What is kWh to Joules Energy Converter?
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Kilowatt-Hours to Joules Converter
kWh to Joules: 1 kWh = 3,600,000 J = 3.6 MJ. Used for electricity bills, battery capacity.
About this converter
The kilowatt-hour (kWh) is a unit of energy equal to exactly 3,600,000 joules. It is the standard billing unit for electricity worldwide. The joule (J) is the SI coherent unit of energy: 1 joule equals one newton-meter, or one watt-second.
How it works
The kilowatt-hour is power multiplied by time. Since the watt is defined as one joule per second (BIPM SI Brochure, 9th ed., Table 4), the conversion is exact and dimensional:
1 kWh = 1,000 W x 3,600 s
= 3,600,000 W*s
= 3,600,000 J (since 1 W*s = 1 J)
= 3.6 MJ
= 3.6 x 10^6 J
1 J = 2.7778 x 10^-7 kWh
The factor 3,600,000 = 1,000 (kilo) x 60 x 60 (seconds per hour). NIST SP 811 Appendix B.8 lists 3.6 x 106 J as the exact conversion.
Worked example
A homeowner wants to compare the energy stored in a 10 kWh home battery to the chemical energy in a tank of gasoline.
- Battery energy: 10 kWh x 3,600,000 J/kWh = 36,000,000 J = 36 MJ.
- Gasoline energy density: roughly 32 MJ per litre (lower heating value).
- Equivalent gasoline: 36 / 32 = 1.125 litres = about 0.30 US gallons.
- But an internal combustion car wastes ~75 percent as heat, so usable wheel energy from that gasoline is ~8 MJ. Electric motors are 85 to 90 percent efficient: usable wheel energy from the battery is ~32 MJ.
Reference table
| kWh | Joules | Real-world equivalent |
|---|---|---|
| 0.001 | 3,600 J | 1 W LED bulb for 1 hour |
| 0.1 | 360,000 J | Microwave on for 4 minutes |
| 1 | 3,600,000 J | 1,000 W heater for 1 hour |
| 10 | 36 MJ | Home backup battery (Powerwall) |
| 30 | 108 MJ | Typical US household daily use |
| 60 | 216 MJ | Tesla Model 3 standard battery |
| 270 | 972 MJ | UK monthly household use |
| 893 | 3.2 GJ | US monthly household use (EIA avg) |
Common pitfalls
- kWh is energy, not power. kW is the rate of energy use (power); kWh is total energy delivered. A "10 kW solar array" describes peak instantaneous output; that array might produce 12,000 kWh per year. Bills are energy; nameplate ratings are power.
- "kW per hour" is almost always wrong. If someone writes "my fridge uses 50 kW per hour", they probably mean 50 W (continuous) or 50 kWh per month. kW already contains time; dividing by hour again creates a unit (kW/h) that means rate-of-change of power.
- Battery capacity in Ah is not energy. A 100 Ah battery does NOT store 100 kWh. Energy in Wh = Ah x voltage. A 100 Ah, 12 V lead-acid stores 1.2 kWh. Always multiply by nominal voltage.
- kWh thermal versus kWh electric. Heat pumps and boilers are rated in kWh of heat output, but consume kWh of electricity at a different rate (COP > 1 for heat pumps, ~0.95 for resistive heaters). Solar thermal collectors are usually rated in kWh thermal per square metre per day.
- Confusing kcal and kWh in nutrition. 1 food calorie (kcal) = 4,184 J. So a 2,000 kcal/day diet equals 2.32 kWh of chemical energy intake per day. Resting human power consumption averages ~80 W ~= 2 kWh/day.
Related tools and references
Frequently asked questions
Why is 1 kWh exactly 3,600,000 joules?
A kilowatt-hour is power times time: 1 kW x 1 h = 1,000 W x 3,600 s = 3,600,000 W*s. Because 1 W*s = 1 J (the watt is defined as one joule per second), the equality is exact, not an approximation.
Is kWh a unit of energy or a unit of power?
kWh is energy (joules), not power. Kilowatt (kW) is power (energy per unit time). A 1,500 W kettle running for 4 minutes uses 0.1 kWh of energy. Utility bills charge by energy delivered (kWh), not by the rate at which it was used.
How big is a typical EV battery in joules?
A 60 kWh Tesla Model 3 battery stores 60 x 3.6 = 216 MJ. An 85 kWh long-range pack holds 306 MJ. By comparison, 1 US gallon of gasoline contains roughly 121 MJ of chemical energy, but combustion engines waste 60 to 75 percent as heat.
How do kWh, calories, BTUs, and joules relate?
All are units of energy and convert by fixed factors: 1 kWh = 3,600,000 J = 860.4 kcal (thermochemical) = 3,412 BTU. A 2,000 kcal daily diet is roughly 8.37 MJ or 2.32 kWh of food energy intake.
Reading kWh on your electricity bill
Electricity suppliers bill in kilowatt-hours because it captures total energy delivered, not the instantaneous rate. Your meter records the running total of kWh consumed; the bill multiplies the difference between two readings by a per-kWh tariff. Converting to joules is rarely needed for billing, but it is the right move whenever you compare household electricity against battery capacity, fuel energy, or food calories, which are all quoted in joules or related SI units.
| Appliance | Power | Run time | Energy used |
|---|---|---|---|
| LED bulb | 10 W | 10 hours | 0.1 kWh (360,000 J) |
| Refrigerator | 150 W average | 24 hours | 3.6 kWh (12.96 MJ) |
| Electric kettle | 2,000 W | 3 minutes | 0.1 kWh (360,000 J) |
| Tumble dryer | 3,000 W | 1 hour | 3 kWh (10.8 MJ) |
To estimate any appliance's energy, multiply its power in kilowatts by the hours it runs: a 2 kW heater for 5 hours uses 10 kWh, which is 36 megajoules. That is the same arithmetic the converter above performs, just expressed in the unit your bill uses.
Sources
- BIPM (2019) The International System of Units (SI), 9th edition, Table 4 (derived units).
- NIST Special Publication 811 (2008) Guide for the Use of the International System of Units, App. B.8.
- US EIA 2024 Residential Energy Consumption Survey, average US household electricity use.