Energy Guide: kWh, BTU, Joules & Unit Conversions
Energy Units Explained
| Unit | Symbol | System | Used For | Equivalent |
|---|---|---|---|---|
| Kilowatt-hour | kWh | SI/Metric | Electricity billing | 3,412 BTU |
| British Thermal Unit | BTU | Imperial | HVAC, natural gas | 1,055 J |
| Joule | J | SI | Scientific | 0.000278 Wh |
| Megajoule | MJ | SI | Gas billing (non-US) | 0.278 kWh |
| Therm | thm | Imperial | US natural gas | 100,000 BTU |
| Calorie | cal | Metric | Food energy | 4.184 J |
| Kilocalorie (food) | kcal | Metric | Food energy | 3.968 BTU |
| Horsepower-hour | HP·h | Imperial | Mechanical | 2,544 BTU |
| Foot-pound | ft·lb | Imperial | Mechanical | 1.356 J |
Quick Conversion Table
| From | To kWh | To BTU | To MJ |
|---|---|---|---|
| 1 kWh | 1 | 3,412 | 3.6 |
| 1 BTU | 0.000293 | 1 | 0.001055 |
| 1 MJ | 0.278 | 947.8 | 1 |
| 1 therm | 29.3 | 100,000 | 105.5 |
| 1 gallon gasoline | 33.7 | 114,500 | 121.3 |
| 1 gallon propane | 27.0 | 91,500 | 96.5 |
| 1 lb coal | 3.0 | 10,200 | 10.8 |
| 1 cu ft natural gas | 0.293 | 1,000 | 1.055 |
Power vs Energy
Power (watts) is the rate of energy use. Energy (watt-hours) is power × time.
- A 100W bulb uses 100W of power
- Running it for 10 hours uses 1,000 Wh = 1 kWh of energy
- A 1,500W heater for 1 hour = 1.5 kWh = 5,118 BTU
Common Energy Calculations
Electricity Cost
Energy (kWh) = Power (W) × Time (h) / 1,000
Cost = kWh × $/kWh rate
Natural Gas Cost
Energy (therms) = Gas volume (cu ft) × heating value / 100,000
Natural gas heating value: ~1,030 BTU/cu ft
100 cu ft = 1.03 therms = 103,000 BTU
HVAC Sizing
Cooling: 1 ton = 12,000 BTU/h = 3.517 kW
Heating: 1 therm = 100,000 BTU = 29.3 kWh equivalent
Energy Efficiency Metrics
| Equipment | Metric | Definition | Good Value |
|---|---|---|---|
| AC/Heat Pump | SEER | BTU cooling / Wh | 16–21 |
| Furnace | AFUE | BTU output / BTU input | 95–98% |
| Water Heater | UEF | Useful energy / total energy | 0.65–0.95 |
| Lighting | lm/W | Lumens per watt | 80–150 |
| Electric Motor | Efficiency % | Shaft power / electrical power | 90–96% |
Common Mistakes
- Confusing power and energy: kW is power (rate), kWh is energy (quantity). Like speed vs distance.
- Confusing BTU and BTU/h: BTU is energy. BTU/h is power. A 36,000 BTU/h AC removes 36,000 BTU per hour.
- Ignoring conversion factors: 1 therm = 29.3 kWh, not 100 kWh. Don't confuse the number with the unit.
Standards Reference
- NIST — National Institute of Standards and Technology (unit definitions)
- EIA — Energy Information Administration (energy data)
- DOE — Department of Energy (efficiency standards)
- ASHRAE — HVAC energy calculations
Frequently Asked Questions
How many BTU in a kWh?
1 kWh = 3,412 BTU. To convert kWh to BTU, multiply by 3,412. To convert BTU to kWh, divide by 3,412. Example: A 3,500W heater for 1 hour = 3.5 kWh = 11,942 BTU.
What's the difference between kW and kWh?
kW (kilowatt) is power — the rate of energy use at any moment. kWh (kilowatt-hour) is energy — total power used over time. A 1,500W (1.5 kW) heater running for 2 hours uses 3 kWh of energy. Think of kW as speed and kWh as distance.
How many kWh in a therm of natural gas?
1 therm = 29.3 kWh of energy equivalent. Natural gas at $1.20/therm = $1.20 / 29.3 = $0.041/kWh equivalent. Compare to electricity at $0.16/kWh — gas is about 4× cheaper per unit of energy for heating.
How do I convert between fuel types?
Use energy content: 1 therm gas = 29.3 kWh = 100,000 BTU. 1 gallon propane = 91,500 BTU = 26.8 kWh. 1 gallon gasoline = 114,500 BTU = 33.6 kWh. Compare costs per BTU or per kWh equivalent to find the cheapest fuel.
What is a BTU exactly?
A BTU (British Thermal Unit) is the energy needed to raise the temperature of one pound of water by one degree Fahrenheit. It's a small amount of energy — about equal to the heat from burning one kitchen match. HVAC systems use thousands of BTU per hour.