Reading Your Energy Bills: How to Compare kWh, BTU, Therms and Calories
By the Super Simple Digital Tools Team · Updated June 2026
Energy hides behind a confusing wardrobe of units. Your electric bill talks in kilowatt-hours, your gas bill in therms or cubic feet, your air conditioner's box in BTU, the kettle's spec sheet in watts, and your breakfast cereal in either Calories or kilojoules depending on the country. They all measure the same thing, energy, but the only way to compare them honestly is to convert them into one shared unit first. The joule is the natural choice because it is the international standard and every other unit is defined from it.
Start with the relationships worth memorizing. One kilowatt-hour is 3.6 million joules, or about 3,412 BTU, or roughly 860 food Calories. One BTU is about 1,055 joules. One food Calorie (the capital-C kind on labels) is 4,184 joules, while the lowercase calorie chemists use is just 4.184 joules. A therm, the natural gas billing block, is 100,000 BTU, which works out to about 29.3 kWh. With these anchors you can sanity-check almost any energy claim you meet in daily life.
Here is where converting changes a decision. Suppose a space heater is rated 1,500 watts and runs two hours, using 3 kWh. A gas furnace produces the same warmth as roughly 10,200 BTU, since 3 kWh times 3,412 is about 10,236 BTU. If your electricity costs more per kWh than gas costs per equivalent BTU, the gas heat is cheaper for the same comfort, but you can only see that after both are expressed in matching terms. The same logic lets you compare a power-bank's watt-hour rating against a battery quoted in milliamp-hours and volts.
Food labels are the other place this matters. A label reading 2,000 kJ is the same as about 478 Calories, found by dividing 2,000 by 4.184. North American labels use Calorie, most of Europe prints both kJ and kcal, and the gap between a calorie and a Calorie trips up countless people because the names differ by only a capital letter yet by a factor of a thousand in value. When in doubt, assume any food figure called a calorie is really a kilocalorie.
The mechanics inside any converter are simple: turn the input into joules, then turn joules into the unit you want. Because each unit has one fixed factor, the calculation is exact and fully reversible, so converting kWh to BTU and back lands on your starting number. The only fuzziness comes from the BTU and calorie having a few near-identical official definitions; for paying bills, cooking, and homework the standard factors are more than accurate enough.
- To compare electric and gas heating, convert both to BTU (multiply kWh by 3,412) before looking at price per unit.
- Watch the capital letter: a food Calorie equals 1,000 small calories, so never mix lab calories with nutrition Calories.
- A quick gas-bill check: multiply your therms by 29.3 to see the equivalent in kWh and judge it against your electric rate.
- For European food labels, divide kilojoules by 4.184 to get the Calories most North American readers expect.