Why does food use Calories with a capital C?

The capital C convention distinguishes kilocalories (1 kcal = 1 food Calorie) from the small calorie (1 cal). Food labels always mean kilocalories — 100 Calories = 100 kcal = 418,400 joules. The notation was introduced to avoid writing 'kilocalories' on consumer packaging, but the capital C is often lost in practice, causing confusion. The EU requires both kcal and kJ on nutrition labels to avoid ambiguity.

What is the difference between energy and power?

Energy is the total amount of work done or heat transferred, measured in joules or kWh. Power is the rate at which energy is used or transferred, measured in watts (1 W = 1 J/s) or kilowatts. A 2 kW electric heater running for 3 hours consumes 6 kWh of energy. An 800 W microwave running for 5 minutes consumes 800 W × 300 s = 240,000 J = 0.067 kWh.

Why are air conditioners rated in BTU?

The US HVAC industry standardized on BTU/hr before SI units were widely adopted, and the unit has remained entrenched in American building codes, contractor training, and equipment catalogs. International markets use watts or kilowatts for the same equipment. 1 ton of cooling = 12,000 BTU/hr = 3.517 kW. When comparing US and international air conditioner specs, divide BTU/hr by 3,412 to get kW, or multiply kW by 3,412 to get BTU/hr.

Energy Units Explained: Joules, kWh, BTU, and Calories

Energy is one of the most unit-fragmented physical quantities. Physicists use joules, electricity bills show kilowatt-hours, HVAC systems are rated in BTU/hr, and food labels use Calories (kilocalories). Each unit evolved in a specific industry context and persists there. This guide explains what each unit measures, where it's used, and the key conversion bridges between them.

Published March 20, 2026

Key takeaways

The joule (J) is the SI unit of energy. 1 J = 1 kg·m²/s² = the energy to move 1 newton through 1 metre.
1 kilowatt-hour (kWh) = 3,600,000 J = 3.6 MJ. Used for electricity billing worldwide.
1 BTU (British Thermal Unit) ≈ 1,055 J. The standard unit for heating and cooling capacity in the US.
1 food Calorie (kcal) = 1,000 physics calories = 4,184 J. Food labels always use kilocalories.
1 kWh ≈ 860 kcal ≈ 3,412 BTU. Knowing these bridges lets you compare energy across domains.

The joule: SI foundation

The joule (J) is the SI unit of energy, named after physicist James Prescott Joule. By definition, 1 J = 1 N·m = 1 kg·m²/s². In practical terms: it takes about 1 joule to lift a small apple (102 g) by 1 meter.

Some useful joule reference points:

A 100 W incandescent bulb consumes 100 J every second (100 J/s = 100 W)
Climbing a flight of stairs (~5 m, ~70 kg person): roughly 3,430 J ≈ 3.4 kJ of mechanical work
A typical smartphone battery: ~15,000–50,000 J (15–50 kJ)
Lightning bolt: ~1 billion joules (1 GJ) of total energy in a very short time

Joules are used in physics equations, engineering thermodynamics, battery specifications, and any scientific context where energy calculations need to be consistent with SI base units.

Kilowatt-hour: electricity's unit

A kilowatt-hour (kWh) is the energy consumed by a 1-kilowatt device running for 1 hour. In joules: 1 kWh = 1,000 W × 3,600 s = 3,600,000 J = 3.6 MJ.

The kWh is the unit on electricity bills worldwide because it scales conveniently for household consumption — joules would give unwieldy numbers (an average US home uses about 900 kWh/month = 3.24 × 10¹² J).

Practical kWh references:

Boiling a kettle of water (1 litre): ~0.1 kWh
Average US household: ~900 kWh/month (about 30 kWh/day)
Electric vehicle (EV) battery: 40–100 kWh typical range; a 75 kWh Tesla Model 3 battery stores enough energy for ~350 miles
Solar panel (400 W residential): produces 1–2 kWh per day depending on sunlight hours
Charging an EV from empty: 40–100 kWh, equivalent to $4–$15 at typical US electricity rates (~$0.15/kWh)

Electricity prices are quoted per kWh. Energy efficiency standards (EU appliance labels, US Energy Star) use kWh/year as the baseline metric.

BTU: heating and cooling

Formula

The British Thermal Unit (BTU) is defined as the energy required to raise the temperature of 1 pound of liquid water by 1 degree Fahrenheit at standard conditions. The International Table BTU = 1,055.06 J.

Key BTU conversions:
1 BTU = 1,055.06 J
1 kWh = 3,412.14 BTU
1,000 BTU = 0.2931 kWh

HVAC (heating, ventilation, air conditioning) equipment in the US is rated in BTU/hr (sometimes written just 'BTU' colloquially). A 'ton of cooling' = 12,000 BTU/hr = 3.517 kW. Air conditioners typically range from 5,000 BTU/hr (small window unit) to 60,000+ BTU/hr (large central system).

Rule of thumb for cooling: approximately 20 BTU/hr per square foot of living space (more in hot climates or poorly insulated homes).

Heating: a gas furnace might be rated at 80,000–120,000 BTU/hr (input) with 80–95% efficiency. A water heater: 30,000–50,000 BTU/hr.

Natural gas is still measured and sold in BTU or therms (1 therm = 100,000 BTU) in the US. International markets increasingly use joules or kilowatt-hours.

Calorie and kilocalorie: food energy

Note

The calorie notation is a source of persistent confusion. There are two different calories:

Small calorie (cal): the energy to raise 1 gram of water by 1°C. 1 cal = 4.184 J exactly.

Kilocalorie (kcal): 1,000 small calories = 4,184 J. This is the food Calorie.

Food labels in most countries list energy in kcal (often written 'Cal' with a capital C or simply 'Calories' in the US). When a package says '200 Calories,' it means 200 kcal = 200,000 cal = 836,800 J.

Energy bridge to other units: 1 kWh ≈ 860 kcal — roughly the energy in 4 chocolate bars 2,000 kcal/day (typical adult recommendation) ≈ 2.33 kWh ≈ 7,950 BTU

In sports and exercise science, 'burning 500 Calories' means expending 500 kcal ≈ 0.58 kWh of metabolic energy — enough to power a 60 W bulb for about 9.7 hours.

Which unit to use when

Choosing the right energy unit is mainly about matching the convention in your field:

Physics and engineering equations: always joules (or multiples: kJ, MJ, GJ). Using other units in equations requires conversion factors that introduce errors.

Electricity billing and EV range: kilowatt-hours (kWh). This is the universal billing unit and matches the way electrical energy is measured.

HVAC and heating appliances (US): BTU or BTU/hr for equipment ratings; therms for gas consumption. Outside the US, kilowatts and kWh are standard.

Nutrition and food: kcal (labelled 'Calories' in the US). The EU requires both kcal and kJ on nutrition labels.

Fuels and combustion: MJ or GJ for individual items; PJ or EJ for national/global energy statistics. Petroleum uses 'barrel of oil equivalent' (boe ≈ 6.1 GJ) as a cross-fuel comparison unit.

Explosives: TNT equivalent (1 tonne TNT = 4.184 GJ).