It’s been a beautiful sunny month in Chippenham and a great month for solar generation. In this post and video our solar admin Matthew takes a look at the different strategies he uses to make the best of that FREE solar electricity and minimise household energy bills.
Introduction
I first installed solar on our house five years ago. I have a micro generation system installed on the flat roof of our garden room and kitchen extension, giving a maximum of 4kW of peak generation. For us installing rooftop solar was one of the best things we did to reduce household energy bills and give a feeling of energy independence. The question is – how can you best utilise that free solar electricity generation to reduce your household energy bills, particularly if you’re at work during the day?
Smart Export Guarantee (SEG) payments.
The easiest option for most people is to get paid for the solar generation you don’t use. To recap i’ll take a quick look at how solar generation works…
How does solar generation work?
Solar Photo Voltaic (PV) is quite simple in its operation. When the sun comes up your solar panels will start to generate DC electricity. In order to use that electricity in your house you will have a so called ‘inverter’ that converts DC to AC (In my case each panel has a micro inverter behind it and these are connected in parallel.) The inverter actually does slightly more than that in that it synchronises to your incoming grid AC supply both in frequency (50Hz), and in terms of the voltage it generates which is at a slightly higher voltage than your incoming supply. By generating at a slightly higher voltage than your incoming supply your solar inverter is your own mini power station, supplying your household loads first, with any excess energy flowing out to the grid. By the grid here we really mean your nearest neighbour connected on the same phase. As most streets have a three phase supply this is likely to be your nearest neighbours two doors up/down the street.
Solar generation typically follows a curve over the course of the day, peaking when the sun is highest. This curve may flatten out a little more if you have east – west panels on two roofs.
The picture below shows my generation on the 30th April 2025.
The Smart Export Guarantee (SEG) scheme introduced a few years ago as a replacement for the Feed In Tariff (FIT) offers payments of typically 15p/kWh for your exported solar.
In order to get these payments you’ll typically need to register for export with your preferred electricity supplier. It doesn’t have to be the same supplier as the one you use for import, but most offer a higher export rate if you use them for both supply and export.
In order to register you will need:
- A Microgeneration Certification Scheme (MCS) certified installation. Note it’s not just the equipment that has to be MCS compliant, but the installation has to be installed by an MCS certified installer.
- Approval from your Distribution Network Operation (DNO) to export electricity. This is usually in the form of an email, and in Chippenham the DNO is Scottish and Southern. Note the DNO is not the same as your electricity supplier.
- Building regulations sign off for your installation. Typically this is a Part P electrical certificate from an NICIEC or NAPIT electrician.
- You will most likely need a Smart meter.
If you’ve been through the Zero Chippenham Community Solar discount scheme you will have all of the necessary certification.
So registering for the SEG is the simplest way of reducing your household energy bill by being paid for the solar you export to the grid.
What other strategies might you employ? What if your overall aim is to reduce the amount of electricity you import and maximise your own generation?
Battery Storage
In the last three years it’s become increasingly common for a solar PV installation to be paired with battery storage. In a new installation this is typically done by a ‘hybrid inverter’ that not only converts the DC generation from your panels into AC, but also manages the charging and discharging of battery storage. At the peak of the day when you have excess solar your home battery is charged. When the solar generation drops your inverter draws stored energy from your battery to maintain your household supply. The cost of home batteries has decreased significantly over the last five years. It is also possible to install a so called ‘AC battery’ which is really a battery and inverter combined. You can even do this without solar, though I personally think its the combination that works for well.
The battery in the video is a 5kWh Alpha ESS battery which i’ve described in a previous post.
If you have an older solar installation when the FIT tariff was in place you may wish to consider an AC battery. The advantage being you don’t need to disrupt your existing installation which might affect your FIT payment.
In the winter you don’t really get enough solar to take full advantage of battery storage, but if you’re on a smart electricity tariff – in my case Intelligent Octopus Go, you can charge your battery at off peak rates and then use the battery energy during peak rate times. This can significantly reduce your winter electricity bills.
Solar predictive Charging using Home Assistant
Home Assistant is free open source software used to automate your home locally. It’s gained a significant user base around the world and has regular software release and a whole ecosystem of developers. You can download it for free and try it on an old lap top or Raspberry Pi computer. In my case I initially ran it on a Raspberry Pi 4 for a year before purchasing Home Assistant Green, which is dedicated hardware that comes preinstalled with Home Assistant. (Note: Though you can find the Home assistant app on the app store, it needs separate hardware acting as a Home assistant server to run).
The clever thing about Home Assistant is it’s not locked to any particular hardware ecosystem or company. When I first plugged mine in it immediately located my Enphase Solar PV system. I had to manually install an integration for my home battery.
So now I have access to my solar generation, and, control over my home battery.
