Common solar payback mistakes

1. Using peak generation as if it were the average

A system does not generate at its rated kW all day, every day. Generation varies by season, weather, roof angle, and shading. This calculator uses an average generation figure per kW per day that you can change — if an installer gives you a local figure, use that, and run a lower figure as a pessimistic case too.

2. Assuming 100% self-use

Unless someone is home using power all day, or flexible loads are shifted to daylight, a large share of solar is exported at the lower feed-in rate. Assuming all solar is self-used overstates saving by a wide margin. Enter a realistic self-use share, and test a lower one.

3. Using the wrong tariff figures

Using a neighbour's feed-in tariff, an advertised rate, or last year's price will give a payback that does not match your bill. Use the buying price and feed-in tariff from your own current plan, and remember both can change.

4. Ignoring what simple payback leaves out

Simple payback — cost divided by yearly saving — leaves out panel degradation, inverter replacement, maintenance, and price changes. It is a useful first comparison, not a lifetime profit figure. The year-by-year view on this calculator also repeats the same yearly saving each year for simplicity; treat it as a scenario, not a forecast.

5. Accepting a quote without a breakdown

If a quote gives only a single payback number, ask for generation, self-use, export, buying price, and feed-in tariff separately, then enter those same figures in this calculator. If the numbers do not reproduce, ask which assumption differs before signing anything.

6. Adding a battery without checking its own payback

A battery can raise self-use and still lengthen overall payback, because its cost is large and its extra value is only the price gap on the kWh it shifts. Check the battery cost divided by its extra yearly value separately — see Battery: pays or not?

Estimates only, not financial advice. See the Disclaimer.