Inputs that make a solar payback estimate useful

Estimate generation for the actual location

Specific yield is annual generation divided by installed peak power, expressed as kWh/kWp. It varies with location, tilt, azimuth, horizon, shading, technology and system losses. Use the European Commission's PVGIS tool for the property coordinates instead of copying a national average from a sales page.

Translate PVGIS output into the calculator

Enter the system peak power in kWp and use PVGIS annual production divided by that peak power as the specific-yield input. Check the assumed system loss and mounting configuration. If trees, chimneys or nearby buildings cause shading that PVGIS does not capture, obtain a site survey or test a lower-yield scenario.

Value self-consumption and export separately

Electricity used while the array is producing normally avoids an import price. Exported electricity is worth the actual compensation, export tariff or accounting value under your contract. Applying the full retail price to every generated kWh can materially overstate savings when self-consumption is low.

Use measured consumption shape

Annual household kWh alone does not show how much solar will be used directly. Daytime loads, EV charging schedules, hot-water control and seasonal occupancy change the overlap between demand and generation. Compare a conservative self-consumption percentage with an improved case rather than assuming a battery automatically makes every solar kWh worth the retail price.

A 7.81 kWp practical starting point

For WattCalc Lab's editor's 7.81 kWp residential array, the correct first step is to model the actual location and roof in PVGIS; the site does not publish an invented production total. For illustration only, a specific yield of 950 kWh/kWp would imply about 7,420 kWh/year before applying self-consumption and export values. Replace that assumption with actual inverter data as a full year becomes available.

Know what simple payback excludes

Simple payback divides installed cost by annual value. It does not automatically model financing, inverter replacement, maintenance, panel degradation, opportunity cost, tariff changes, tax or equipment residual value. Use it to compare scenarios, not as a guaranteed investment return.

Review the estimate after commissioning

Compare monthly inverter production with the original model and check import/export meter data. Update self-consumption and export values after a representative year. A calculator becomes more useful when assumptions are replaced by measured figures rather than left at their defaults.

Audit self-consumption with a balanced energy ledger

For a hypothetical month with 800 kWh PV generation and 500 kWh export, direct self-consumption is 300 kWh, or 37.5% of generation. If household demand is 600 kWh, that direct use covers 50% of demand. Those percentages answer different questions. Enter 37.5%, not 50%, in the solar-payback self-consumption field.

Check that generation, import and export cover identical dates and units. Where a battery exists, account separately for charging, discharging and losses: subtracting export from generation is not necessarily direct household use. A national yield average cannot replace a roof assessment, and an annual percentage cannot prove that a proposed battery cycles every day.

Sources and scope

European Commission JRC: PVGIS · EIA: measuring electricity. References explain units and technology; hypothetical examples use the site’s own model. Owner observations are not laboratory tests.