Energy Converter

Energy is the same physical quantity whether it's in a meal, a battery, a gust of wind, or a fuel tank — but it shows up in at least six different units depending on the industry. A kilowatt-hour (KWh) on your electric bill, a Calorie on a nutrition label, a BTU on an air-conditioner spec, a joule in a physics textbook, and a kilowatt-hour on an EV battery are all measuring the same thing. Converting between them makes 'a Big Mac' and 'a hair dryer' comparable.

The joule (J) is the SI unit — one newton-meter of work. It's tiny: lifting an apple one meter takes about 1 J. Daily human food energy is ~2,000,000 J per meal, which is inconvenient, so nutrition uses kilocalories (kcal). One 'nutritional Calorie' (capital C) equals one kcal equals 1,000 'small' calories. A 2,000 'Calorie' diet is 2,000 kcal = 8.4 million joules.

The BTU (British thermal unit) is the energy to raise 1 lb of water by 1°F, about 1,055 J. Used in US HVAC specs, ice machines, and stove burners. A '24,000 BTU/hr' air conditioner moves 7 kW of heat. The 'ton' of HVAC refrigeration is 12,000 BTU/hr — originally the cooling power to melt one ton of ice in 24 hours.

Calories (food) to joules: multiply by 4,184. Kilowatt-hours to joules: multiply by 3,600,000. BTU to joules: multiply by 1,055. kWh to kcal: multiply by 860. A 2,000-kcal daily diet is 2.32 kWh of chemical energy — about the same as running a hairdryer for 2 hours.

For electricity bills: US average residential rate is 16-18¢ per kWh. A 100-watt bulb for 10 hours is 1 kWh = 17¢. A 1,500 W space heater running 8 hours is 12 kWh = $2.04. A dryer cycle (3 kWh) costs about 50¢.

• Joules
• Kilojoules
• Calories (small)
• Kilocalories (food)
• Watt-hours
• Kilowatt-hours
• BTU
• Foot-pounds

• Calorie confusion. Nutrition 'Calorie' (capital C) = 1 kcal = 1,000 'small' calories (lowercase c). Scientific 'calorie' is the small one. Food labels only ever mean the big one. Never mix.
• Watts vs kilowatt-hours. Watts is power (rate); kWh is energy (quantity). A 100 W bulb draws power; running it for 10 hours uses 1 kWh of energy. 'My solar panel generates 400 W' is a power rating; 'my house uses 30 kWh/day' is energy.
• BTU vs BTU/hour. Heater and AC specs are usually BTU/hr (rate of heat). 'A 24,000 BTU AC' means 24,000 BTU per hour. Miss the /hour and you'll dramatically underestimate.
• Energy density of batteries. EV batteries quote kWh; laptops quote watt-hours (Wh); phones quote milliamp-hours at a voltage. A 100 Wh laptop battery is 100 Wh = 0.1 kWh. A 5,000 mAh phone battery at 3.7 V is 18.5 Wh.
• Gasoline energy. One US gallon of gasoline contains 33.7 kWh of chemical energy. An EV that gets 3 mi/kWh gets the equivalent of 100 MPG (gasoline-equivalent) from that energy.

• 1 Big Mac: 550 kcal = 2.3 MJ = 0.64 kWh.
• Daily adult food intake: 2,000 kcal = 8.4 MJ = 2.3 kWh.
• AA alkaline battery: ~4 Wh = 3,400 J.
• iPhone 15 battery: 17.3 Wh (3,349 mAh at ~5.17 V nominal).
• MacBook Air M3 battery: 52.6 Wh.
• Tesla Model 3 Long Range battery: 75 kWh.
• 1 gallon of gasoline: 33.7 kWh = 120 MJ = 114,000 BTU.
• US household daily electricity: 30 kWh average.
• Hiroshima bomb (Little Boy): 15 kilotons TNT = 63 TJ = 17.5 GWh.
• 1 kilowatt-hour: enough to run a 1000-W toaster for 1 hour, or charge a Tesla Model 3 for ~3 miles of range.

• Compare appliances by kWh/year, not watts. A 1,500 W heater used 2 hours/day costs much less than a 100 W server running 24/7.
• For fitness tracking, calories burned on cardio machines are rough. Actual metabolic measurement uses indirect calorimetry (oxygen consumption) — gym treadmill numbers are typically 20% high.
• EV range math: kWh in battery ÷ kWh/mile = range in miles. A 75 kWh Tesla Model 3 using 0.26 kWh/mi gets 288 miles of range (matches EPA).

Related: Power Converter · Specific Heat Converter · Electric Charge Converter.