Power is the rate energy is used
Energy is a total; power is how fast you spend it. Divide the energy used by the time it took and you get power, in watts.
Using 1000 joules in 10 seconds is 100 watts. One watt is one joule per second, so a 60 W bulb turns 60 joules into light and heat every second it is on.
The same power from voltage and current
For anything electrical, power is also voltage times current.
A 230 V kettle drawing 9 A pulls about 2,070 W. The calculator handles both routes, so you can work from whichever numbers you have.
Power is not the same as energy
| Thing | Power |
|---|---|
| LED bulb | ~9 W |
| Phone charger | ~20 W |
| Electric kettle | ~2,000 W |
| Small car engine | ~100,000 W |
A watt is a rate; a watt-hour is an amount. Your electricity bill is in kilowatt-hours because it charges for energy, not power.
Frequently asked questions
What is the power formula?
Power is energy divided by time: P = W ÷ t, measured in watts. Using 1000 joules in 10 seconds is 100 W. For electricity, power is also voltage times current, P = V × I.
What are the units of power?
The watt (W), equal to one joule per second. Larger amounts use kilowatts (1000 W) and megawatts. One mechanical horsepower is about 746 W.
What is the difference between power and energy?
Power is a rate; energy is an amount. A 2000 W kettle uses energy fast, but the total energy depends on how long it runs. Your electricity bill charges for energy, in kilowatt-hours, not power.
How do I calculate electrical power?
Multiply voltage by current: P = V × I. A 230 V appliance drawing 9 A uses about 2070 W. If you know resistance instead, P = V²/R or P = I²R.
How many watts is one horsepower?
About 746 watts for one mechanical horsepower. So a 150 hp engine is roughly 112 kW. Metric horsepower is slightly different, about 735 W.