Turning Last Winter's Therms Into the Electricity a Heat Pump Would Draw
Anyone weighing up a heat pump eventually reaches the same wall: the gas bills are in therms and everything about the replacement — the equipment sheet, the panel, the tariff — is in kilowatt-hours. Converting the therm figure is the first move, but the raw number is only a starting point. What the heat pump draws depends on how much of that gas became heat and how efficiently the compressor makes heat from electricity.
The Two Divisions That Sit After the Conversion
AFUE Shrinks the Target
COP Shrinks It Again
Annual Energy Is Not Peak Demand
Working From a Year of Bills to a Realistic kWh Number
Start with one number — total therms for twelve consecutive statements — and take it through the chain in order.
Total a full year of therms
Add the therm column across twelve statements rather than scaling one winter month up, since heating loads are wildly seasonal. If you want the heating portion alone, subtract twelve times your flattest summer month before you go any further.
Convert the total and write the raw kWh down
Type the annual therms on the left; the kWh appears beside it as you type, so trying 620 instead of 600 costs nothing. Type it with spaces or a comma as the decimal mark if that is how your notes read. This raw figure is gas energy, not electricity — do not stop here.
Apply AFUE, then divide by your COP
Multiply the raw kWh by your old furnace's AFUE — 0.80 for a mid-efficiency unit, 0.95 for a condensing one — then divide by the seasonal COP you expect. If your quote lists HSPF instead, HSPF ÷ 3.412 gives you the COP: an HSPF of 10.5 is about 3.08.
Swap to test a kWh estimate someone handed you
Contractors and utility programmes often quote the switch in kWh. The swap arrow puts kWh on the left so you can convert their figure back into therms and see whether it resembles the gas you genuinely burn.
Annual Gas Use Converted to Heat-Pump Electricity by COP
Each row starts from a year's gas consumption, applies an 80% AFUE furnace to get the heat that was actually delivered, then divides by four seasonal COP values. The delivered-heat column doubles as the COP 1.0 case, which is what plain resistance heat would cost you in kWh.
| Annual gas use | Raw energy (kWh) | Heat delivered / COP 1.0 | COP 2.0 | COP 2.5 | COP 3.0 | COP 3.5 |
|---|---|---|---|---|---|---|
| 300 therms | 8,792 | 7,034 kWh | 3,517 | 2,813 | 2,345 | 2,010 |
| 500 therms | 14,654 | 11,723 kWh | 5,861 | 4,689 | 3,908 | 3,349 |
| 700 therms | 20,515 | 16,412 kWh | 8,206 | 6,565 | 5,471 | 4,689 |
| 900 therms | 26,376 | 21,101 kWh | 10,551 | 8,440 | 7,034 | 6,029 |
| 1,200 therms | 35,169 | 28,135 kWh | 14,067 | 11,254 | 9,378 | 8,039 |
| 1,500 therms | 43,961 | 35,169 kWh | 17,584 | 14,067 | 11,723 | 10,048 |
Read across the 700-therm row and the whole electrification argument is visible in one line: the same house needs 16,412 kWh of resistance heat but only 5,471 kWh from a heat pump holding a seasonal COP of 3.0. Read down instead and you can see how much the COP assumption matters — the gap between COP 2.0 and COP 3.5 is roughly a factor of 1.75 on every row, which is why a mild-climate estimate and a cold-climate one are not interchangeable.
What Helps While Sketching an Electrification Estimate
Try a Range of Annual Totals in Seconds
Because the conversion follows every keystroke, walking a therm total from 600 up to 1,000 to see how the kWh moves takes about as long as typing the digits.
Clean kWh Figures for a Load Sheet
The copy control gives the bare number without the unit or thousands spacing, so the raw kWh drops straight into the cell where you divide by COP. Ctrl+C inside the field works the same way.
Sanity-Check a Contractor's kWh Claim
Reversing the direction turns a proposed annual kWh back into the therms it implies, which is the quickest way to spot a quote built on somebody else's house.
Other Units When the Audit Report Changes Scale
Home-energy audits and modelling tools may report in MJ, GJ or BTU. Both dropdowns search all 23 energy units, so a report in another scale still meets your therms on one screen.
Electrification Questions Behind the Therm-to-kWh Number
How many kWh does a heat pump actually need to replace one therm?
Between about 7 and 12 kWh for most households, not the 29.3 kWh the straight conversion prints. Start from 29.3071 kWh of raw gas energy, keep 80% of it as delivered heat because that is what a mid-efficiency furnace managed, and divide the remaining 23.4 kWh by the seasonal COP. At COP 3.0 that is 7.8 kWh per therm replaced; at a colder-climate COP 2.5 it is 9.4 kWh. If your old furnace was a condensing model at 95% AFUE, raise those figures by about a fifth, since less of the gas was being wasted in the first place.
Why does the raw 29.3 kWh per therm overstate a heat pump's consumption?
Two separate reasons stack. First, the therm you were billed for was never all useful — the flue took a share of it, so the house never needed 29.3 kWh of heat. Second, and much larger, a heat pump does not manufacture heat from electricity; it pumps existing heat indoors, so every kilowatt-hour on the meter arrives as two to four kilowatt-hours of warmth. The raw conversion answers “how much energy was in that gas”, which is a genuinely different question from “how much will the electric meter turn”.
What does cold-weather COP degradation do to the annual estimate?
A great deal, because a heat pump's efficiency falls exactly when you need it most. Units commonly test near COP 3.5–4.0 at 47 °F but land closer to 2.0–2.5 at 17 °F, and lower again below that, with defrost cycles taking a further bite. A seasonal average of 3.0–3.5 is reasonable in a mild climate; in a cold one, 2.2–2.8 is more honest, and any hours covered by resistance backup run at COP 1.0. Pick your COP from the seasonal rating rather than the headline test point and the table above stops flattering the estimate.
Will a 100-amp panel handle the added winter load?
Often yes, but the answer comes from a load calculation, not from the annual kWh. A ducted cold-climate heat pump of about three tons typically lists a minimum circuit ampacity in the mid-twenties, which a 100-amp service can usually absorb. Electric backup is what breaks the budget: a 10 kW resistance strip alone draws about 42 amps at 240 volts, and adding it on top of an existing range, dryer and water heater is what pushes many homes over. Sizing the heat pump to carry more of the winter, or keeping the gas equipment as backup, is usually cheaper than a service upgrade.
How do I turn twelve gas bills into one therm total?
Pull twelve consecutive statements and add the therms column; most utilities also publish a downloadable usage history that saves the typing. If your statements report CCF, multiply each month's volume by that month's therm factor before adding, because the factor is not constant across the year. Then decide what you are actually replacing: a heat pump only takes over space heating, so if the water heater, range and dryer stay on gas, subtract twelve times your lowest summer month before converting. Feeding the whole-house total into an electrification estimate is the single most common way people overstate the kWh they will need.
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