Calculate energy in kWh or joules from power and time, find watts from joules and seconds, and add an electricity rate to estimate the cost.
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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 = energy
- P = power
- t = time
For one continuously operating device, the calculator can solve one missing value from the other two. For multiple identical devices, it uses E = P × t × N × duty fraction, where P is on-state power per device and N is device count. A unique inverse requires a nonzero divisor. Optional energy cost is total kWh multiplied by your rate; fixed charges and taxes are excluded.
- 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 thermochemical 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 | Input electrical power may exceed the cooking-output rating; use measured or rated input watts |
| 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.
Substitute the values:
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.
Substitute the values:
The power is 120 W.
Electrical Work and Net Energy
Constant-DC electrical work uses E = V × I × t, with volts, amperes and seconds giving joules. Its inverse modes divide work by the other two quantities. This model does not account for varying waveforms, AC power factor or conversion losses. Net energy = produced energy − consumed energy, with both measured over the same interval and system boundary; a negative net means consumption exceeds production.
Conversions use mechanical horsepower, thermochemical calories and International Table Btu. The second/minute/hour/day conversions use 60/3,600/86,400 seconds; a day here is a fixed duration.
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.
