A lower ratio is an observation under stated conditions, not proof of retrofit savings, building heat loss or heating-system efficiency.
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.
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.
EIA: degree-day definitions and US base convention (English; not a local design standard) ↗Calculate the index, not a savings certificate
Fictional E1 = 2400 kWh, H1 = 600 °C·day; E2 = 2100 kWh, H2 = 700 °C·day. Indices are 2400 / 600 = 4 and 2100 / 700 = 3 kWh/(°C·day). Signed index change = (3 / 4 − 1) × 100 = −25%. Raw energy change is −12.5%, a different comparison.
Negative means a lower index, positive a higher index; neither identifies the cause. Zero E2 gives a zero index and −100%, which may reflect no heating input or changed use, not certified savings. Zero HDD cannot be divided into and is rejected. Do not extrapolate a near-zero-HDD or very short period into an annual result.
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.
ASHRAE Handbook Chapter 37: energy records and comparison context (English; no compliance or savings certification) ↗A comparable period record
| Record | Keep together | Do not substitute |
|---|---|---|
| Heating input | Dates, kWh, equipment boundary, measured/estimated status | Money, delivered heat or a mixed bill |
| Weather | Same dates, station, base, °C·day and method | CDD, another base or a normal year |
| Conditions | Setpoint, schedule, occupants, area, equipment | An unexplained efficiency claim |
Worked example
Fictional comparison, not retrofit proof
Two documented space-heating input records are 2400 kWh over 600 °C·day and 2100 kWh over 700 °C·day, with compatible weather definitions.
I1 = 2400 / 600 = 4 I2 = 2100 / 700 = 3 ΔI = (3 / 4 − 1) × 100 = −25%
The index changes from 4 to 3 kWh/(°C·day), or −25%. It does not certify savings or separate other changes; keep the underlying records.
What to have ready
- Space-heating input energy isolated and documented
- Energy dates exactly match each HDD interval
- Compatible station, base, units and daily method
- Operating and equipment changes recorded
- Index kept separate from verified savings and cost
Common mistakes
- Dividing a whole-home mixed bill by HDD without separation.
- Mixing °F·day with °C·day or changing base temperature.
- Using calendar-month weather for different meter dates.
- Treating a lower ratio as proof of a retrofit effect.
- Dividing by zero HDD or extrapolating a tiny period.
Use and limitations. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.