BTU to Watts Calculator

To convert BTU to watts, multiply BTU per hour by 0.293, or divide by 3.412. So 10,000 BTU/h is 2,931 watts. That figure is thermal output, not electrical draw: a 10,000 BTU/h window air conditioner made since 2014 draws roughly 625–920 watts at its rated efficiency, because it moves heat rather than making it.

The BTU to watts formula, and what each term means

One BTU, or British thermal unit, is 1,055 joules — the energy needed to raise a pound of water by one degree Fahrenheit. A watt is one joule per second. Spread 1,055 joules across the 3,600 seconds in an hour and you have the only constant on this page:

watts = BTU per hour × 0.29307

BTU per hour = watts × 3.41214

• BTU per hour (BTU/h) is a rate: energy moved every hour. Almost every appliance label that prints a bare “BTU” means BTU/h.
• Watts is the same rate in metric units. 3,412 BTU/h is 1 kilowatt, and that one pairing is worth memorizing.
• BTU on its own is a quantity of energy, the twin of a watt-hour rather than of a watt. The same constant applies: 3,412 BTU is 1kWh, so a 2,000Wh battery holds about 6,824 BTU.

Nothing there asks for a voltage, a power factor or an efficiency figure. It is pure unit swapping, which is why the errors people make with this conversion are never arithmetic errors.

Conversion table: BTU per hour into watts

Thermal watts at 0.29307 watts per BTU/h. The last column is the current that wattage would draw at 120V US household voltage if the appliance were an electric resistance heater, the one case where output and input are the same number. Anything that moves heat rather than generating it draws far less.

The highlighted row is the whole table in one line: 3,412 BTU/h is 1 kilowatt. Divide any BTU figure by 3,412 and you have kilowatts directly, and 12,000 BTU/h — one “ton” of cooling — is 3.5kW. The amps column also shows where a wall outlet gives up. Above about 4,900 BTU/h (1,440W), a resistance heater exceeds the continuous budget of a 15A circuit under the 80% rule, and a 20A circuit raises that ceiling to about 6,550 BTU/h (1,920W).

Why this conversion decides your battery and inverter

Batteries are sold in watt-hours, inverters in continuous watts, solar panels in watts. Air conditioners, RV furnaces, propane heaters and diesel heaters are sold in BTU. This conversion is the only bridge between the two kinds of spec sheet, and it sits underneath most bad sizing decisions.

Work one through. A small 5,000 BTU/h window air conditioner built to the 2000–2014 federal minimum draws about 500W while the compressor runs, and a newer one less — not the 1,465W the raw conversion suggests, for the reason in the next section. Allow 85% of rated capacity after inverter losses, assume the compressor runs continuously, and you get:

• 1,000Wh power station: about 1.7 hours
• 2,000Wh: about 3.4 hours
• 3,000Wh: about 5 hours
• 5,000Wh: about 8.5 hours

In a well-insulated room the compressor cycles rather than running flat out, and those figures stretch by half again. In a metal van in August they do not. Either way the honest conclusion is that a portable power station is not an overnight air conditioning solution. Eight hours of that same small unit needs roughly 4,700Wh of battery, and replacing the 4,000Wh you drew takes something like 1,200W of solar the following day in good summer sun. If cooling through the night is the goal, a fixed home backup battery is the honest answer, not a box with a handle on it.

Once you have a real wattage the rest is ordinary sizing. Our guide to what size power station you need turns a load list into a capacity figure, the solar sizing walkthrough handles the panel side, and the portable power station listing lets you sort by continuous output and watt-hours — the two numbers this conversion exists to produce.

Air conditioners: BTU is the output, not the input

A refrigeration cycle does not manufacture cold, it relocates heat, so it moves far more thermal energy than it consumes as electricity. The ratio is the energy efficiency ratio — EER, or CEER on current US labels — which is BTU/h of cooling divided by watts of input. Federal minimums for a cooling-only window unit under 20,000 BTU/h are EER 9.7–9.8 for units made October 2000 to May 2014, CEER 10.7–11.0 from June 2014, and CEER 13.1–16.0 from May 26, 2026, depending on size (10 CFR 430.32(b)). So the working formula for a cooling appliance is input watts = BTU per hour ÷ EER, not the 0.293 constant.

Certified models sit well above the 2014 floor. On ENERGY STAR’s certified list (October 2026), cooling-only window units of 5,000–12,000 BTU/h are rated CEER 13.1–17.0, over four in five at 15.0–16.0, and every model on the list is variable-speed. DOE rates a variable-speed unit from its full-speed efficiency at 95°F outdoors, then raises the figure for its part-load savings (10 CFR 430, appendix F). So BTU ÷ CEER suits an energy estimate over a season, which is how ENERGY STAR’s yearly kWh figures are worked out, but flat out on a hot afternoon the unit draws more: size the inverter from the watts or amps on its rating label.

Two further things decide whether a battery will actually run one of these. First, a fixed-speed compressor surges to roughly two to three times its running wattage for a fraction of a second at startup, so a 1,000W air conditioner wants an inverter rated near 2,000W continuous to start reliably. Variable-speed inverter compressors ramp up instead and barely surge, which is why they are worth hunting for if you are running off a battery.

Second, the circuit sets a ceiling the appliance cannot argue with. A 15A circuit at 120V is 1,800W on paper, but the 80% rule limits continuous loads to 1,440W, and a 14,000 BTU/h unit made between October 2000 and May 2014, at its 9.7 minimum EER, works out to about 1,440W and sits right on that line. To get the current figure for any load, run it through the watts to amps calculator. Heat pumps bend the conversion the same way, delivering two to four watts of heat for every watt of electricity depending on the outdoor temperature. Their electrical draw is BTU per hour × 0.293 ÷ the coefficient of performance (COP), so a 24,000 BTU/h heat pump at a COP of 3 draws about 2,345W.

Heaters, where the conversion is exact

Electric resistance heating is the one case where the main table is literally true. A resistance element turns essentially all of its input electricity into heat, so its BTU output and its electrical draw are the same energy in two different units. A 1,500W space heater is 5,118 BTU/h, and its 750W low setting is 2,559 BTU/h. Nothing is lost and nothing is multiplied.

That also explains why every plug-in heater on the shelf is rated 1,500W: it is the largest single load that fits sensibly on a standard 15A circuit.

Plug-in 120V electric heaters top out at 1,500W, which is 5,118 BTU/h, so a plug-in electric heater that claims much more than about 5,100 BTU/h is overstating. A power station’s AC outlets are built to the same standard, so the ceiling carries across: a unit rated for 2,000W of output will run a 1,500W heater. How long it runs depends on its watt-hours, not its watts, at about 1.1 hours per 2,000Wh of capacity (2,000 × 0.85 ÷ 1,500).

The fuel that spares the battery brings its own hazard. EPA lists unvented kerosene and gas space heaters as indoor sources of carbon monoxide, nitrogen dioxide and particles, and CPSC warns never to use a fuel-burning portable heater or lantern while sleeping in a tent, camper or vehicle. In an RV or van, use a heater built and vented for the vehicle, installed to the maker’s manual; Dometic’s RV furnace manual, for one, requires all combustion air to come from outside and the exhaust to vent outdoors. CPSC also advises battery-operated or battery-backup CO alarms on every level of a home and outside sleeping areas.

For the trip in the other direction, use the watts to BTU calculator. To turn a running wattage into a daily energy figure and a running cost, the watts to kWh calculator takes it from there.

Frequently asked questions