How fast is the work getting done?
Power is the rate of doing work, measured in watts. Enter energy and time, or force and speed, and this gives you the power in watts, kilowatts and horsepower, plus how much energy that rate delivers over time.
The numbers
Power = energy divided by time. Enter both and the rate is calculated for you.
What the result means
Same energy, different times
Power needed vs time to deliver one hour's worth of energy
Power scorecard
Energy delivered at this power
Hold this power steady and this is how much energy it moves over each stretch of time. Energy = power times time.
| Run time | Energy (J) | Energy (Wh) | Energy (kWh) | Food calories |
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Power, explained
Power vs Work vs Energy
Work is energy transferred, measured in joules (J). Power is the rate at which work is done, measured in watts (W = J/s). Energy and work are the same units; power is the rate. Two engines can do the same total work, but the more powerful one does it faster, that is the only difference.
Watts, Kilowatts, and Horsepower
1 kW = 1000 W. 1 HP = 745.7 W (mechanical horsepower) or 746 W (electrical). A typical car engine outputs 75-300 HP (55-220 kW). A toaster uses about 800 W. A human in steady-state exercise produces around 75 W (0.1 HP). World-class cyclist sprints can hit 1500-2000 W briefly.
Power in Electrical Systems
For electrical devices: Power = Voltage × Current (P = V × I). A 120V appliance drawing 10A consumes 1200W. This is why home circuits are rated in amps, they limit how much power you can draw at the wall outlet. A 15A 120V circuit can handle 1800W max safely.
Average vs Instantaneous Power
This calculator returns average power (total work divided by total time). Instantaneous power (P at one moment) requires differentiating work with respect to time. For a sprinter, peak instantaneous power is much higher than the average over a full race.
Common questions
How is power different from force?
Force is a push or pull (newtons). Power is the rate of energy transfer (joules per second). A strong but slow lift produces less power than a moderate lift performed quickly.
Why are car engines rated in horsepower?
Historical convention from James Watt comparing steam engines to horses. 1 HP = 745.7 W is the rate at which one strong draft horse could do sustained work. Modern engines list both HP and kW in most countries.
What does 'wattage' mean on appliances?
It is the rate at which the appliance consumes electrical energy. A 100W bulb uses 100 joules per second. Over one hour, that is 360,000 J or 0.1 kWh, the billing unit on your electric bill.
How is mechanical power different from electrical?
Same unit (watt) but different formulas. Mechanical: P = W/t or P = F × v. Electrical: P = V × I. Both measure energy transferred per unit time, just in different contexts.
Can power be negative?
Yes, if work done is negative (force opposes motion). A car engine produces positive power when accelerating; the brakes produce negative power (removing kinetic energy from the wheels).
Results use classical mechanics and average power (total energy divided by total time). One horsepower is taken as 745.7 W (mechanical). Instantaneous or peak power can be far higher than the average. Verify against your own reference values for engineering work.