MotionLab

Power Calculator (Physics)

P = W / t, in watts.

Work / energy (J)

Time (s)

Power (W)

P = W / t

What you entered

Work / energy (J): 500Time (s): 10

1
Power: P = W ÷ t
500 ÷ 10= 50

Result

Power (W): 50

Delivering 500J of work over 10s is 50 watts of power.

Power from work done over time

Power is work divided by time - the rate of energy transfer. One watt is one joule per second.

Work (J)	Time (s)	Power (W)	Context
100	5	20.00	Slow steady effort
746	1	746.00	One horsepower
1000	1	1000.00	1 kW burst
1000	10	100.00	Same work, ten times slower
5000	2	2500.00	Brief hard effort
60000	60	1000.00	1 kW sustained for a minute

Rows three and four do exactly the same 1,000 joules of work, but one takes a second and the other ten - power is the difference between them, and it is why a strong person and a weak one can both lift the same load while only one does it quickly. 746 W is the mechanical horsepower, defined by James Watt from what a dray horse could sustain. A kilowatt-hour, the unit on an electricity bill, is 1,000 W for 3,600 s, or 3.6 million joules.

Power is how fast, not how much

Two engines can do the exact same total work, but the one that does it faster has more power — power measures the rate of energy transfer, not the total amount transferred.

Watts, horsepower, and everyday use

A watt is one joule per second — light bulb wattage, car horsepower (746 watts each), and your electricity bill's kilowatt-hours all trace back to this same rate-of-energy concept.

Frequently asked questions

A motor does 15,000 J of work in 30 seconds. What's its power output?

P = W/t = 15,000/30 = 500 W. That's about 0.67 horsepower — comparable to a small electric drill.

How are watts and horsepower related?

1 horsepower = 746 watts. A 150 hp car engine produces about 112 kW. Horsepower is still used for engines and motors, but the watt is the SI standard used everywhere else.

A 60 W light bulb runs for 8 hours. How much energy does it use?

Energy = Power × time = 60 × (8 × 3600) = 1,728,000 J = 0.48 kWh. At $0.12/kWh, that costs about 5.8 cents. See the work calculator for problems starting from force and distance.