Power Converter

Your generator is rated in kilowatts, the appliance you want to run lists wattage on its plate, and the AC unit you're sizing comes spec'd in BTU per hour because the HVAC industry decided to stay loyal to British thermal units forever. Reconciling those units while you're standing in the garage with your phone isn't anyone's idea of a good time.

The conversion factors here are exact: 1 horsepower is 745.7 watts (mechanical), 1 BTU/hr is 0.293 watts, and a kilowatt is a thousand watts no matter how you slice it. The mechanical horsepower figure is what most American spec sheets use; metric horsepower (PS, CV) is slightly different at 735.5 watts, which matters more often than you'd think when you're cross-shopping European tools.

The SI unit for power is the watt, defined as one joule per second. Everything else converts back to watts. Mechanical horsepower originated when James Watt wanted to sell steam engines and needed a unit pony farmers could relate to — he picked the average sustained output of a draft horse, which became 33,000 foot-pounds per minute, which works out to 745.7 watts. BTU per hour is the heat-flow unit; one BTU is the energy needed to raise one pound of water by one degree Fahrenheit, and dividing by 3,600 seconds per hour gives you the watt equivalent.

A worked example: you're sizing a backup generator for a 12,000 BTU/hr window AC unit and a 1,500-watt space heater. Convert the AC: 12,000 × 0.293 = 3,516 watts. Add the heater: 3,516 + 1,500 = 5,016 watts. That's your minimum continuous draw. Multiply by 1.25 to handle startup surge on the AC compressor (roughly 6,250 watts) and you need a generator rated for at least that on its surge spec. A 5 kW continuous / 6.5 kW surge unit barely covers it; a 7 kW unit gives you headroom and can run a fridge at the same time.

Where the math gets imprecise: AC power has reactive components that watts alone don't capture. A motor with 5 kW of real power often draws several extra kVA of apparent power because the inductive load shifts current out of phase with voltage. Generators care about apparent power (kVA) for sizing because that's what stresses their windings, but the watt rating on the device tells you real-power consumption. For pure resistive loads like heaters and incandescent lamps, watts and kVA are equal. For motors, transformers, and modern switching power supplies, the gap can be 20-30% — read both numbers if both are listed.