Solar / Battery Storage considerations

Hi Phil

Will do my best to answer but not sure of the technical details so my answers may not be reliable.

We have an LG 7kW Therma V ASHP, installed July 2020.
The temperature set for the rads is 45 degrees max according to one doc I have but the mcs cert says 50 with scoop 3.18.
Rads - No over size. We have “made do” with the original radiators. The assessment said they would be enough except in main living room where we originally undersized it as we have a range oven and wood burner in the same room


No idea :man_shrugging: :wink: it’s very hard to know - and I haven’t decided yet whether or not I willl go forward with a system. The forecasts I have seen assume continual price rises at 9% pa which I will not rely on. But the outline quote provided is for pylontech batteries which have a 6000 discharge cycle guarantee which is 16 years at one a day. So am not too worried about short life - and I am aware that EV batteries are generally lasting much better than expected- rate of discharge will be hugely slower on domestic batteries than car batteries.

We have an advantage with the wind turbine - that should give extra return from batteries compared to most - but a downside in shared supply that cheaper overnight charging rates are not an option for me, reducing potential benefit

For us there is some value in uninterrupted supply as we have a powercut at least once a year for a few minutes and occasionally up to an hour or two

I am not going to apply a cost of capital to the decision as the cost of borrowing is pretty low, am already putting max into pension, and additional investments aren’t v interesting to me. There will have to be a bit of a leap of faith in going for it - though I do think there will come increasing opportunities for arbitrage with the grid that will help an entirely unspecified life

that said I have never seen anyone show calculations proving it pays (apart from EV man on YouTube). Most people start making excuses if you ask for detail. But that was all before the likely doubling of cost for us this year - currently paying 14.6p per kWh but two year contract up in September and as a business supply (farm) we don’t benefit from the cap


Yes - I think the only way to know it’s definitely worthwhile (or not) is actually to do it but then it’s too late!

Thanks Tim for the detailed/personalised reply - much appreciated!

I think it supports my thoughts/findings on the solar/battery studies…

In pure economic terms, it is probably not a good investment. However, if one can afford/are willing to commit say £10-20K to the installation costs it becomes a ‘lifestyle choice’, i.e. you want to do it on the expectation that you will get some cost savings in the future (unknown) and some security of supply (although minimal) in these uncertain times.

For me, my thinking is that I will not progress at this stage and will keep a close eye on developments…

Good luck in your deliberations.


This may not help, as every house situation is different, but I do chart everything, and I have got back 9.5-10% of the full cost each year for the first 3 years. This year that will increase due to cost rise of fuel. My system is just Solar panels, and a Solar surplus diverter for the immersion. I purchased the panels just before I retired, so the money came out of working income so I sort of wrote it off at the time (in my head), and now every day is a benefit.


Our place is about 275m2 first occupied 1990. We have 6 bedrooms although one is a sort of gym and muddles room. The triple glazing has been brilliant with zero maintenance (Internorm powder coated aluminium to wood frames all factory finished). The new EPC says C78. The firms quoting all demand dealing with -1oC and estimate about 14kW heat loss under that scenario. The oversize factor is 2.7 meaning at 45oC surface temperature the required heat can be emitted. I expect to turn the water down lower as the COP should rise to 4. This would mean an input energy of 14/4 or 3.5kW.

Given the average outside temperature since 27 Sep 2021 has been 8.13oC as measured by my weather station we might use 12/21 with everything flat out to achieve 20oC day and running 24hours. Or 2kW average. The gas has been about 60kWh/day so 15kWh/day looks consistent given we don’t heat unused bedrooms. We like having space! We use 14,000kWh/year of gas.

We average about 14kWh/day at the moment without the ASHP. These figures are supposed to inform decisions on the Solar/Battery. It is proposed we have 27m2 which at 50W/m2 for 6 hours at mid winter produces 1.8kWh/day! GreenShop’s simulation is 200kWh for December and January. Right now it’s more like 300W average over 8 hours giving 14.26kW/day. I have to use GreenShop monthly figures though rather than a sunny day like today. They expect 470kWh in March. They recommend we do batteries once the system has been running for 9 months. I will have to see what Naked Solar say in a few weeks.

