Worth getting solar pv? 15 May 2021 at 3:23PM in Green & ethical MoneySaving 21 replies 506 views

DaisyH8

There are some very knowledgeable people on this forum and I’d be grateful for your thoughts. My gas boiler is ancient and I’m planning to replace it with an air source heat pump. I’m wondering whether it’s also worth getting solar pv. I have an end terrace house with a hipped roof so the surfaces are quite small and more-or-less triangular so I don’t think I’d be able to fit many panels on it. The front of the house faces south (roof about 5m wide at widest point) and the side faces west (8-9m wide). I’m in NE England. My aims are reducing my carbon footprint and hopefully reducing my energy costs. Am I likely to generate enough electricity to make the outlay worthwhile?

Reed_Richards

The problem with solar panels (and I write as someone who has solar panels) is that they generate electricity when you need it the least.  That's much much more in the summer than in the winter and most in the middle of the day with little or nothing in the early morning or in the evening (my panels face south, other directions will differ).  At the moment it is still quite cold in the morning and cools off in the evening.  To match my ASHP (I also have one of those) to my panels I would need to put up with a cold house in the morning, make the house too hot in the middle of the day so by evening it has cooled down to a reasonable level.  That way I would match the output of my panels (on a sunny day) to the ASHP.  I'm afraid instead I choose to pay extra for greater comfort.

I should also warn you that replacing gas heating with an ASHP will increase your energy costs.  

Verdigris

You'll need to replace more than the boiler because the water temperature from ASHPs is lower than from a gas boiler. You'd either need larger conventional radiators or underfloor heating installed. You roof doesn't sound promising, either, although the east-facing end would give you a head-start first thing in the morning when you usually need a heating boost.

I applaud you wanting to reduce your carbon footprint and every little helps, as the saying goes. I think your best bet, if you wish to pursue ASHP is to go for a TOU (time of use) tariff. You would need a smart meter to be able to take advantage of TOU. Home storage batteries would also assist in being able to time-shift cheap electricity price/surplus solar generation to more useful times of the day.

Good luck!

Martyn1981

Hiya. If you are thinking about getting PV anyway, then anything that increases your leccy consumption when the PV is generating, will mean you 'save' more, by importing less.
The bulk of your PV generation will be in the warmer months, and for myself, with E/W panels, our winter gen is even smaller (it's a summer specialist), however we still export at times in the winter months, and when that happens, on goes our small A/C unit to reduce GCH. We've also been able to use the small unit for all house heating since mid Mch, and by late Mch almost all heating has been done from PV generation - though of course the amount of heat needed, even in a cold Apr and May, is tiny compared to Jan/Feb.

So, PV won't give you enough juice in the winter, but it'll still contribute, and of course do what PV does the rest of the year.

PS. Haven't seen nor mentioned these for ages, by if you Google Trienergia, then you may find 'triangular' solar panels, which can be combined for install on roofs with a sloping edge, or full hip, just like you are thinking off. They used to be a bit more expensive, but allowed for a smidge more Wp to be installed, but more importantly, might look nicer.

DaisyH8

Verdigris said:

I think your best bet, if you wish to pursue ASHP is to go for a TOU (time of use) tariff. You would need a smart meter to be able to take advantage of TOU. Home storage batteries would also assist in being able to time-shift cheap electricity price/surplus solar generation to more useful times of the day.

Good luck!

Thank you. I hadn’t heard of TOU, I’ll look into that. I’ve been thinking about batteries too, definitely lots to think about. 

QrizB

DaisyH8 said:

The front of the house faces south (roof about 5m wide at widest point) and the side faces west (8-9m wide). I’m in NE England.

Typical domestic solar panels are rectangular, about 1m x 1.7m, and produce around 360-400 watts each (higher wattages are available but more expensive, lower wattages are older technology and not much cheaper). If you can fit seven (four in one row, with three above) on the south face of your roof you'll have 2.52 kilowatts. You could supplement this with a further eight panels on the west face of your roof (possibly a single row) for another 2.88 kilowatts.

