An 800W array can make a useful dent in a home’s background electricity use. It cannot erase a typical bill, and it will not produce 800W all day.
Start with annual energy
Britain’s solar resource varies by region. Direction, tilt and shading then move the result. Our checker uses rounded annual-yield bands from 850 to 1,050kWh for each installed kilowatt.
At 0.8kW, that creates a broad unshaded planning range of 680 to 840kWh a year before orientation adjustments.
Self-use controls the cash result
Electricity used while the panels generate avoids the retail tariff. Exported electricity may earn less or nothing, depending on metering and supplier arrangements.
Suppose 75% of 760kWh is used at home. At 27p per kWh, the avoided purchase is about £154 a year. This excludes export, degradation, finance and maintenance.
A good estimate shows the assumptions beside the answer. A sales number hides them.
Payback is not fixed
An £850 installed cost and £154 yearly saving gives a simple payback near 5.5 years. A shaded site, lower tariff or poor daytime use can stretch it. A lower kit price can shorten it.
Run the calculator with your own tariff and a cautious self-use estimate. Treat the result as a comparison tool, not a quotation.