HTHOMETALLYTHE PRACTICAL HOME GUIDE

Heating/ CALCULATOR

Compare heating input energy per degree-day

Compare two space-heating input-energy/HDD ratios and their signed percentage change. No baseline regression, normal-year forecast, cost or verified savings.

Positive kWh attributable to space heating on a documented input-energy boundary. Not a whole-home bill including hot water/appliances, currency, kW or delivered heat.
Positive °C·day for exactly period 1’s energy dates. Record station, base temperature, daily method and source; use matching definitions for both periods.
Same input-energy boundary and kWh basis as period 1. Zero means recorded zero heating input, not proof of 100% efficiency improvement.
Positive °C·day for exactly period 2’s energy dates; same station, base and calculation convention as H1. Not cooling degree-days or a normal-year total.
Period 1 input-energy/HDD index

4.000 kWh/(°C·day)

Period 2 input-energy/HDD index

3.000 kWh/(°C·day)

Index change: period 2 versus period 1

-25.00 %

E1 > 0, E2 ≥ 0, H1 > 0 and H2 > 0. Same documented space-heating input-energy boundary and compatible °C·day data. The index uses a proportional zero-intercept comparison; no base-load separation, fitted balance temperature, regression, uncertainty, normal-year adjustment or causal savings claim. Keep occupancy, heated area, schedules, setpoints and system changes visible. Fictional defaults.

FROM ESTIMATE TO ACTION

Use the result, not just the number.

01

Define the heating energy boundary

Record energy dates, meter/source, kWh conversion and which space-heating equipment is included. Use existing safe records; do not open meters, electrical panels or fuel equipment. Keep input energy separate from delivered heat, peak kW and money. Domestic hot water, cooking and appliances are not automatically space heating. If only a mixed whole-home total is available, obtain a defensible separation or a more appropriate model; do not invent a percentage subtraction. A switch of fuel, meter boundary or heat-pump COP also changes what the ratio can mean.

02

Match dates and degree-day definitions

EIA describes heating degree-days as outdoor temperature deficits relative to a chosen base. Its US 65°F convention is not a mandatory base for this tool. For each energy period use HDD from the identical dates, recording station, base, units and calculation method. Both periods need compatible definitions; lengths can differ. Convert temperature-difference units consistently: 100 °F·day = 100 × 5/9 ≈ 55.556 °C·day, not (100 − 32) × 5/9. A changed base temperature is not fixed by unit conversion.

03

Keep base load and other changes visible

ASHRAE recommends retaining meter dates, estimated readings and operating/weather information when comparing energy records. This tool is not its measurement-and-verification procedure. A simple algebra check shows the limitation: if total E = B + aH, then E/H = a + B/H. With unchanged a = 4 and base energy B = 400 kWh, H = 600 and 700 give mixed totals 2800 and 3200 kWh, and misleading indices about 4.667 and 4.571. The tool does not remove B. Keep setpoints, hours, occupants, heated area, solar gains and equipment changes; a qualified multi-period analysis is needed before attributing a change to a measure.

Inputs to verify

Period 1 space-heating input energy E1 (kWh)
Positive kWh attributable to space heating on a documented input-energy boundary. Not a whole-home bill including hot water/appliances, currency, kW or delivered heat.
Period 1 heating degree-days H1 (°C·day)
Positive °C·day for exactly period 1’s energy dates. Record station, base temperature, daily method and source; use matching definitions for both periods.
Period 2 space-heating input energy E2 (kWh)
Same input-energy boundary and kWh basis as period 1. Zero means recorded zero heating input, not proof of 100% efficiency improvement.
Period 2 heating degree-days H2 (°C·day)
Positive °C·day for exactly period 2’s energy dates; same station, base and calculation convention as H1. Not cooling degree-days or a normal-year total.

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