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BTU to Watts Calculator
Turn a heating or cooling BTU rating into the watts your battery, inverter and breaker actually have to supply.
BTU per hour to watts, for sizing a battery to an air conditioner or heater.
Watts = BTU/h ÷ 3.412142
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 air conditioner pulls roughly 835–1,250 watts from the socket, 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 memorising.
- 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.
The mistake that costs real money. The watts this conversion gives you are thermal watts — heat moved. On an air conditioner or a heat pump that is the output, not the input. Size a battery from it and you overstate the load by three to four times. It is the single most common reason people buy an inverter twice the size they needed.
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 mains 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.
| BTU per hour | Watts | Kilowatts | Amps at 120V, resistive |
|---|---|---|---|
| 1,000 BTU/h | 293.1 W | 0.293 kW | 2.4 A |
| 2,000 BTU/h | 586.1 W | 0.586 kW | 4.9 A |
| 3,000 BTU/h | 879.2 W | 0.879 kW | 7.3 A |
| 3,412 BTU/h | 1,000 W | 1.000 kW | 8.3 A |
| 4,000 BTU/h | 1,172 W | 1.172 kW | 9.8 A |
| 5,000 BTU/h | 1,465 W | 1.465 kW | 12.2 A |
| 6,000 BTU/h | 1,758 W | 1.758 kW | 14.7 A |
| 8,000 BTU/h | 2,345 W | 2.345 kW | 19.5 A |
| 10,000 BTU/h | 2,931 W | 2.931 kW | 24.4 A |
| 12,000 BTU/h | 3,517 W | 3.517 kW | 29.3 A |
| 14,000 BTU/h | 4,103 W | 4.103 kW | 34.2 A |
| 15,000 BTU/h | 4,396 W | 4.396 kW | 36.6 A |
| 18,000 BTU/h | 5,275 W | 5.275 kW | 44.0 A |
| 24,000 BTU/h | 7,034 W | 7.034 kW | 58.6 A |
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. Past about 5,000 BTU/h, no ordinary 120V socket can deliver that much heat resistively, because 6,000 BTU/h already asks for 14.7A on a 15A circuit.
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 catalogues, and it sits underneath most bad sizing decisions.
Work one through. A small 5,000 BTU/h window air conditioner draws around 500W while the compressor runs — 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. Window and portable units generally land between 8 and 12. So the working formula for a cooling appliance is input watts = BTU per hour ÷ EER, not the 0.293 constant.
| Cooling rating | Thermal equivalent | Running watts at EER 8–12 | Amps at 120V |
|---|---|---|---|
| 5,000 BTU/h | 1,465 W | 415–625 W | 3.5–5.2 A |
| 6,000 BTU/h | 1,758 W | 500–750 W | 4.2–6.3 A |
| 8,000 BTU/h | 2,345 W | 665–1,000 W | 5.6–8.3 A |
| 10,000 BTU/h | 2,931 W | 835–1,250 W | 7.0–10.4 A |
| 12,000 BTU/h | 3,517 W | 1,000–1,500 W | 8.3–12.5 A |
| 14,000 BTU/h | 4,103 W | 1,165–1,750 W | 9.7–14.6 A |
| 18,000 BTU/h | 5,275 W | 1,500–2,250 W | 12.5–18.8 A |
On the assumptions. The running-watts column is the cooling rating divided by an EER of 12 at the efficient end and 8 at the inefficient end, which is the band that covers the window and portable units people actually plug into a battery. Single-hose portable units sit at the bottom of it or below; inverter-driven mini splits sit above it and draw less again. If your own label prints a watts or amps figure, use that and ignore the estimate.
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 12,000 BTU/h unit at the inefficient end of the table 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 and further still, delivering two to four times more BTU than they consume in watts depending on the outdoor temperature: divide by the coefficient of performance, not by anything on this page.
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.
| Circuit at 120V | Continuous limit at 80% | Most heat it can deliver |
|---|---|---|
| 15A | 1,440 W | 4,913 BTU/h |
| 20A | 1,920 W | 6,551 BTU/h |
If a listing claims more BTU than that from a 120V outlet, the claim is wrong. A power station’s AC outlets are built to the same standard, so the ceiling carries across: a 2,000W unit will run a 1,500W heater, and will do so for well under an hour.
Combustion heaters are a different animal. A propane, diesel or kerosene heater rated at 9,000 BTU/h takes none of that from your battery — the fuel supplies the heat. Only the blower, igniter and controller are electrical, typically a few tens of watts. Converting the BTU rating tells you nothing at all about the electrical load, and doing it anyway is the fastest way to oversize an RV or van electrical system by two orders of magnitude.
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
How many watts is 12,000 BTU?
12,000 BTU per hour is 3,517 watts of thermal power. If that is an air conditioner’s cooling rating, it describes the heat the unit moves, not the electricity it consumes: expect roughly 1,000–1,500 watts of actual draw, or 8.3–12.5 amps at 120V. If it is an electric resistance heater, the full 3,517 watts is the real load.
How do I convert BTU to watts?
Multiply BTU per hour by 0.29307, or divide by 3.412. The constant comes from the definition of a BTU as 1,055 joules spread over an hour’s 3,600 seconds. No voltage, power factor or efficiency figure is involved. The easiest anchor to remember is that 3,412 BTU/h is exactly 1 kilowatt.
Does a 10,000 BTU air conditioner really use 2,931 watts?
No. 2,931 watts is the heat it moves, not the power it draws. Divide the cooling rating by the unit’s EER, usually somewhere between 8 and 12, and you get a realistic 835–1,250 watts. Sizing a battery or an inverter from the 2,931 figure costs about three times more than it needs to.
What size power station do I need for a 5,000 BTU air conditioner?
Around 500 watts of running draw, so look for at least 1,000W of continuous output to absorb the compressor surge. Capacity is the harder half: a 2,000Wh unit gives roughly three and a half hours of continuous cooling once inverter losses are counted, and running one overnight needs closer to 4,700Wh.
Is one ton of cooling the same as 12,000 BTU?
Yes. One ton of refrigeration is defined as 12,000 BTU per hour, which is 3,517 watts of thermal capacity. A two-ton system is 24,000 BTU/h, or 7,034 watts. The unit is a leftover from the ice trade and has nothing to do with the weight of the equipment.
How do I convert BTU to kilowatts?
Divide BTU per hour by 3,412. So 10,000 BTU/h is 2.93kW, 18,000 BTU/h is 5.28kW and 24,000 BTU/h is 7.03kW. Going back the other way, multiply kilowatts by 3,412. It is the same constant as the watts version with the decimal point moved three places.
Why do heaters and air conditioners convert differently?
Because one makes heat and the other moves it. A resistance heater turns electricity into heat at close to one-to-one, so its BTU rating and its wattage are the same energy described twice. An air conditioner pumps heat that already exists from one side of a wall to the other, which takes far less energy than creating it.
Next step. Once the BTU rating is a wattage, and the wattage is a daily watt-hour figure, the power station finder narrows the catalogue to the units that clear both numbers.