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Payback is system cost ÷ annual savings — the number of years for the electricity you no longer buy to repay the installation. It is the headline figure most people want when weighing solar.
This ignores the time value of money, tariff inflation, subsidies, panel degradation, and maintenance. Rising electricity rates usually shorten real payback, while degradation slightly lengthens it — a fuller model captures both.
That is the promise behind every tool here. Files are read in your browser tab, processed there, and discarded when you close it — never uploaded to our servers.
