Calculate SimplyHow to use
Start with the worked example to understand the inputs, then replace every relevant value with your own figures. Use one currency and consistent units throughout. Add or remove rows where offered; irrelevant conditional fields are disabled and excluded. Select Calculate after editing. Review negative values and any “not determined” result rather than assuming every scenario must be profitable. Download a text summary or print the page for your records.
Formula and assumptions
Enter a solar-plus-battery quote and independently estimated first-year generation, for example from PVWatts. The model values self-consumed energy at the import tariff and exported energy at the export rate. Apply annual production degradation and price escalation, subtract maintenance and a chosen battery replacement. The year-by-year table starts with net installation cost after upfront incentives. Payback is the first interpolated cumulative crossing; later replacement costs can reduce the surplus again. This undiscounted cash-flow model does not size storage, simulate hourly dispatch, calculate tax credits or guarantee generation.
cash(y) = generation × (1−degradation)^(y−1) × [self × import rate + (1−self) × export rate] × (1+escalation)^(y−1) − maintenance − replacement
Worked example
Example only: replace rates with your own figures.
Inputs
- Total solar and battery installation cost: 15000
- Upfront grants or incentives: 0
- First-year solar generation: 7000 kWh
- Solar self-consumption share: 70 %
- Import electricity rate per kWh: 0.25
- Export payment per kWh: 0.05
- Annual maintenance cost: 150
- Annual generation degradation: 0.5 %
- Annual electricity price escalation: 0 %
- Projection years: 20
- Battery replacement year (0 = none): 10
- Battery replacement cost: 4000
- Net upfront cost: 15000 USD
- First-year net cash flow: 1180 USD
- Final net benefit: 3373.61 USD
- Simple payback: 16.835 years
- Return on upfront cost: 22.4907 %
Frequently asked questions
Are the example rates current market prices?
No. Example inputs demonstrate arithmetic, not live prices, tax rules, salary surveys or sponsor benchmarks. Enter a dated quote, bill, manufacturer specification or verified fee schedule that applies to your situation. Compare a conservative and an optimistic scenario separately.
Why is a result not determined?
Some calculations have no meaningful value when a denominator is zero, savings are non-positive, a target cannot be reached or your data do not bracket an intersection. The tool leaves that result undetermined instead of displaying infinity or inventing a recommendation. Check the method and your inputs.
Sources
Sources explain the underlying concepts, not an endorsement of this website or its results. Method review: 20 October 2026.