more adventurous.
My decision was a significant change – not just a move from Economy 7 to a smart meter, but also the addition of a smart tariff (one that changes rate multiple times per day), and indeed my chosen tariff is dynamic so it potentially changes every half hour and day-to-day. As I don’t yet have a smart meter then I’ll continue on Economy 7 until the meter is replaced, but then adopt the dynamic tariff. With flexible loads like electric car charging and my storage battery then I should be well equipped to make the most of such a tariff.
On the dynamic tariff rates are published each day at 4:00 PM for the next day. Some times (not very often!) prices even go negative so one is being paid to consume. At other times electricity is relatively expensive (early evening’s principally) but the battery should help me minimise purchases during such times.
I’ve already checked the battery storage and it has the ability to be programmed very flexibly around different electricity prices at different times of day so that it doesn’t just absorb surplus solar but charges at lower cost times to discharge at higher cost times.
I also want to explore opportunities to automate the response to tariff changes – potentially linking storage battery, car charging, and water heating to tariff as well as self-consumption.
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.
The image above shows my solar electricity generation and usage in 2017 by calendar month.
The purple line shows the use of electricity (excluding gas replacement) by month. It’s relatively stable through the year, although it does rise in the autumn as my daughter started school and so my electric car mileage increased.
The green line shows the output from the solar panels. From April to August (5 months) solar panel output exceeded usage giving the potential not to buy electricity with sufficient smart capability – be that electricity storage or alignment of consumption with availability.
The red line shows import of electricity from the mains. It tends to be the reverse of the solar panel output. It’s never zero indicating potential (at cost) to improve smart usage. Solar power is a more significant energy source than imported power from March to September (7 months).
The blue line shows diversion of surplus electricity to water heating as gas replacement.
The turquoise line shows export of electricity to the grid. This occurs when there is insufficient energy smart resource available to store or self-use the surplus power. Export amounts to about 12% of total solar panel output. While this potentially free energy, the economics of storage or smart controls make using this remainder increasingly costly from an investment perspective. In my case this surplus occurs on particularly sunny summer days when the electric car is not at home or is already fully charged – which might be vacation periods when the house is unoccupied for example.
Here we have the system in action earlier today. With 1542 Watts coming from the solar panels, the house (including the PowerVault storage battery) is running at a maximum of 1097 Watts, with the balance of the available power controlled by the ImmerSUN- 410 Watts to water heating and at the point of this snapshot 35 Watts into the grid. At this moment that’s 97% of generation used as such self-consumption and 100% of energy being consumed coming from the solar panels.
if the ImmerSUN had priority then it would have taken all the available power leaving nothing for the PowerVault storage battery.
In a prior post I described the use of current clamps to prioritise smart loads that are enabled by surplus solar power to maximise self-use of this ‘free’ electricity. That’s free in the sense that a deemed export tariff doesn’t pay any more for an extra kWh exported, or pay any less for an extra kWh used, and so the marginal cost of using that (and every other) kWh is zero. In that post three current clamps were visible in the picture – though I described only the function of the right-most.
In fact my home currently has 6 current clamps which is probably more current measurement than the substation that supplies my area. The six clamps are as follows:
Solar panel output
Measure sum (3)
Import / Export
Battery In / Out
I’ve tried to distinguish between their functions as follows:
Control – is an output current actively controlled by a device so there’s generally no need to measure it with a clamp.
Measure – a clamp whose output is analysed automatically to create a control action such as divert more or less power to some device.
Report – it’s just reporting something for the purposes of understanding, but it’s not used to directly control anything.
So the 6 clamps are:
The optional ImmerSUN monitoring package adds a clamp to measure the output of the solar panels which then enables the charts and self-use calculations that I’ve used before to illustrate system behaviour.
The ImmerSUN fundamentally operates by measuring export via this clamp and then responding to minimise that export.
The operation of the PowerVault uses this clamp. In most installations that’s simply around the live of the incoming supply, but for me it’s also around the live feed to the immersion heater to set priorities.
I also have 3 data loggers at my home to provide a year’s data for UKPN around self-usage so that they can assess the impact on the grid from large scale battery adoption. Each logger measures one of the fundamentals for battery behaviour: output from the solar panels, ..
.. import to / export from my home, ..
.. and current to / from the PowerVault. From those three you can infer what is being used by my home in its entirety, but not how power is divided between (for example) car charging and water heating.