I've had solar panels on my house (it's a medium size, detached 3 bed) for the last year and this is my calculation:
Initial capital outlay
12 solar panels (400 W nominal output, £300 per panel). £3600
3 Kw inverter (Growatt 5000) £1300
9 KWh batteries (900£ per KWh, Growatt) £7000
Installation (including scaffolding) £2000
Remote immersion heater £500
Total £14400
Over 25 years, expect to replace the invertor twice and the batteries once (and assume battery cost is halved by then)
So the 25 year outlay is 14.4 + 2*1.3 + 3.5 = say 20k.
Last year my usage was:
1.25 MWh imported from the grid.
Cost of electricity units: £188
Standing charge for the year: £121
So the total bill was about £400 (I was on a cheap contract up to the middle of the year, but that expired).
Overall I consumed 3.7 MWh in the year (I do not have an electric or hybrid car). I generated rather more than that and exported 1.7 MWh back to the grid (at 4p a unit I think, ffs, so this generated about £80 of revenue).
So assuming I use about 4 MWh per year and assume that 75% of this is now supplied by the solar and based on my current OVO contract (
48p per day standing charge and 28p per unit) then:
My savings yearly would be (4000 * 0.75 * 28p) = £840 per year.
To recoup the 20k outlay takes 20/0.84 = 24 years - by which time the solar panels, batteries and inverter will all need replacement.
So basically I expect to just about break even.
Of course, we could make the calculation more complicated by assuming energy costs go up over the 25 years, but then we ought to also consider that the value of money also goes down during that period due to inflation - not sure how to account for this properly.
I don’t see how sub 10-year pay back periods can be achieved (that would mean having a savings of more than £2k a year which would mean I would need to be consuming 5-6 MWh a year, but that would mean a bigger solar inverter and more solar panels and more batteries, which means more capital cost - and to do that I would either need an electric car or to use electricity in place of gas for heating).
What am I doing wrong? Did I get ripped off on the initial installation (which was a council supported scheme, with allegedly cost savings due to scale)? Do I have too few/too many batteries - or do I need to just consume more electricity per year? Do I need to buy an electric car and get a cheaper tariff?
Don't get me wrong. Solar power is (imho) great and necessary.
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