Select energy, elapsed time, or average power as the unknown, then enter the two known values. Convert between J, kJ, thermochemical cal and kcal, IT Btu, s, min, h, W, kW, and IT Btu/h.
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Energy To Power Conversion Formula
Energy change divided by a positive elapsed time gives average power over that interval. For constant power, this also equals the instantaneous power:
The same relationship can be rearranged to solve for energy or time:
- P = power, usually measured in watts (W)
- E = change in energy, usually measured in joules (J)
- t = change in time, usually measured in seconds (s)
The calculator uses base SI units internally. Energy is converted to joules, time is converted to seconds, and power is converted to watts before the formula is applied.
- To calculate power: enter energy and time. The calculator uses P = E / t.
- To calculate energy: enter power and time. The calculator uses E = P × t.
- To calculate time: enter energy and nonzero average power. The calculator uses t = E / P. Energy and power must have the same sign for nonnegative time; zero energy gives zero time when power is nonzero.
Energy, Time, and Power Unit Conversions
These are the unit conversions used to convert values before applying the formula.
| Energy Unit | Equivalent in Joules |
|---|---|
| 1 J | 1 J |
| 1 kJ | 1,000 J |
| 1 cal | 4.184 J |
| 1 kcal | 4,184 J |
| 1 Btu (IT) | 1,055.05585262 J |
| Unit Type | Conversion |
|---|---|
| Time | 1 min = 60 s |
| Time | 1 h = 3,600 s |
| Power | 1 kW = 1,000 W |
| Power | 1 Btu (IT)/h ≈ 0.293071 W |
| Power | 1 W ≈ 3.412142 Btu (IT)/h |
Example Calculations
Example 1: Convert energy and time to power
You use 12,000 J of energy over 60 s. Find the power.
The power is 200 W.
Example 2: Find energy from power and time
A device runs at 2 kW for 3 minutes. Find the energy in kJ.
First convert the values:
Then calculate energy:
The energy is 360 kJ.
FAQ
What is the difference between energy and power?
Energy is the total amount of work, heat, or transferred ability to do work. Power is the rate at which that energy is used or transferred. For example, joules measure energy, while watts measure joules per second.
Why does time need to be in seconds for the formula?
A watt is defined as one joule per second. Because of that, the standard formula P = E / t uses joules for energy and seconds for time to get watts. If you enter minutes or hours, they must be converted to seconds before calculating power in watts.
Can power be calculated from calories or BTU?
Yes. Calories, kilocalories, and BTU are energy units. They can be converted to joules first, then divided by time in seconds to calculate power in watts. The calculator handles these conversions before applying the formula.
