I don’t calculate it that way. Dodgy salesmen use shoddy math like that to prove people can make money out of solar that way; they then find it’s pie in the sky, based on total guesswork.
The way I work it is, mental arithmetic. A flat rate 14.4p per kWh is typical, but it is creeping up. If I use on average 30kWh every day that’s £4.50 a day, or £31.50 a week, or £96 a month, £1150 ish per year - and that’s consistent with my quarterly bills before getting solar.
In other words, if I didn’t get solar, and if prices stay static for 7 years, after 7 years I’d have paid as much to the grid as I (overspent) last year on solar.
If I generate 60% of my own power over that seven year period, without reducing consumption, it’ll take just around 11.5 years to get to a point where the solar has paid not just for itself but for all the other works I had done at the same time, and for its maintenance.
In reality, the system is rated for 16 years minimum, and if it’s well maintained it should last for 22 years.
So, TCO over the lifetime amounts to a 20% cost reduction.
That’s if I generate only 60%, AND the price of using the grid doesn’t rise.
If I can get to a point where
A rate of 8 or 9 p per kWh is typical here in the US. It's not creeping up as long as we aren't required to do stupid things like get rid of cheap coal electric plants prematurely.
Your numbers are fine and I have no problem with your motivations. What people need to understand is that your grid electric costs are at least 50% higher than the US and you heat with wood heat Those are the the most important facts.
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the efficiency is above 60% and it’s also charging the car, then I’ll save even more.
The only fly in the ointment is batteries. Pylontech are good but not cheap. Tesla is bigger but more expensive. The alternative to storage is, sell those unused kilowatts to the power company... For 3p a k. And buy it back at 14p when I need it.
I know a guy who knows a guy who can build a 18kWp battery array for about the same price as expanding my current setup to a 9kWp Pylontech packaged modular battery solution with BMS.
If I had 21kWp of offline storage, and switched to a ToU tariff, I’d buy power in at 9p off-peak in winter, instead of 14p, and I’d still be able to sell the excess for 3p.
Panels: Canadian Solar. The CSU-370MB-AG ones can go up to 481w, have 30/12 year warranties instead of 25/10, and weigh 1.4kg more than the CS3W-41SPs (415w). I did look at other manufacturers. Your mileage may vary.