BTU Calculator

Definitional, not measured

A kilowatt-hour is exactly 3.6 megajoules. A BTU is defined as exactly 1055.05585262 joules. Dividing gives 3,412.14 BTU per kilowatt-hour, with no rounding anywhere in the chain.

A ton of refrigeration is exactly 12,000 BTU per hour. It comes from the heat needed to melt a short ton of ice over 24 hours — a nineteenth-century definition that outlived the ice trade.

Because these are definitions rather than measurements, they do not drift and do not need sourcing.

In practice

1 kWh is 3,412.14 BTU. A ton of cooling running for an hour is 12,000 BTU, which is 3.517 kWh of heat moved.

That last figure is worth holding on to: a one-ton air conditioner moves about 3.5 kW of heat, while drawing far less than that in electricity — because it moves heat rather than making it.

Power against energy, again

BTU is energy; BTU per hour is power. Appliance ratings frequently print BTU when they mean BTU per hour, and the ambiguity causes real confusion when comparing products.

A 12,000 BTU air conditioner means 12,000 BTU per hour — one ton. A 12,000 BTU of gas is an amount of fuel energy. Same number, different quantities.

When comparing appliances, check which one the rating is, then convert both sides to the same footing before doing anything else.

Where the figure deceives

Conversions say nothing about efficiency. A gas appliance rated at 100,000 BTU input delivers less than that as useful heat; the difference goes up the flue.

A heat pump goes the other way, delivering more heat than its electrical input because it is moving heat from outside. Coefficients of performance of three or four are ordinary, and that is not a violation of anything.

So an energy conversion is the first step of a fuel comparison, never the whole of one.

Acting on it

Convert to a common unit, then apply each appliance's efficiency or coefficient of performance, then compare cost per unit of delivered heat.

Check whether a rating is input or output before comparing two appliances — manufacturers are not consistent.