Sizu Riatto
Watt Happens Next · Chapter 6

Reverse the Slope: Totals

The heating line's slope is a rate; the month's total is that rate added up over the month's cold. Degree-days do the adding, so next month's gas is one multiplication away.

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

Degree-days from the daily mean outdoor temperatures

Your heating line (from the heating-line calculator)

Next period (optional): its normal degree-days and its days

Enter the daily temperatures or next month's degree-days, then press Calculate.

The formula

  • A day's degree-days = base − the day's mean temperature, or 0 if the day is warmer than the base
  • Degree-days for a period = Σ of the days' degree-days: the area under (base − outdoor), in day units
  • Heating gas = gas per degree-day × degree-days; the rest = the no-heating gas × days
  • This is the Fundamental Theorem at work: the total is the area under the rate
  • US degree-days use 65 °F; °F-degree-days = °C-degree-days × 1.8
The book's worked example, Chapter 6. The book's 28 days add up to 298.5 degree-days (537 °F-days). Next month's normal is 290 degree-days over 31 days: heating 8.066 × 290 = 2,339.1 kWh, plus 31 × 7.30 = 226.2 kWh, 2,565.3 kWh of gas (87.5 therms).
Check it. Run the logged month through it: the heating line's gas should come back to the meter's total for the same days.

Honest limits. Normal degree-days are an average: a cold snap can add 10% or more. That is why the Bill Predictor carries a band (Chapter 10). 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.