Calculate heating degree days for energy use.
Heating degree days measure how much heating a building needed, and they are the mirror image of cooling degree days: each day contributes the shortfall of its mean temperature below a balance point rather than the excess above one. The conventional base differs by country — 15.5 C in the United Kingdom, 18.3 C (65 F) in the United States — because it represents the outdoor temperature below which a building starts losing more heat than its occupants and appliances put in. Since fabric heat loss is proportional to the inside-outside temperature difference, fuel consumption tracks heating degree days closely, which is why they underpin boiler sizing, fuel budgeting and any honest year-on-year comparison of a heating bill.
Heating degree days
Mean temperature = (daily high + daily low) / 2; heating degree days per day = max(0, balance point - mean temperature); total = per-day figure x days in the period
Convention, not physics. The UK settled on 15.5 C and the US on 65 F (18.3 C). The base changes the numbers substantially, so never compare degree-day totals without checking that both used the same balance point.
Divide fuel used by heating degree days for a period before the work and again after. That gives fuel per degree day, which strips the weather out of the comparison — a fall in that ratio is real efficiency, whereas a fall in raw fuel use might just be a mild winter.
No. The balance point is lower than the indoor temperature you want, because occupants, lighting, cooking and appliances contribute several degrees of free heat. Using 21 C would badly overstate the heating requirement.