Feb 16, 2023

Where to use solar PV generated electricity – hot water storage or not?

One of our SHIFT clients recently asked us a question that is no doubt on lots of people’s minds – so here’s the question and our response:

Question:  We are delivering housing (retrofitted) for some of the poorest homes in the UK. With this in mind, is it more beneficial for the residents if we pair any main PV generation to heat water first (tank etc.) or would it be better to power the sockets etc. and use a combi boiler? 

Here are our thoughts:  Thanks and a great question.  I have done a little research on this on my own home – but as you know each home is very different. A lot depends on occupancy and getting ready for the future. Here are some general points which may help: 

  • if people are at home all day, using any PV generated at that time is likely to be beneficial 
  • if they are working during the day, I’d say it’s best to ‘store’ the generated energy in the water cylinder
  • future homes are likely to be more efficient if you have hot water cylinders – heat pumps will use it, dynamic electricity tariffs will likely use them (e.g. use cheap electricity if the wind is blowing in the night)
  • below is the solar profile and actual electric usage for my home. I have a 4kWp solar array, the house is about EPC B and heated by a gas boiler – we are generally careful about electricity use and  ~95% of the time turn on the dishwasher and/or washing machine during daylight hours to maximise free solar energy. You can see there is far, far more generation than we actually use and we divert some surplus energy to the hot water storage. I guess in the future this surplus energy could be used to power some form of electric heating, but I’m waiting for my boiler to fail (in about 10 years) before I make any decisions on that!
  • I’ve also added a line on likely air source heat pump demand – to do this I looked at my monthly kWh of gas usage and divided by 2.5 to estimate likely kWh of electricity needed to run a heat pump to heat my home. It looks like solar PV, when connected to electric heating, will certainly take a massive chunk out of heating bills but not reduce the bills to nothing, especially during the winter months. 


Given that each of your homes will have people with different occupancy patterns, I’d suggest that you can only assume ‘general occupancy’ (i.e. out most of the day and at home in the evening using heat and power). Also the future is likely to be electric heating which means that hot water cylinders will play a key role and combi boilers will become redundant. Therefore as some guiding principles I’d suggest the following to you which would reduce CO2 and save money for residents: 

  • maximise insulation (don’t rely solely on PV to get to EPC C) 
  • install maximum PV on each roof – this will also prepare you for being a so-called ‘Virtual Power Plant’ 
  • divert surplus power to hot water cylinders
  • when electricity prices are more favourable compared to gas consider switching to electric sources of heating – maybe in about 2 years time 

One final point. I think combi boilers get an extra couple of SAP points more than a hot water cylinder. But my colleagues back at Sustainable Homes did some research on actual bills in social homes and found practically no difference in running costs between hot water cylinders and combis.

If you would like an EPC analysis of your stock, then please be in touch.

Image by Bill Mead on Unsplash


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