BTU to Watts Calculator

How to use the BTU to watts calculator

Divide by 3.41214. That is the number of BTU per hour in one watt, and it is the whole conversion. The reason this needs its own page rather than sitting alongside the kilowatt-hour conversion is that the two are not the same calculation, even though the digits look identical.

A BTU is a unit of energy and a watt is a unit of power. They are not comparable. What is comparable is BTU per hour against watts, because both are rates: energy divided by time. One kilowatt-hour is 3,412.14 BTU, and one kilowatt is 3,412.14 BTU per hour. The hour cancels on both sides, which is why the number repeats and why the units get mixed up.

The most important thing this figure is not is a power consumption. An air conditioner rated at 12,000 BTU/h moves that much heat out of a room; it does not draw 3,517 watts to do it. A reasonably efficient unit might draw around a thousand watts to move three and a half kilowatts of heat, because it is pumping heat rather than creating it.

The tons figure is worth understanding because HVAC equipment is priced and specified in it. One ton is 12,000 BTU per hour, and the unit comes from the rate at which a ton of ice absorbs heat as it melts over a day. It is an unusual survival, but it is what a contractor will quote in.

The mistake this page exists to prevent

There are two BTU conversions and they differ by a factor of a thousand. One kilowatt-hour is 3,412.14 BTU. One watt is 3.41214 BTU per hour. Using the first factor where the second belongs makes a domestic appliance look like industrial plant, and the digits are similar enough that the error survives a glance.

The reliable check is to say the units out loud. If both sides of your conversion end in "per hour" or neither does, you want 3.41214. If one side is a quantity of energy and the other is a rate, something has already gone wrong upstream.

Where the wattage figure is actually useful

The converted wattage is a comparison figure, not a consumption one. It lets you put a BTU-rated system beside an electric heater, a heat pump and a boiler on one scale, which is otherwise impossible because each trade quotes in its own unit.

For running cost you need the efficiency rating rather than the capacity: the SEER or EER figure for cooling, or the coefficient of performance for a heat pump. Those say how much electricity buys how much heat movement, and that ratio is where the money is, not in the BTU number on the box.

The formula, worked line by line

One division by 3.41214. The arithmetic is the trivial part; keeping the units straight is not, and that is what the notes below are for.

Everything else follows from holding those apart. Once the units on each side match, the conversion cannot go wrong by three orders of magnitude.

watts = (BTU per hour) / 3.41214
BTU per hour = watts x 3.41214
12,000 / 3.41214 = 3,517 W = 3.52 kW
tons = (BTU per hour) / 12,000
energy, NOT this: 1 kWh = 3,412.14 BTU

Where the ton comes from

It is the rate at which one short ton of ice absorbs heat while melting over twenty-four hours. The latent heat of fusion works out to 288,000 BTU across the day, which is 12,000 an hour. The unit predates mechanical refrigeration and survived because it described exactly what the machines replaced.

The four units on one scale

The habit worth keeping: whenever a BTU figure appears without "per hour" attached, work out from context whether it is a rate or a quantity before converting anything. Most equipment labels mean the rate.

Questions people ask