Any good solar calcs/rules of thumb?

[Mr Whippy]

I currently use about £125pcm on leccy.

I’m guessing about £7,500-£10,000 for a Tesla powerwall 3 with inverter built in.

What is the cost of just panels going to be, roughly, to cover roughly 2/3rd I’m winter and 4/3rd in summer?
Ignoring install etc as I’d do all that separately.


I assume ROI (sans inflation of energy and kit) is going to be ~ 10yrs, but possibly 7-9 years given prevailing economics etc.


I’m struggling to find a calculator or any website that doesn’t just want me to get a free quote blah blah or simplify assumptions so much you can’t even add a battery.

Assume roof is optimally average.


Is this a no brainer these days given prevailing inflation, economics, power generation stuff etc… if you have £10-20k to spend on “savings” on a long term home?

[OutInTheShed]

Rough guide is equivalent 1000 hours of sun a year, so 4000kWh from your 4kW system.
Assuming a vaguely South facing roof.

And about 3:1 ratio of Summer:Winter generation

Beyond that, everyone is different when it comes to how much it actually impacts their bills.

[superpp]

I'm of the opinion a battery will not be a ROI.
Just solar with an EV tariff & SEG export makes sense though.
EV charge overnight at around 7.5p kWh (plus household washing/dishwasher etc..)
Daytime use as normal but excess is exported at 15p kWh.

I have 4.8kw array SSE facing in the NW and generate as follows (average over 5 yrs, in kWh):
Jan 114
Feb 192
Mar 443
Apr 664
May 634
Jun 626
Jul 571
Aug 561
Sep 424
Oct 275
Nov142
Dec 102

Winter use will not be covered by solar generation.

Use those figure against your usage to work out your own situation.

Edited by superpp on Friday 1st November 14:07

[Tesco]

This calculator will give you a good idea of output from an array. How that stacks up against usage and equipment costs you'll have to work out as everyone is different.

https://re.jrc.ec.europa.eu/pvg_tools/en/tools.htm...

I've had my system installed since December 2022, and its actually performing better than the calcs I did for it.

We are heavy users (circa 6000kWh per year) with electric cooking and some electric heating (no gas available as semi rural. Have oil for main heating). I have a 6.4 kWp system spread over 2 strings of 3.2 kWp each, one facing east one facing west. With that we have 12kWh of battery storage. I had about 8 years worth of historic electricity usage data which I used to help me calculate whether a system was actually worth it.

System install cost (bungalow, so no scaffolding) in the East of England was around 13K. During darker months I'm filling the battery overnight on a cheap rate which plays into any costings calculations. I reckoned on 8-9 years effective payback, but if the past couple of years data continues forward, this will likely come down.

Panels and batteries might not work for everyone, really depends on use.

[4Q]

The formula you use is

kWp (the size of your array) x kk = kWh per annum.

You can find your kk from downloading the irradiance dataset from MCS here
https://view.officeapps.live.com/op/view.aspx?src=...

Choose your location (on the bottom tabs) and then orientation from south and the pitch of the roof to give you the correct figure

To find your proportion of self use use the MCS Self consumption tables MDG003 at https://mcscertified.com/wp-content/uploads/2022/0... they are quite pessimistic but ALL MCS installers should be using the same figures.

Edited by 4Q on Friday 1st November 14:10

[Mr Whippy]

Ok, looks like I’m in ~ 850kk range.

And assuming 10 x 400w panels is about 28m.2, that’d get me about 3400kwh a year.

Prob need double that so 56m.2, or 2x (4x7m) ish arrays.


So £10k ish panels and £10k ish battery.


But battery might not be great VFM vs just doing spread deal with grid and letting them be the battery.

But battery could be very “useful” for power cuts, etc, but yes in bigger picture it’s gonna be expensive vs grid feed in take out spread.

Ie, if battery is worn after say 10yrs and will cost £6,000 in todays money to replace, it’s about £1.64/day for that convenience.
Probably a lot cheaper to just buy a generator if you want power coverage in a powercut?

[4Q]

The thing you're missing with your battery calcs is the ability to buy cheap rate electricity to run your home instead of at full price. I have 25kWh of batteries and charge them fully overnight at 7.5p per kWh saving me around £6 per day.

[Mr Whippy]

I think in the end it makes sense having the battery to be grid independent if you wish, but also to benefit from off-peak purchase/storage.

Even at £2 a day it’d pay off in about half the battery life, while also allowing independence/power cut coverage etc.

[Cupid-stunt]

Mr Whippy said:

Ok, looks like I’m in ~ 850kk range.

And assuming 10 x 400w panels is about 28m.2, that’d get me about 3400kwh a year.

Prob need double that so 56m.2, or 2x (4x7m) ish arrays.


So £10k ish panels and £10k ish battery.


But battery might not be great VFM vs just doing spread deal with grid and letting them be the battery.

But battery could be very “useful” for power cuts, etc, but yes in bigger picture it’s gonna be expensive vs grid feed in take out spread.

Ie, if battery is worn after say 10yrs and will cost £6,000 in todays money to replace, it’s about £1.64/day for that convenience.
Probably a lot cheaper to just buy a generator if you want power coverage in a powercut?

£10k for battery?
Which ones are you looking at as I am in the process of trying to evaluate similar and the cost of solar and battery is £9k.
That is for
10kW Solar (11x AIKO A455 MAH54Mb/2s) 455w panels
(produce 6,500kWh/yr)

5.76kWh Battery storage (Fox ESC series)

10kWInverter - Fox K Series Hybrid

The £9k inc MCS certification, scaffold, HIES workmanship warranty - I am awaiting some other quotes, but the other one I got was circa £13k - (different PV, Inverter and battery)

[Mr Whippy]

Well there are always extras.

Chances are property I’m scoping is in need of other works in/around electrics, roof etc, so by the time I’m done…

Also thinking doubling batteries for a decent capacity… may ultimately eventually do something like GSHP too?!

Deffo seems sensible to go bigger on battery for covering winter usage, so a full battery and avg solar daily power covers a 24hr period?

Edited by Mr Whippy on Friday 1st November 18:33