Energy units vs power — why BTU/hour and watt are different
Watt is a power unit, watt-hour is an energy unit. The h matters.
One of the most common conversion traps: confusing power (rate of energy use) with energy (total work done). Watt (W) is power. Watt-hour (Wh) is energy.
The relation
\text{energy} = \text{power} \times \text{time}
A 100 W light bulb running for 10 hours uses 1,000 Wh (or 1 kWh) of energy. A 1,000 W microwave running for 1 minute uses about 17 Wh.
Why BTU/hour exists
The BTU was defined first (British thermal unit, ~1,055 J). When HVAC engineers needed a power unit, they took BTU/hour instead of converting everything to watts. The result: two parallel worlds.
- 1 watt ≈ 3.412 BTU/hour - 1 BTU/hour ≈ 0.293 watts
When you see "BTU" alone, it's usually energy. When you see "BTU/hour", it's power (a rate of use, useful for sizing an air conditioner).
What to use when
- Home energy bills: kWh (kilowatt-hour). - Food labels: kcal (kilocalorie, sometimes written as "Calorie" with a capital C). - HVAC sizing: BTU/hour for cooling capacity; tons of refrigeration (12,000 BTU/hour ≈ 3.5 kW). - Physics / chemistry: joules (J) or kilojoules (kJ). - Engines, motor torque: horsepower (1 hp ≈ 746 W) or foot-pound.
The site lists energy units only (joule, calorie, kWh, BTU, etc.) — power units (watt, horsepower) are a separate family.