Enter how much electricity you use in a year (it’s on your power bill or your retailer’s app), your area’s average sunshine, and local prices. Money amounts work in any currency; just keep them consistent.
| Year | Generation (kWh) | Savings that year | Total saved | Net gain |
|---|
Peak sun hours are the number of hours a day that sunlight would need to shine at full strength (1,000 W per m²) to deliver the same energy as the whole day actually does. A 1 kW system in a 4.2-hour location therefore makes about 4.2 kWh on an average day, less losses.
System size (kW) = yearly kWh to cover ÷ (peak sun hours × 365 × (1 − losses)). To cover 80% of 7,000 kWh at 4.2 hours with 14% losses: 5,600 ÷ (4.2 × 365 × 0.86) ≈ 4.25 kW, or 10 panels of 440 W (4.4 kW). Each panel is about 1.95 m² (21 ft²).
Savings depend on how much solar you use yourself (worth the full power price) versus export to the grid (worth the lower buy-back rate). Running the dishwasher, washing machine, hot water or EV charging during the day raises your self-use share, and your savings with it.
Payback is the year in which total savings first exceed the system cost, allowing for power prices rising and panels slowly losing output.
Divide your yearly electricity use by what one panel makes in a year: panel watts × peak sun hours × 365 × 0.86 ÷ 1,000. A 440 W panel in a 4.2-hour location makes about 580 kWh a year, so covering 5,600 kWh takes about 10 panels.
Typically 6 to 12 years, depending on system cost, local sunshine, power prices and how much solar you use yourself. Panels usually carry 25-year performance warranties, so most of their life is profit after payback.
A way of measuring how much solar energy a place receives in a day, expressed as hours of full-strength (1,000 W/m²) sunshine. It’s lower than daylight hours, and varies by season: a place averaging 4 might see 6+ in summer and 2 in winter.
A battery lets you use more of your own solar in the evening, raising your self-use share, but batteries are expensive and usually take longer to pay back than the panels themselves. Try setting self-use to 80–90% to see the potential gain, then compare it with the battery’s price.
Real systems deliver less than panels’ rated output because of heat, inverter efficiency, wiring, dust and small amounts of shading. 14% is a standard default used in solar modelling tools such as NREL’s PVWatts.
