Retrofit #6: First impressions

Everything we needed doing on the retrofit was completed back in January 2022. We’ve been living with the system for a couple of months, so here are some thoughts on the different components.

Solar panels

Love ’em. They look pretty sleek and there are a stack of them (20 in total) and they have an awesome 6 kW inverter that chucks out a fair proportion of their peak power. Basically they sit there doing their amazing thing, and are barely noticeable. As the days have become more sunny, we really understand the benefit of over specifying on solar panels: additional panels are comparatively cheap and the more you have the earlier and later in the day the property is off grid.

One array of the solar panels. Free energy from daylight – what’s not to like?

Givenergy battery

This might be my new favourite retrofit/energy-efficiency piece of kit.

Initially I wasn’t sure on how to use it most efficiently – there are 4 modes and a charge timer and what I thought I wanted was a big red ‘charge me now’ button. But the system is great. It charges and discharges based on the needs of the house and balances the our energy usage. It also discharges at 3 kW so covers most household appliances and has no noticeable lag (turn on the vacuum cleaner, or the oven, or the iron and it just draws from the battery – unless all are on at the same time, in which case it draws the maximum 3 kW and tops up from the grid). Excess solar energy fills the battery during the day (for use in the evening), and we use the cheaper tariff on Octopus Go to top up to 100% at night (for use in the morning). As the days have become brighter and longer, we no longer need to even take from the grid at night – the panels do all the charging. The battery then runs the heat pump in the morning and evening.

Mixergy hot water cylinder

Apart from a small quirk with one of the thermostats that we are looking into, the Mixergy cylinder is doing what it does so well. We don’t have it in machine learning mode, so 90% of the time we just dump excess solar energy into it. You may ask why we went with Mixergy. Well, we like….

  • the app and its interface
  • the fact you can boost the cylinder through the app, the hardware and the Eddi,
  • the fact that it’s a cool little business that is based in the village we used to live in
  • the fact that we can use excess solar, the heat pump and direct electricity to heat the water depending on what we need
  • that it’s versatile and simple to use.

Zappi EV charger

It’s great to have the Zappi back. Having no access to it for 18-24 months was an odd feeling. Now we can charge the car from the solar panel excess electricity again and that’s a brilliant feeling. We use the car infrequently these days so in most cases can just use the solar to charge, without the need to pull from the grid. That’s free, green fuel.

Heat pump

OK, this was the component I was most unsure about, having very little experience of even seeing one let alone understanding how they are used. First, the less positive aspects:

  • It’s big. Much bigger when installed than I was expecting. We had measured the dimensions that had been given in the specs but once the standing legs and pipework is in place then it’s bigger than just the size of the box. However, it has almost all of the heat pump kit in it, unlike other systems that have a split between the outdoor unit and an internal unit containing the electronics. And there are things that we can do to shield our outside unit from view, so it forces me to get out gardening. Which is good.
  • It was pretty noisy the first few days. In fact we almost wondered whether we had made the right decision. However, the house had had no central heating for almost 2 months so was cold, and the outside temperature was below 5C so the pump needed to work hard. We also live in a very quiet area so we think the noise aspect was exacerbated by what we were used to. However, in normal running the pump is quieter than our gas boiler was.

However on the more positive side:

  • It’s provided more than 3 times as many units of heat compared to the input electricity. This means for every 1 kWh of electric used by the pump, more than 3 kWh of heat is put into the house. For comparison, for every 1 kWh of gas burnt in a boiler, at best you get 1 kWh of heat back (and ‘best’ rarely happens). Although I hate saying this, as winters become generally warmer due to climate change, the heat pump will also become more efficient.
  • Running off the battery further reduces the cost of heating the home. Especially when the battery is filled from the solar panels.
  • Any noise (and there is some, but not a lot) is of little consequence because in the majority of cases if the house is heating then we have the windows closed, as do the neighbours and we wouldn’t be sat out in the garden. So overall, the lived experience is one where it’s quieter in the house than when we had the gas boiler.


Overall the system is amazing, and exactly what we were after. It seems to be working as expected, now that a couple of settings have been tweaked and we are confident that when the Mixergy thermostat is fixed then we can just leave the system to work itself – which is how it should be. For wider uptake of technology like this we need to have self-running systems that don’t need tweaking to operate efficiently.

The noise issue around the heat pump is an interesting one: noise perception and response is personal, subjective and emotional. Objectively our heat pump is not noisy and we are confident that even in our very quiet area that it’s less noisy than neighbouring gas boiler flues. However, it is a different sound and so people will naturally tune into that. I also think some online discussions are potentially misleading, saying that noise from heat pumps is imperceptible, although maybe that’s true if the compressor component is inside the house (i.e. in a split system). If you have any background traffic noise where you live then I’d agree that you would not hear the heat pump at all.

If you have available funds, or are able to access grants, then I would recommend that you investigate these technologies. I’d look at solar PV panels first (generating free electricity is better than paying for it from the grid) and then maybe a battery (costly outlay currently, but offsets the usable time of the solar PV) or hot water tank (can be used as a ‘heat battery’, and is cheaper) and finally a heat pump.


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