1.5 kWh transferred in 2 hours
P = E ÷ t
1.5 kWh ÷ 2 h = 0.75 kW, or 750 W. In SI units, 5,400,000 J ÷ 7,200 s gives the same 750 W.
Energy over time
Power Calculator
Find average power, energy transferred, or elapsed time from the other two values. Convert joules and kilowatt-hours without mixing up power and energy.
Calculate power, energy, or time
Average power
Enter the two known values, then select Calculate.
- Power in watts
- Energy in joules
- Time in seconds
- Formula
Calculation steps
≈ marks display rounding. Calculations run in your browser. Press Enter to calculate. Successful results appear in this tab's Recent calculations.
Three worked examples
A 60 W lamp running for 3 hours
E = P × t
60 W × 3 h = 180 Wh, or 0.18 kWh. This equals 648,000 J. It assumes the lamp averages 60 W for the full three hours.
Transferring 3 kWh at 1.5 kW
t = E ÷ P
3 kWh ÷ 1.5 kW = 2 hours. The estimate uses a constant 1.5 kW transfer rate. It does not automatically account for energy losses, pauses, or a changing charge rate.
Formulas, unit definitions, and limits
Average power is the energy transferred divided by the duration of that transfer: P = E/t. For mechanical work, that energy can be work done. Rearranging gives E = Pt and t = E/P. For changing power, the average must cover the same interval as the energy. These relations do not recover a peak power or a time-varying power curve. Reference: OpenStax — Power.
The SI units are the watt for power, joule for energy, and second for time. The conversions used here are exact: 1 W = 1 J/s; 1 kW = 1,000 W; 1 MW = 1,000,000 W; 1 kJ = 1,000 J; 1 MJ = 1,000,000 J; 1 min = 60 s; 1 h = 3,600 s; 1 Wh = 3,600 J; and 1 kWh = 3,600,000 J. References: NIST — SI units and prefixes and NIST — conversion factors by quantity.
This tool accepts nonnegative energy and power magnitudes; it does not track the sign of energy flow. A supplied time must be positive. Zero energy over a positive interval gives zero average power, and zero power over a positive interval gives zero energy.
When solving for time, zero power with positive energy cannot give a finite answer. Zero power and zero energy do not determine a unique duration. A zero energy target at positive power gives an ideal duration of zero. That last result is a solution of E = Pt, not a definition of average power over a zero-length interval.
Use the energy and power measured at the same part of a system. Input electrical power and useful output power may differ because of losses. The calculator does not add efficiency factors, appliance duty cycles, battery capacity changes, or charging taper. A device's rated power may differ from actual average consumption.
Active inputs accept at most 64 characters, 15 integer digits, and 10 decimal places. Expressions, scientific notation, and units inside numeric fields are not accepted. Hidden inputs are disabled and ignored. Arithmetic and unit conversions retain exact fractions, and only the displayed values are rounded. Halfway values round away from zero; trailing zeros are omitted and ≈ identifies rounding. A tiny nonzero value may display as ≈ 0, with a notice. Result-unit and decimal-place changes update the display without adding a recent-calculation entry.
Common questions
What is the difference between kW and kWh?
A kilowatt is a power, or rate of energy transfer. A kilowatt-hour is an amount of energy. One kilowatt sustained for one hour transfers one kilowatt-hour. Multiplying power by time gives energy; dividing energy by time gives power.
Is this only for electricity?
No. Watts also describe mechanical and thermal energy transfer. Select the appropriate energy and time units and keep both values tied to the same process. For voltage and current calculations, use the linked Ohm's Law Calculator.
Does the result show average power or maximum power?
Energy divided by elapsed time gives average power. If the rate stays constant, average and instantaneous power match. A process with changing power can have the same energy and duration as a constant-power process while reaching a higher peak.
Can I use this to estimate battery charging time?
You can estimate an ideal duration from energy to be added and the power actually reaching the battery. Real charging can take longer because of losses, reduced charging rates, temperature, pauses, and other conditions. A charger's advertised maximum is not necessarily the average power reaching the battery.
Why is zero time rejected when calculating power?
Average power divides energy by a positive elapsed time. Positive energy divided by zero has no finite result, and zero divided by zero has no unique result. The time mode separately permits a zero energy target at a positive power, which needs no elapsed time in the ideal model.