Batteries are probably not an investment likely to give a return, but maybe today’s zero rating will make the difference.


What make have you got?
What did it cost and to install?
I think this is the way I want to go.

The one I got was “Apollo Gem”, although I think there are a few different makes of the same idea. I paid about £250 at the time (fitting was part of the whole Solar Panel system), and so far it has pointed 3900kWh to the immersion and a convection heater. Last year it was equivalent to £250 worth of energy diverted. Effectively the immersion is being used like a battery to store energy. I have hot water from it pretty much all year, and surprisingly most of winter.

I turn the thermostat on the tank down to 55degrees, and then set the thermostat for the immersion to 65-70degrees, that way the boiler only heats the water when temperature is low. I’ve also fitted a couple of £3 LCD thermometers with a probe to the top of the immersion, and by the boiler thermostat half way down so that I can see what its doing - not necessary, but nice to watch the temperature go up.

As an extra saving, I found that if I pause my washing machine just as it starts to fill, I can then open the drawer and pour in 6 litres of hot water from the tap into the pre-wash tray, then the washing machine tends not to need to heat the water at its usual 2.4kW. This is useful when you have lots of hot water, but those pesky clouds stop the sun on and off all day.

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Many thanks for all the information. I think the prices have now increased and would cost about £480 now including the wireless sender plus installation. It may however still be a good option.
How long have you had yours installed?

What’s the difference between the regular hot water tank (is an electric element) and the immersion?

I’ve had mine for 3 years plus 2 months, and even if it were £500, I still think its a no brainer, especially if you don’t ago for batteries

Sorry I wasn’t clear. On the side of the hot water tank is a thermostat, which is used by the boiler to decide when to heat up the water. This is the one that I set to 55 or lower. e.g.

The Immersion element itself, has its own thermostat with a control at the top (see diag), and this I set to 65+

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Thanks. We’ve a lad coming round next week to assess out house for solar panels, so I need to scramble to build my understanding.

The options seem to be panels for power and sell surplus to the grid, or adding batteries (at double the cost). They have given me a ball park of needing 14 panels based on our annual power bills. Our house is northwest facing, so ideal here in New Zealand.

I want to be sure not to adversely affect the Hifi of course, and it may give an opportunity to look at a dedicated main spur again.

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I noted this issue but have never noticed any impact. Our solar system was installed nearly eleven years ago.

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I don’t believe my panels have any effect on SQ, but then again I tend to only listen intently at night. In terms of panel numbers, then the cost of an extra panel is a small proportion of the overall cost, so I would suggest consider filling your roof (of course do the maths to check) if you are looking at an EV

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The only issue I have had with my solar panels (and it depends upon your roof and location of panels) is that if pigeons can get between roof and panels they think it is a new home. We had about 50 and they make a hell of a mess and a noise. You need to install a net or spikes around the panels to prevent ingress.


I’ve got a Solar Boost + box that diverts electricity generated to power an element in my hot water tank when the system is generating more electricity than the house uses.

I’ve had it for about 8 years.

It’s a good system and we rarely turn on water heating from now until autumn.

Shame that we’re probably moving out just when the system has probably just about paid off the nominal £6k we spent on it, leaving 14 years of index-linked FiT payments for our buyer to collect, plus the panels should work for decades after that.

What make of divert “box” do you have?
Was it installed with your system or added later?
I need to add one.

14 years of FIT payments should be added to selling price. That could be worth £28,000 plus inflation.

I’ve not had any bird issues, but a friend of mine who lives by some restaurants and take-aways, suffered a lot with pigeons and had to put up some deterrents. So it may depend on what’s around you.

@JimDog Thanks for the info on Solar iBoost.