An estimate. Generation depends on roof orientation, pitch, shading and location (UK yields are roughly 850–1,000 kWh per kWp). Self-consumption, SEG export rates and electricity prices vary, and it excludes inverter replacement and panel degradation over time.
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This calculator estimates the payback period and annual benefit of a solar PV system. Enter the system cost, its size in kWp, the annual generation per kWp, how much you use yourself, and your import and export prices, and it works out the yearly saving and how long the system takes to pay for itself.
It is a useful tool when quoting solar and a customer asks the inevitable question: how long until it pays back? A clear annual saving and payback figure sits alongside the installation quote and helps them judge the investment.
The benefit comes from two sources: the electricity you generate and use yourself, which you no longer buy, and the surplus you export for a payment. Add them up and divide the system cost by the annual benefit to get the payback in years.
Benefit = (self-used × import price) + (exported × export price); Payback = system cost ÷ annual benefit
Annual generation is the system size times the yield per kWp. The share you use yourself saves you buying that electricity at the import price; the surplus is exported at the SEG rate. The two together are the annual benefit, and the system cost divided by that benefit gives the payback period in years.
Type the installed cost and the system size in kWp. A typical domestic system is around 4 kWp.
Enter the expected generation per kWp per year. UK systems produce roughly 850–1,000 kWh per kWp depending on orientation, pitch and location; 900 is a reasonable default.
Enter the share of generation you use yourself (often 30–50% without a battery), your electricity import price, and the SEG export rate you are paid for surplus.
The calculator shows annual generation, the combined saving and export income, and the payback period in years.
Self-consumption is the biggest lever. Electricity you use yourself is worth the full import price you would otherwise pay, while exported surplus earns only the lower SEG rate — so the more you use as you generate, the faster the payback. Batteries, timed loads and daytime usage all raise self-consumption and shorten the payback.
Generation depends on the roof. A south-facing, unshaded roof at a good pitch will hit the top of the yield range; east-west or shaded roofs produce less. Use a realistic per-kWp figure for the actual roof rather than a best-case number, or the payback will look rosier than it is.
This is an estimate, not a guarantee. It excludes inverter replacement partway through the system's life, gradual panel degradation, and future changes in electricity and export prices — all of which affect the real return. Treat it as a guide to inform the conversation, and note the assumptions to the customer.
An estimate. Generation depends on roof orientation, pitch, shading and location (UK yields are roughly 850–1,000 kWh per kWp). Self-consumption, SEG export rates and electricity prices vary, and it excludes inverter replacement and panel degradation over time.
Typically somewhere around 8 to 14 years, depending on the system cost, how much of the generation you use yourself, and your electricity and export rates. High self-consumption and high electricity prices shorten it; a shaded roof or low self-use lengthens it. Enter your figures above for an estimate.
The Smart Export Guarantee is the scheme under which licensed electricity suppliers pay you for surplus solar electricity you export to the grid. Rates vary by supplier, so the export price you are offered matters to the payback — the higher it is, the more your surplus generation is worth.
Significantly. Electricity you use yourself saves the full import price, whereas exported surplus only earns the lower SEG rate. Raising self-consumption — by using appliances during the day or adding a battery — increases the annual benefit and shortens the payback period.
In the UK, roughly 850–1,000 kWh per kWp of system per year, so a 4 kWp system generates around 3,400–4,000 kWh. The exact figure depends on the roof's orientation and pitch, shading and where in the country it is. Use a realistic per-kWp yield for the specific roof.
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