The third part of the puzzle was to install a free solar forecast integration. This is a bit like a weather forecast and predicts how much solar you’ll generate tomorrow based on time of year, weather, angle of panels etc. It’s quite accurate with a bit of a tweaking.
Then I use a very simple algorithm to decide what percentage to charge my battery overnight based on the next day’s solar forecast.
The above shows a screenshot from Home Assistant showing how this is done.
At 23:06 at night the solar forecast is checked, and if it’s over 20kWh I only set the battery overnight charge to 30%.
If this type of algorithm is understandable to you then Home Assistant might be for you. If it looks too complicated then Home Assistant may not be your best choice, but don’t let this put you off. For most people Home battery storage is controlled with a simple app. I’ve optimised my battery storage to make maximum use of my own solar generation.
Home Assistant also gives you a great overall picture of your household energy usage in almost real time. This might appeal even if you don’t use the automations. It’s a great help if you have solar from one manufacturer, battery from another, an ev charger from another, and want to see everything in ne place…
Charging an Electric Vehicle (EV) with Solar
I covered my MyEnergi Zappi charger in a previous post here.
Charging your car with excess solar is extremely satisfying, though realistically is only possible during the summer months. Most charging is done using low rate off peak electricity.
Heating Hot Water
We changed our heating and hot water system to an Octopus Installed Daikin Altherma 3 Air Source Heat Pump in October last year. I covered the installation process in this post.
I then did a follow up on running costs in this posts.
As part of the installation our hot water is now supplied by a 180litre Daikin hot water tank. Most of the time this is heated by the heat pump on a schedule such that hot water is heated at night on off peak rates. The tank does also have an immersion heater, though I only use this once a week for the anti-legionella cycle. Typically the heat pump can heat the hot water to 50 degrees C and the immersion adds the extra 10 degrees C taking it to 60 degrees C for Anti-legionella.
Some people opt for another MyEnergy Solar diversion product – The Eddie. This is similar to the Zappi in that it uses excess solar to heat the hot water via the immersion. As immersion heating is 1kW of electricity = 1kW of heat, it can be quite energy intensive. Hence the Eddie can proportionally control the immersion to only use excess solar.
I didn’t opt for the Eddie and use the Heat Pump to heat the hot water when I have excess solar. The advantage here is 1kw of solar electricity produces 3kW of heat.
I use Home Assistant to integrate with the Daikin Onecta cloud api to control the heat pump. That’s not as complicated as it sounds!
Home Assistant is clever enough to map the heat pump to its built in ‘Water Heater’ functionality.
When the exported power rises above a threshold of 1kW for more than 2 minutes I set the Water Heater to ‘Performance’ mode. For my heat pump this is equivalent to pressing the strong man button on the controller when I want to heat hot water outside of the schedule.
There is a separate automation to turn it off if the power drops below 1kW for half an hour. I use half an hour so I don’t continuously turn the pump on and off.
It works really well and means we can either have extra hot baths at night, or reduce the amount of electricity needed to heat the hot water on the night time schedule.
The insulation on the Daikin hot water tank is so good that it hardly loses temperature throughout the day.
The image below from Home Assistant shows the excess solar automation turning on the heat pump and reheating the water at around 15:30 outside of the normal nighttime schedule.
Scheduling the Dishwasher
So our dishwasher is quite old now and doesn’t have any intelligent functionality. But, if you start it, turn the power off, wait a while, then turn the power back on, it does continue its cycle. So I decided to use a wifi smart plug (TpLink)* in my case purchased from Argos to remotely switch the dishwasher on and off.
*a word of caution. Most smart wifi plugs are rated for resistive loads only. Equipment with motors such as dishwashers and in particular washing machines have inductive loads. So you need to check the load rating of your switch carefully. Even though it’s rated for a 13A resistive load the contacts of the switch may burn out or weld together if you try to use it to turn off a large inductive load. Check the VA rating of the switch and its suitability for inductive loads.
I used a low cost button (£6) next to the washing machine that connects over a zigbee* network to my Home Assistant.
*zigbee is local network used for smart devices. It avoids clogging your wifi network with lots of remote devices.
When pressed the button turns off the wifi switch to stop the dishwasher. The switch is then turned on if we have excess solar generation, or at 11-30pm at night when we have cheap rate electricity.
The first part of the automation is quite easy… each time the button is pressed toggle the dishwasher plug from on to off, or off to on.
The second automation turns the dishwasher smart switch on. This is either occurs at 11-30pm in the evening, or when we’re exporting electricity beyond a few 100 watts.
You can use much simpler manual timer plugs to achieve a similar thing. James Bradbury wrote a post here on how he does this.
The washing machine
Our Washing machine isn’t a smart device. We simply use the time delay function to schedule the end of wash time. In the winter this will be overnight to make use of the cheap rate 7p/kWh. In the summer we’ll delay it until mid afternoon on a sunny day when we know we’ll have maximum solar.
