Climate Friendly Retrofit Part 8: Solar, Microgeneration, and Energy Usage

Josiah Lockhart
5 min readSep 21, 2021

The news over this past week about the troubles facing the UK energy market has made it abundantly clear that, despite the rhetoric, electrification is not necessarily equivalent to decarbonisation. With a grid that is currently 60.5% fed by fossil fuels as I type this, any attempt at net-zero retrofits of our houses have to rely on various types of localised micro-generation and/or district heating until we are able to decarbonise the grid.

For our home, the middle of our three main stages of retrofit is solar combined with various smart energy management systems/devices. And, while there are still elements to be installed, we are coming up on three months of our micro-generation set-up.

Because we are in the process of electrifying our house, have both a 58 kW EV (VW iD3) and an 18.6 kW electric moped (NIU MQi+), are planning to electrify our heat, and work from home 90% our usage could potentially be high depending on the frequency we charge the car and moped. It is because of this that we slightly oversized the system to 5.01kW (UK average 3.5kW-ish). And, according to paper-based assessments, our system will generate between 4,094 (Installer estimate) and 4,559 (Energy Savings Trust estimate) kWh per year depending on site variables, annual cloud cover, and shading.

For those keeping track, our system (currently) includes;

But the big question is how it has been in practice (keeping in mind that we missed good generation months of Apr, May, June and part of July)? Between 12 July and 21 Sept, we generated 967 kWh, exported 396 kWh when we were producing more than we could use, and imported 400 kWh when we weren’t generating ouch (e.g. night time). And, because we had electrified our hot water in that time, we lowered our gas consumption from 881 kWh per month to 22 kWh for our gas hob, which has just been replaced (Sept should be zero).

As far as specific energy usage is concerned here is a breakdown of what it looks like today (a big shout out to both MyEnergi, Mixergy, and Octopus who have great energy monitoring).

  • When excluding hot water and vehicle charging, our home uses between 6 and 9 kWh a day depending on oven and washing machine usage. Sadly, oven usage is hard to shift out of the dreaded 4–7 pm window, but we’ve been able to shift our clothes and dishwashing to times where we know/see excess solar energy thanks to scheduling functions. The main point here is we haven’t changed our lifestyle/times we do things, only that we use the schedule function rather than starting things straight away.
  • Our hot water has nearly exclusively been heated by solar during this period other than a few places where I was still working out the automation. Our setup is such that the MyEnergi Eddi will divert excess PV into the immersion heaters on our Mixergy, and while this might be as low as .1 kWh at times, the tank can scrape 3–6 KWh over the day leaving us with a full tank by the end of most days (occasionally over 2 days). As we get to winter, we’ll see how we can get the solar to interact with Mixergy’s smart charging, and his interplay is going to require a blog of its own.
  • Our electric moped, which my wife almost exclusively uses to drive 16mi to/from work, needs roughly a 5.5kwh charge (30% of battery) per every commute, but because she goes in 2–3 days a week, we can normally grab that from excess solar if there is a day in between, otherwise, we can schedule charging to time with Octopus Go overnight tariff at 5p per kWh.
  • Our BEV (ID.3) has been the biggest change from what we expected. With our previous EV, when we lived in the city, we did not have a charger/driveway at home and had to get used to what EV driver’s call “grazing.” That is grabbing small amounts of electricity when out and about, at supermarkets, train stations, cinema, etc… We had anticipated charging primarily at home in this house, but with a battery that is nearly 1/3 larger than our last EV, our habits mean that we only really charge at home when there is LOTS of excess energy, or we need to leave at 100% charge for a long drive. In August, for example, this has only been 78 kWh (the equivalent of 283 miles or so of driving.

The only final thing to discuss is cost and payback. The total cost of the solar/MyEnergi system was £7,837 including install. We did, however, get £600 in grants towards our EV charger elements and a £5,000 interest-free loan from the Energy Savings Trust. This means we were out of pocket £2,237 and will be paying off the loan for the next 5 years. As a caveat, it will be very hard to calculate the payback period without much pre-system data to compare to, and it will take some time to calculate this as the system has an estimated 25-year life and we can’t yet account for the whole system approach, effect on property value, savings from charging car/moped on solar, how net-zero policies to housing will change in the coming years, changing energy prices, nor the current crisis affecting the UK grid. All of those known unknowns aside, our system designer calculated it as a 9-year payback simply from annual savings, but we are already on track to beat by a few years.

This is all lots of information, but the overriding data points to this being a good decision for us, regardless of whether we were going for a net-zero retrofit. The big barriers for people will be what is suitable for your specific home and whether you have spare cash to pay the difference between the max loan and system cost. We were lucky to have retained some cash from the sale of our last home and the purchase of this home to allow us to carry out our retrofit, but that won’t be the case for everyone. Navigating the bureaucracy of the incentives has also been a nightmare (see my earlier blog), but we are just about there and starting to focus on phase 3 which includes space heating and further energy storage.