PRACTICAL GUIDE / REVIEWED
Understanding energy units
Energy measures the capacity to do work or transfer heat. The joule is the SI unit. Kilowatt-hours describe energy accumulated from power over time, calories and kilocalories describe heat or food energy, and BTU remains common in heating and cooling contexts.
Where these units are used
Electricity bills use kWh, batteries may use Wh, nutrition labels use kilocalories or kilojoules, heating equipment uses BTU, and physics uses joules or electronvolts depending on scale.
Rounding and precision
Billing and nutrition should follow the precision of the original label. Scientific work should preserve significant figures. Small electronvolt values converted to joules may be clearer in scientific notation.
Regional differences
Food labels may write 'Calories' with a capital C to mean kilocalories. Kilojoules are prominent on labels in many countries, while food Calories are common in the United States. BTU conventions can vary slightly; this site states its stored factor.
Common mistakes
A watt is power, not energy. Multiply power by time to obtain Wh or joules. Also distinguish the small calorie from the nutritional kilocalorie, which is 1,000 small calories.
USEFUL ANSWERS
Energy conversion FAQs
Is a food Calorie a kilocalorie?
Yes. In nutrition, one capital-C Calorie normally means one kilocalorie, or 4,184 joules.
How is kWh related to kW?
Energy equals power multiplied by time; using 1 kW for 1 hour consumes 1 kWh.
Why use electronvolts?
Electronvolts give convenient-sized numbers for atomic and particle-scale energies.