I don't know exactly where you are in the NE but I looked at Durham in PVGIS. 2.52kW facing south is projected to generate 2280kWh per year and 2.88kW facing west gives 2030kWh.

If you installed both systems it would cost you about £5000 (for details see below) and generate 4310kWh per year. If you sold them all to the grid at 5.5p per kWh you would earn £237 a year and pay for itself in 23 years. If you used them all instead of mains elextricity at 15p/kWh the benefit would be £646 a year and the system pays for itself in 8.5 years. In practice you'll be somewhere in-between; I would guess payback in ~15 years.

Logic for the price: I recently installed a system. The installer's scoping estimate was £4200 for 12 370-watt panels, adding or subtracting a panel was £200. I installed 9 panels for ~£3600 but you might want 15 panels for £4800. (You can see from this estimate how that installer covers their fixed costs; fitting zero panels would cost £1800.)

DaisyH8

Martyn1981 said:

So, PV won't give you enough juice in the winter, but it'll still contribute, and of course do what PV does the rest of the year.

PS. Haven't seen nor mentioned these for ages, by if you Google Trienergia, then you may find 'triangular' solar panels, which can be combined for install on roofs with a sloping edge, or full hip, just like you are thinking off. They used to be a bit more expensive, but allowed for a smidge more Wp to be installed, but more importantly, might look nicer.

That’s encouraging, thanks. I didn’t know about triangular panels, which would be helpful on my roof. I think I’ll go ahead and get some quotes and see if it’s viable. 

Verdigris

Another thing to consider is fitting a large, well insulated, hot water cylinder to dump surplus solar electricity, particularly in summer. You should get free hot water for a good proportion of the year. You can get a control unit that ensures surplus power goes to water heating, rather than the grid, when all other uses have been satisfied.

DaisyH8

QrizB: thanks for the facts & figs. The cost of the system is lower than I expected. We use a little under 2000kWh a year and we’re at home most of the day so we’d use even a measly amount generated in winter. We’re close to Durham so spot on there! 

I’m glad I posted, really helpful comments, thank you. 

Reed_Richards

DaisyH8 said:

Verdigris said:

I think your best bet, if you wish to pursue ASHP is to go for a TOU (time of use) tariff. You would need a smart meter to be able to take advantage of TOU. Home storage batteries would also assist in being able to time-shift cheap electricity price/surplus solar generation to more useful times of the day.

Good luck!

Thank you. I hadn’t heard of TOU, I’ll look into that. I’ve been thinking about batteries too, definitely lots to think about. 

Time of Use tariffs tend to charge you premium rates between about 16:00 and 19:00 and you would want to minimise your use of electricity then.  So I am going to stick my neck out and say that this is not compatible with an ASHP (or an electric boiler) unless you are prepared to wear a coat indoors for three hours.  If I am wrong  I'm sure others will be along to tell you this but I bet they don't actually have electrical heating (other than Night Storage Heaters which should work very well with that sort of tariff).

Reed_Richards

Out of interest I looked at my electricity consumption recorded on my meter last night (15th May) since 30th April which is 243 kWh.  In that time the power consumed by my 12 kW ASHP was 260 kWh.  There is a little bit of uncertainty in these figures because I did not read both meters at the same time in April.  This is despite the fact that I have a 4.8 kW solar PV array with a 6.5 kWh battery!  Sadly I have not been diligent in reading and recording my generation meter so I don't know how much I have generated in that time.  I live so far north in the NE of England that we regard people in Durham as southerners.

So my solar panels are managing to cover the rest of the house demand for electricity but are barely touching the requirement of the ASHP.  One good thing is that on most days I have heated the water in my cylinder by directing the surplus electricity to my immersion heater so the ASHP would have used more power if I had not been doing that.  I am suing the immersion heater because its power requirement is better matched to my panels than that of the heat pump and it allows me to get the tank up to about 75 C whilst the heat pump stops at 55 C.     .