How many watts does a furnace use?
A furnace uses 78–836 watts while running, which is 0.65–7 amps at 120 V.
Furnace watts, amps and circuit
| Running watts | 78–836 W |
|---|---|
| Voltage | 120 V |
| Running amps | 0.65–7 A |
| Circuit | dedicated 120 V circuit, breaker size from the nameplate |
Also called: gas furnace, furnace blower, propane furnace, oil furnace, electric furnace.
Furnace watts by size and type
| Size or type | Running watts, W | Volts, V | Amps, A | Starting watts, W |
|---|---|---|---|---|
| Furnace | 78–836 | 120 | 0.65–7 | — |
| Electric furnace | 10,000–25,000 | 240 | 41.7–104.2 | — |
What size generator runs a furnace?
A furnace needs a generator that supplies at least 836 running watts. Starting a furnace takes a short surge of several times its running watts, and the surge is not listed here: size the generator from the locked-rotor amps (LRA) on the nameplate or the maker's generator guidance, not from the running watts.
How long a portable power station runs a furnace
| Battery capacity, Wh | At 836 W, h | At 78 W, h |
|---|---|---|
| 500 | 0.5 | 5.4 |
| 1,000 | 1 | 10.9 |
| 2,000 | 2 | 21.8 |
| 3,600 | 3.7 | 39.2 |
A 500 Wh portable power station runs a furnace drawing 836 W for about 0.5 hours at 85% inverter efficiency. Runtime = capacity × 85% ÷ watts; the 85% efficiency is an assumption. The power station's AC output must be at least 836 W.
What it costs to run a furnace
Running a furnace at 457 W, the middle of its 78–836 W range, for one hour a day uses 166.8 kWh a year. That costs about $30 a year at $0.18 per kWh, the US average residential price for August 2026.
Your bill's total ÷ kWh used. 0.18 is the US residential average.
- Per day
- $0.08
- Per year
- $30
- Energy per day
- 0.46 kWh
- Energy per month
- 13.9 kWh
How it is calculated
- kWh per day = 457 W × 1 h ÷ 1,000 = 0.457
- Per month = 0.457 kWh × 30.4 days × $0.18 = $2.50
What affects furnace power use
A gas, propane or oil furnace burns fuel for heat and uses electricity for the blower, the draft inducer and the controls. The running watts here are the blower motor alone, from the maker's blower tables; they rise with blower speed and furnace size, and the inducer and controls add to them. An electric furnace heats with resistance elements, runs on 240 V and needs much larger circuits than a gas furnace.
How the amps are calculated
Amps = watts ÷ volts. For 836 W at 120 V that is 7 A. The 78–836 W range covers typical models; the nameplate on a furnace gives the exact figure.