Peek at the label on your microwave, your space heater, or that old window air conditioner, and you will see a wattage rating — 1,200 watts, 1,500 watts — but your electric bill never talks about watts. The utility company bills you in kilowatt-hours, which measures energy instead of power, and the gap between those two ideas trips up almost everyone. A watt-hour is simply one watt of power running for one hour, and a kilowatt-hour is 1,000 of them. This converter starts with the number you type into the Watt-hours (Wh) box and instantly shows the same energy as Kilowatt-hours (kWh), Joules (J), Megajoules (MJ), and BTUs. Type 100 and you will see 0.1 kWh, 360,000 joules, 0.36 megajoules, and about 341.2 BTUs. Those extra units matter more than they look: joules and megajoules are the scientist's units, and BTUs are how American furnaces, water heaters, and air conditioners are rated, so converting lets you compare a space heater's appetite to your gas furnace's output on equal terms. Once you see that a 1,500-watt heater burns 1.5 kWh every hour, estimating its cost on your bill becomes simple arithmetic instead of guesswork.

With the pre-filled defaults — Watt-hours (Wh) 100 — the Watt Hour Converter returns Kilowatt-hours (kWh) 0.1000, Joules (J) 360,000.00, Megajoules (MJ) 0.360000, BTUs 341.2141. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.