Every energy unit leads back to the joule
Heat, food, electricity and motion look different, but they are all the same quantity, so each can be converted to the SI unit, the joule (J), and compared. One joule is the energy needed to push something 1 m with a force of 1 newton, a very small amount in everyday terms. That is why each field uses a unit of a convenient size: kilocalories or kilojoules for food, kilowatt-hours for electricity. However different the units look, once you know how many joules each one is, you can convert between them.
Calories and kilocalories
The calorie originally came from the heat needed to raise 1 g of water by 1 degree. Because that amount varies slightly with water temperature, the calorie is now defined in joules; the commonly used thermochemical calorie is exactly 4.184 J. Engineering sometimes uses the international steam table calorie of 4.1868 J, so the two differ slightly. The calorie on food packaging and exercise machines is in fact the kilocalorie, a thousand times larger. When a food is said to have 300 calories, it means 300 kilocalories, about 1,255 kilojoules. If a nutrition label shows both kcal and kJ, the kJ figure is about 4.2 times larger.
How food energy is calculated
Food energy is usually calculated by multiplying the weight of each nutrient by a set factor and adding them up. The widely used general factors are 4 kcal per gram for carbohydrate and protein, 9 kcal per gram for fat and 7 kcal per gram for alcohol. That is why fatty foods come out higher for the same weight. These are average approximations, though: some components such as dietary fibre are only partly absorbed, and the energy a body actually uses varies from person to person. Use labelled energy as a basis for comparison, and if you need to manage weight or a medical condition, consult a doctor or a nutrition professional.
Watts and watt-hours are different
The most common confusion in electricity is between watts (W) and watt-hours (Wh). A watt measures how fast energy is used, that is power, and 1 W is 1 J per second. A watt-hour is that rate multiplied by how long it runs, the amount of energy. Run a 1,000 W heater for an hour and it uses 1 kWh; run it for 30 minutes and it uses 0.5 kWh. An hour is 3,600 seconds, so 1 Wh is 3,600 J and 1 kWh is exactly 3,600,000 J, or 3.6 megajoules. Usage on an electricity bill is in kilowatt-hours, while the power rating printed on an appliance is in watts.
Food and electricity on one scale
Going through the joule lets you compare energy from different fields. Decimals are rounded, and calories use the thermochemical definition.
- 1 kWh = 3.6 MJ ≈ 860 kcal
- 1 kcal = 4.184 kJ ≈ 1.16 Wh
- 2,000 kcal ≈ 8.37 MJ ≈ 2.32 kWh
- A 1,000 W appliance for 1 hour = 1 kWh
Reading battery capacity
The mAh printed on phones and power banks is electric charge, so on its own it cannot compare energy. Energy requires multiplying by voltage, which means batteries at different voltages hold different amounts of energy even with the same mAh figure. Converting to watt-hours is simple.
- Wh = mAh × voltage (V) ÷ 1,000
- Example: a 3.7 V battery of 10,000 mAh = 37 Wh
- Charging involves losses, so not all of the rated capacity is transferred
- Airline carry-on limits are set in Wh, so check the airline's official guidance
Other units you will meet
Foreign sources often use other energy units. The British thermal unit (BTU), used for heating and cooling capacity, is about 1,055 J and is usually quoted per hour, so it works like a power rating. In the world of atoms and particles, the tiny electronvolt is used. A car's horsepower is not energy but power, a rate, putting it in the same family as the watt. When you meet an unfamiliar unit, first decide whether it measures energy (an amount) or power (a rate), then convert to joules or watts for easy comparison.
Common mistakes
Most energy unit errors come from mixing amount and rate, or dropping the kilo in front.
- Treating food calories as plain calories and ending up a factor of 1,000 out
- Treating power rating (W) and usage (kWh) as the same thing
- Comparing batteries by mAh alone, ignoring voltage
- Mixing the two calorie definitions (4.184 J and 4.1868 J) in precise calculations
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