Making use of Intelligent Octopus Slots
As we roll out renewables across the UK there are more and more times when excess renewable energy is available from the grid itself. I’m on the intelligent Octopus Go tariff, and this means that if our car is plugged in during the daytime, it sometimes gets free cheap rate ad hoc daytime charging slots when the grid has an abundance of electricity.
With Home Assistant it’s possible to detect these extra slots and make use of that low cost electricity. In my case I use it to give our home battery a boost if it’s below 50% charge. It could also be used as a way of stopping your Home battery discharging when your car is plugged in.
The automation below detects that Octopus has started a charging slot for one minute. (The enormous Home Assistant community has written a free integration for Intelligent Octopus).
If it’s daytime and the battery is below 50% charge then I set the home battery to charge. A separate automation sets it back to normal operation when the slot turns off.
How do all these separate automation work together?
You might wonder how all these separate devices all trying to use excess solar work together. Well fortunately very well. Even though they’re all from separate manufacturers and aren’t particularly aware of each other, they mostly cooperate. Essentially if the home battery is charging from excess solar then it uses all the excess solar to charge the battery. The Zappi has a time delay to make sure excess solar has been steady for 20 seconds before charging. As soon as the Zappi starts charging it uses all the excess. So if we’re charging battery or car or both there isn’t any excess for the other devices to detect.
How has this affected my electricity bill?
The graph below is a snapshot for a typical days usage in early May 25. You can see that the majority of electricity is used at night during off peak hours and the daytime and evening hours have almost no grid draw.
For the month of April our total household energy bill for everything, including charging the car, was £30 excluding the standing charge. We managed to use the majority of our electricity off peak and maximise solar usage.
How much did this all cost?
This is the question that always gets asked. How much did it cost, when will it pay back?
I always find this slightly odd given we never ask when a new kitchen will payback, or a gas boiler, or double glazing? Essentially you do things like heating replacement or car replacement when they’re nearing end of life.
But here’s how much i’ve spent on the technology discussed.
- Solar Panels – Installed myself in two parts – 3kW 10 panels system – £3500, additional 1kW 3 panels a year later £1500. Total £5000. Under the Zero Chippenham Community Solar scheme it is possible to get a typical 4kW solar PV system installed for under £5000.
- Home Battery – Installed myself – Alpha ESS 5kWH AC Battery – £3500. It’s possible to get solar and battery or a separate battery install through the Zero Chippenham scheme. Battery costs have fallen since I installed mine three years ago.
- MyEnergi Zappi EV Charger – Self installed £600. Our Solar scheme contractor also installs EV chargers. (Mitsubishi outlander PHEV purchased used £16k 5 years ago, now done over 100k miles, and yes the battery is still going strong).
- Heat Pump Installation – Octopus Energy Installation £2180 paid after government £7500 grant.
- Home Assistant – Home Assistant Green £85. Plus TP Link Smart Plug £9, Remote button £6, Zigbee dongle £29. I have other smart home gadgets not mentioned here. Software free open source.
So for this technology around £11,500 in total. I have undertaken other house insulation measures and diy installed ventilation and heat recovery, so total household retrofit spend is circa £13,000 over 7 years.
Conclusions
Once you start on the electrification and smart home journey it’s really empowering – pun intended. It’s the first step towards ‘The Internet of Energy‘.
I’ve adopted a slightly different solar power strategy to many, in that I try and use every available electron of home generate electricity through load shifting or storage, to be as energy independent as possible. Realistically these strategies are in use for the summer six months. My panels are on a flat roof with some tree shading, so in the winter I don’t generate enough solar generation to have any spare. In the winter I try and make as much use of our off peak 7p/kWh night time rate by ‘load shifting’ appliances to run at night.
Of course the more of your home generated energy you can use and the lower the grid import, the greater the carbon saving, at least whilst the grid is still partly reliant on gas generators. Similarly by detecting Intelligent Octopus slots you can help balance the grid by adding extra loads such as Home batteries or charging your car when there is excess grid renewable energy.
I know the counter argument is often put – what about all the mining and resources used to make those panels and batteries? Of course this isn’t perfect. I’ve calculated the embodied carbon payback of my house battery to be circa 1 year, solar panels payback embodied carbon in circa one year.
Whilst the environmental impacts of EV mineral mining are not negligible, the scale of pollution and greenhouse gas emissions from fossil fuel extraction far outweighs those from EV mineral extraction. Therefore, transitioning to electric vehicles can significantly reduce overall emissions compared to relying on fossil fuels, even when considering the mining and processing of EV minerals.
However it is crucial to address the environmental challenges associated with mineral extraction through responsible sourcing, sustainable mining practices, and improved recycling technologies. The majority of battery materials are likely to be recycled at end of life.
For me the electrification of our home over several years has allowed us to reduce our bills, reduce operational carbon, and have a sense of control and understanding of our home energy. I hope this post has helped – Matthew