Solar Calculator

Estimate annual solar output, self-consumption, export value and simple payback using assumptions you control. No address, lead form or automatic tariff is required.

Solar Calculator

Your inputs are calculated in your browser. No account is required.

Guide updated 6 October 2026

How to estimate solar generation and savings

Enter the array’s rated capacity in kW, average daily peak sun hours and combined system losses. Then add average monthly electricity use, the percentage of generated electricity you expect to use directly, retail electricity value and export value per kWh.

Complete the installed cost, confirmed upfront incentive and annual maintenance cost to estimate simple payback. Keep every financial value in one currency. Default technical values are illustrative starting assumptions, not a location-specific assessment.

Solar output formula: kW, sun hours and losses

Estimated annual output = capacity × daily peak sun hours × 365 × (1 − losses ÷ 100). Peak sun hours are equivalent hours of full solar irradiance, not the number of daylight hours. A 5 kW system with 4 sun hours and 20% combined losses produces an estimated 5,840 kWh per year.

This simple annual model does not retrieve roof geometry, weather records or monthly irradiance. PVWatts provides a more detailed weather-based model. Do not treat our entered loss factor as identical to a PVWatts setting: this tool’s combined loss allowance must account for all reductions you intend to include.

Self-consumption and export compensation

Directly used solar electricity is valued at the retail rate you enter. Exported electricity is valued at your entered export rate. The tool caps annual self-used energy at your entered annual consumption so it cannot claim that you avoided buying more electricity than you use.

An annual cap does not model hourly matching, batteries or seasonal shortages. Your expected self-consumption percentage should already reflect those limitations. Export rules, credit caps and fixed bill charges are not automatically applied.

Installation cost, incentives and simple payback

Net upfront cost = installed cost − confirmed upfront incentive. Annual net benefit = self-used kWh × retail value + exported kWh × export value − annual maintenance. Simple payback = net upfront cost ÷ annual net benefit, provided that benefit is positive.

If generation is 5,840 kWh, half is used directly at 0.25 per kWh and half exported at 0.05, the annual energy value is 876. With maintenance of 100 and net cost of 8,000, annual net benefit is 776 and simple payback is about 10.31 years. These are fictional flat-rate examples.

What this solar calculator does not forecast

The estimate excludes financing, electricity-price escalation, panel degradation, inverter replacement, discount rates and tax treatment. It does not apply US credits, Australian rebates, German feed-in rules or Spanish compensation limits automatically. Enter only benefits you have verified.

A payback result is not a savings guarantee or an installer quote. Roof suitability, shading, structural condition and electrical design need a site-specific assessment. A zero or negative annual benefit produces no finite simple-payback result.

Common solar calculator mistakes

Do not enter panel wattage as total system kW: ten 400 W panels total 4 kW. Use a per-kWh tariff, not your entire electricity bill. Avoid using full daytime hours as peak sun hours or assuming every generated kWh earns the retail rate.

Try several scenarios for sun hours, self-consumption and export value. If the economics only work under optimistic inputs, obtain a more detailed generation and tariff model before deciding.