The payback period is the moment your savings have repaid what you spent. After that, the electricity is effectively free. It's the single most useful number in solar — and you can estimate yours on the back of an envelope.
The simple formula
Take your net system cost, then divide by what you save each year on electricity (bill savings plus any export payments). A typical £7,000 system that saves around £700–£900 a year pays back in roughly eight to ten years — and then keeps going for two more decades.
What moves the number
Higher electricity prices, a south-facing roof, self-using more of what you generate, and adding a battery to dodge peak-rate evenings all shorten payback. The panels carry a performance warranty of around 25 years (and many now a 25-year product warranty too), so most of their life is pure savings.
The quick version
- Payback = net cost ÷ yearly savings.
- Self-consumption and high power prices shorten it.
- Panels last ~25 years, so most of that is profit.