Calculate power, time, or energy from any two values with conversions for watts, kW, hp, seconds, minutes, hours, J, kJ, cal, and kWh.
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Energy Formula
The calculator uses the relationship between energy, power, and time. In base SI units, energy is measured in joules, power is measured in watts, and time is measured in seconds.
E = P*t
P = E/t
t = E/P
- E = energy
- P = power
- t = time
The calculator can solve for any one missing value when you enter the other two:
- Calculate energy: enter power and time. The calculator multiplies them using E = P*t.
- Calculate power: enter energy and time. The calculator divides energy by time using P = E/t.
- Calculate time: enter energy and power. The calculator divides energy by power using t = E/P.
Before calculating, values are converted to base units: watts, seconds, and joules. The final answer is then converted back to the unit you selected.
Energy, Power, and Time Unit Conversions
Use these conversions to understand how the calculator changes units before applying the formula.
| Quantity | Unit | Conversion to base unit |
|---|---|---|
| Power | 1 watt (W) | 1 W |
| Power | 1 kilowatt (kW) | 1,000 W |
| Power | 1 horsepower (hp) | 745.7 W |
| Time | 1 minute | 60 s |
| Time | 1 hour | 3,600 s |
| Energy | 1 kilojoule (kJ) | 1,000 J |
| Energy | 1 calorie (cal) | 4.184 J |
| Energy | 1 kilowatt-hour (kWh) | 3,600,000 J |
Common Power and Energy References
| Item or situation | Typical value | Meaning |
|---|---|---|
| Small LED bulb | 8 W to 12 W | Uses 8 to 12 joules each second |
| Microwave oven | 700 W to 1,200 W | Uses energy quickly over short cooking times |
| Space heater | 1.5 kW | Uses 1.5 kWh in one hour |
| 1 kWh | 3,600,000 J | Energy used by a 1 kW device running for 1 hour |
Example Problems
Example 1: Calculate energy
You run a 1.5 kW space heater for 2 hours. Find the energy in kWh.
E = P*t
Substitute the values:
E = 1.5*2 = 3
The energy used is 3 kWh.
Example 2: Calculate power
A device uses 7,200 J of energy in 60 seconds. Find the power in watts.
P = E/t
Substitute the values:
P = 7200/60 = 120
The power is 120 W.
FAQ
What is the difference between power and energy?
Energy is the total amount of work done or electricity used. Power is the rate at which energy is used. For example, a 100 W device uses 100 joules of energy every second.
Why does the calculator convert everything to joules, watts, and seconds?
The formula works cleanly in SI base units. Since 1 watt equals 1 joule per second, the calculator first converts power to watts, time to seconds, and energy to joules. After calculating, it converts the result to your selected unit.
How are kilowatt-hours related to watts?
A kilowatt-hour is a unit of energy, not power. It means one kilowatt of power used for one hour. Since 1 kW is 1,000 W and 1 hour is 3,600 seconds, 1 kWh equals 3,600,000 J.
