Power converter

What it's for

Power is the same quantity for an engine, an oven, an air conditioner and a bulb, but every trade writes it with its own unit: watts for electricity, horsepower for engines, BTU per hour for air conditioning, calories per hour in heating. Converting them lets you compare things that look different and aren't.

Horsepower, but which horsepower

There are two, and they're not the same: the continental CV is 735.5 watts, the anglosphere HP is 745.7. The difference is 1.4%, which seems little until you compare two spec sheets: a 100 CV engine is 98.6 HP, and a car «with 150 horsepower» in a British catalogue has 152 in continental ones. In practice the number to look at is always the kilowatt, the only one that's the same everywhere and the one that ends up on the registration document.

The BTU of air conditioners

Air conditioners are sold in «BTU», which are really BTU per hour: a BTU on its own is an energy, a BTU/h is a power. The conversion to remember is 1 kW ≈ 3412 BTU/h, so the famous 9000 BTU is about 2.6 kW and 12,000 is 3.5 kW.

Careful, though: that's the heat moved, not the energy used. A modern 9000 BTU unit draws around 800 watts from the socket, because it doesn't produce cold, it moves heat outside. It's why on the bill an air conditioner costs far less than an electric heater of the same «power».

Power and consumption are not the same thing

The watt is a power, how much is being used at every instant; the kilowatt-hour is an energy, how much has been used in total. A 2000 W oven on for half an hour uses 1 kWh, and that's what the bill charges for. Confusing the two is the most expensive mistake you can make when choosing an appliance.

Nearby tools

For an appliance's cost on the bill there is Electricity cost, and for energy units Energy converter. To see how many BTU a room actually needs the page is Air conditioner BTU.