Sizu Riatto
Watt Happens Next · Chapter 8

The Setback

Letting the house coast overnight saves the heat it does not lose while it is cooler. How much depends on how fast it cools: one formula, your τ, and the answer is in your own kWh.

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

The night

The house (from the cooling log and the heating line)

The money

Enter the night and the house, then press Calculate.

The formula

  • Held all night: degree-hours = (thermostat − outdoor) × hours
  • Coasting: degree-hours = gap × τ × (1 − e^(−hours/τ)), the area under the cooling curve
  • Saved heat (kWh) = heat loss (kW per °C) × the degree-hours saved; gas = heat ÷ boiler efficiency
  • The coldest point = outdoor + gap × e^(−hours/τ)
  • A short-τ (light) house cools further, so coasting saves more
The book's worked example, Chapter 8. Holding 20 °C for 8 hours on a 4 °C night costs 128 °C·h; coasting costs 114.78, so it saves 13.22 °C·h: 3.97 kWh of heat, 4.41 kWh of gas a night, and the house is down to 16.81 °C by morning. Over 31 nights: 136.7 kWh, 10.93, 5.8% of the month's heating gas. A light house (τ 12 h) saves 34.58 °C·h.
Check it. Read the gas meter at bedtime and at breakfast on held nights and on coasting nights of similar weather: the difference is your own setback saving (the book's Chapter 11 checks it).

Honest limits. A one-box model; the morning reheat is not free, and a heavy house saves less than a light one. Government figures such as "up to 10%" are ceilings for some homes, not promises. 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.