Sizu Riatto
Watt Happens Next · Chapter 12

Is It Worth It?

Every energy decision is the same three numbers: what it costs, the kWh it saves, and what a kWh costs you. Put in your own and the payback is one division away.

Home › Calculators › Is It Worth It?

Opens on the book's house. Change any number to your own and press Calculate.

Or from the old and new use (optional, replaces the kWh it saves)

Enter the cost, the kWh saved and the price, then press Calculate.

The formula

  • kWh a year = kWh a day × 365, or a month × 12; or (old − new kWh a day) × 365
  • Money a year = kWh a year × the price of a kWh (heat from gas: the gas rate ÷ the boiler's efficiency)
  • Payback (years) = the cost ÷ the money a year; over a life of L years: money a year × L − the cost
  • Do not add decisions up: they overlap. Run the model again with all of them in
The book's worked example, Chapter 12. The book's fridge: 1.20 against 0.62 kWh a day saves 211.7 kWh, 55.04 a year; at 600 it pays back in 10.9 years, so replace it when it fails unless it is failing. The loft top-up: 480 for 73.45 a year, 6.5 years. The setback: 136.7 kWh a month, 10.93. The standby: 307 kWh, 79.72 a year.
Check it. Before you buy, measure the old appliance for a week on a plug meter: the saving is only as good as the old number.

Honest limits. Every saving here is a worked example, not a promise: your habits, your tariff and the weather decide it. Published savings figures are their sources', with their conditions. Installation work goes to a qualified installer. The book's house, its meters and its bill were worked out on paper from published figures, not measured by us: read your own meters and log your own night. Tariffs are illustrative, as seen in November 2026 (shaped like the GB price cap of October to December 2026, with US prices beside), shown without a currency sign; put in your own. No saving is promised. Everything stays in your browser: no sign-up, nothing stored.

These calculators go with Watt Happens Next (Make It Add Up, Book 3), where every number on this page is worked by hand.