How payback is calculated
Monthly generation = kW × sun-hours × 0.8 × 30. Units you use yourself save your full tariff; surplus exported earns the export rate. Payback = system cost ÷ monthly savings. The 25-year figure uses today's tariff — conservative, since tariffs usually rise.
Read your quotation with this
Run the quote you were given, then re-run with a size closer to your daytime usage. Under net billing, the smaller system frequently pays back faster — capacity that only exports at a low rate is slow money.
What is a good solar payback period?
In high-tariff, sunny markets, 2–4 years is common for well-sized systems. Beyond 6–7 years usually signals an oversized system or a poor export tariff.
Why does oversizing hurt payback under net billing?
Exported units are often paid a fraction of what imported units cost. Energy you can't use yourself earns little — so capacity beyond your daytime usage pays back very slowly.
Is this calculation in my currency?
Yes — enter cost and tariffs in your own currency; the result is currency-neutral arithmetic.