Energy Converter

Energy is the capacity to do work, and the beauty of the joule (J) is that it puts every form of energy on one scale: the calories in food, the kilowatt-hours on your power bill, the BTU on a furnace, all the same quantity in different clothing.

Key relationships:

• 1 kJ = 1,000 J
• 1 cal = 4.184 J
• 1 kcal = 4,184 J (a food Calorie)
• 1 Wh = 3,600 J
• 1 kWh = 3,600,000 J
• 1 BTU = 1,055.06 J
• 1 therm = 100,000 BTU = 105,506,000 J (how US natural gas is billed)

For scale:

• A food Calorie: 4,184 J
• A 100 W bulb for one hour: 100 Wh (360,000 J)
• A typical US household per day: about 30 kWh
• A US gallon of gasoline: about 120 MJ

That last one comes with a caveat that catches people out. A gallon of gasoline is quoted at roughly 120 MJ on the lower heating value, which is the energy you actually get out of an engine, and at about 131 MJ on the higher heating value, which counts the energy still locked in the water vapour the flame produces. Both figures are correct; they answer different questions. Anything to do with fuel economy uses the lower one. The US EPA’s 33.7 kWh per gallon, the number behind every MPGe rating, is 121 MJ, so it sits on the lower side too.

The calorie causes more confusion than any other energy unit. The “calorie” on a food label is really a kilocalorie, 1,000 of the small calories used in chemistry, which is why it’s sometimes written with a capital C. A 200 Calorie snack holds 200,000 chemistry calories, about 837 kJ.

Don’t mix energy up with power. Energy (joules, kWh) is the total amount; power (watts) is the rate of using it. A kilowatt-hour is energy, what you get from one kilowatt running for one hour. Your electric bill charges for energy in kWh, not for power, which is why leaving a small device on for days can cost more than a big appliance used briefly.