Power and energy are different things, and the difference is the key to every calculator on this site: power (watts) is how fast electricity is being used right now, while energy (watt-hours) is how much has been used over time. Utilities bill for energy, so cost is always wattage × hours — never wattage alone.

A watt (W) is the base unit. A kilowatt (kW) is 1,000 watts, and a kilowatt-hour (kWh) is one kilowatt sustained for one hour; it is the billing unit because a typical US home uses ~10 million watt-hours a year — 10,000 kWh reads better.

A megawatt (MW) is 1,000 kilowatts — one million watts — and a megawatt-hour (MWh) is that power sustained for one hour. These are the working units of data center developers and utility-scale energy projects: an AI campus is negotiated in MW of grid capacity, a solar plant is nameplated in MW, and battery storage is bought and sold in MWh.

The BTU (British Thermal Unit) measures heat: 1 kWh equals 3,412 BTU. It's how an air conditioner's rating connects to its electrical draw, and how a data center's IT load becomes a cooling problem.

Horsepower is 745.7 W (mechanical) or 735.5 W (metric), thanks to James Watt, who invented the unit to sell steam engines against draft horses — rounded, one horsepower is 746 watts.

And the ton of cooling — 12,000 BTU per hour — is sized for everything from a home AC to a data center chiller plant. One ton of cooling typically requires about 1–1.2 kW of electricity.