Watts to BTU Calculator

To convert watts to BTU per hour, multiply by 3.412. A 1,000 watt load is 3,412 BTU per hour; a 1,500 watt space heater is 5,118 BTU per hour. That figure is heat, not cooling capacity — an air conditioner drawing 1,000 watts delivers far more BTU than 3,412.

The formula, and what each term means

Watts and BTU per hour are two units for the same thing: a rate of energy. One BTU is 1,055 joules, the energy that raises a pound of water by one degree Fahrenheit. A watt is one joule per second. Divide the 3,600 seconds in an hour by 1,055 and you get the constant:

BTU per hour = watts × 3.41214 and watts = BTU per hour ÷ 3.41214

• Watts is a rate — energy every second. It is what your inverter, your breaker and your solar panels are rated in.
• BTU per hour (BTU/h) is the same rate in the units the heating and cooling trade uses. Labels that print only “BTU” almost always mean BTU/h.
• BTU on its own is a quantity of energy rather than a rate, and it pairs with watt-hours, not watts. The constant is identical: 1 kWh is 3,412 BTU.
• One ton of cooling is defined as 12,000 BTU/h, which is 3,517 watts of thermal capacity.

Notice what is missing: no voltage and no power factor, which is what separates this from turning watts into amps. It works identically at 12V, 120V or 240V, on AC or DC, and the answer is exact.

Watts to BTU conversion table

No voltage assumption is needed, so these rows hold for any system. The example column is the kind of load that typically sits at each wattage, not a specification for any appliance.

The highlighted row is the one to carry around: 293 watts is 1,000 BTU/h. Everything else here is that ratio applied to something — multiply any kilowatt figure by 3,412 and you have BTU/h.

Why this conversion matters for portable power and solar

Batteries are sold in watt-hours, inverters in continuous watts and panels in watts. Heaters, air conditioners, RV furnaces and diesel heaters are sold in BTU. Anyone building an off-grid system stands with one foot in each catalogue, and this conversion is the bridge. It gets used in two directions, and they lead to very different conclusions.

• Heat you intend to make electrically. Convert the BTU/h you want and the wattage says immediately whether a battery can supply it. Usually the answer is no.
• Cooling for the heat your own gear produces. Convert the wattage of everything running inside a van or a small room and you have the load your air conditioning must remove first.

Take the first case. A 1,500W space heater is 5,118 BTU/h and it draws that continuously — there is no duty cycle to save you, because a heater only stops when the room is warm enough. Allowing a realistic 85% of rated capacity after inverter losses:

• A 1,000Wh power station: about 34 minutes
• A 2,000Wh power station: about 68 minutes
• A 3,000Wh power station: about 1 hour 40 minutes
• A 5,000Wh power station: about 2 hours 50 minutes

That is the whole argument against electric heat on a battery, in four lines. Even a 750W setting overnight is 6,000Wh for eight hours, and refilling that at four peak sun hours and a realistic 75% system yield takes about 2,000W of panels. Our solar sizing walkthrough covers the panel arithmetic, and the sizing guide the battery side.

The second case is the one people forget. Every watt your fridge, inverter, laptop and lighting consume inside an enclosed space comes back out as heat, and in a van or RV that heat is not free. To turn the same wattage into daily energy instead, use the watts to kWh calculator; the power station finder filters the catalogue by continuous output.

Electric heat, where the conversion is exactly true

Resistance heating is the one case where the table above is literally the appliance’s output. An element converts essentially all of its input electricity into heat, so a 1,500W heater really is 5,118 BTU/h and a 750W setting really is 2,559 BTU/h. There is no efficiency figure to look up, because there is nowhere else for the energy to go.

That symmetry also sets a hard ceiling on how much electric heat a household circuit can deliver.

The 80% figure is the continuous-load rule: a breaker is not meant to be held near its rating for hours, and a heater is the definition of a continuous load. Note where that leaves the standard 1,500W heater — 12.5 amps at 120V, above the 1,440W continuous limit of a 15A branch, which is why two of them on one circuit trips the breaker rather than warming the room.

Heat pumps break the symmetry the other way. Because they move heat instead of generating it, they deliver two to four times more BTU than they draw in watts, depending on outdoor temperature. Divide by the coefficient of performance, not by one. For the reverse trip, use the BTU to watts calculator.

Every watt you use also becomes heat

Apart from the light that escapes through a window, everything electrical you run in an enclosed space ends up as heat in that space. That is not an approximation; it is where the energy goes. So this conversion also answers a question nobody thinks to ask: how much cooling capacity is my own equipment spending?

The right-hand column is measured against a 5,000 BTU/h unit, the smallest common window or portable air conditioner size.

A modest van setup — fridge, laptop, lights, a fan, a router — sits in the 200 to 400W band with everything running, which is a quarter of a small air conditioner’s output spent before the sun has touched the roof. Two people add another 200W of body heat on top.

The same happens inside the power station. A 1,000W AC load drawn through an inverter running at 90% efficiency means the battery supplies about 1,111W, and the missing 111W leaves as heat — 379 BTU/h out of the vents. That is why the fans run under load, why a unit in direct sun derates, and why cupboard installations of portable power stations and home batteries need clear air rather than a tidy enclosure.

Frequently asked questions