Power is the rate at which work is done or energy is used — not the amount of energy itself. This works it out from energy and time, and converts the result into horsepower for comparison.
How it works
Power
P = W / t
W is work or energy in joules, t is time in seconds, P comes out in watts. One mechanical horsepower is 745.699871582 watts exact, derived from the exact pound-force and foot already used elsewhere on this site.
The same job, done faster, needs more power
Lifting a box a fixed height takes the same total energy whether it happens in one second or one minute — but the one-second lift needs sixty times the power, because power divides that fixed energy by a much smaller time. A power rating in watts or horsepower is a statement about how quickly energy is delivered, not about how much total work eventually gets done — a low-power motor can still do a large amount of total work, it just takes longer.
How to use this calculator
- Enter the work or energy in joules.
- Enter the time in seconds.
- Read the power in watts, or in horsepower for a more familiar comparison.
Frequently asked questions
Why can’t time be zero?
Dividing energy by zero time is mathematically undefined, not an infinite power figure — no real process delivers a finite amount of energy in literally zero time, so this calculator declines the case rather than showing a meaningless result.
Is horsepower an exact conversion from watts?
The mechanical horsepower used here is exact — 550 foot-pounds-force per second, converted using the same exact foot and pound-force definitions this site’s other calculators already use, giving 745.699871582 watts precisely.
Why do electric motors and light bulbs get rated in watts?
Because watts describe the rate of energy use directly — a 100 W bulb uses 100 joules every second, which is exactly the quantity a power rating is meant to communicate.
What’s the difference between power and energy on an electricity bill?
An electricity bill charges for energy (kilowatt-hours — power sustained over time), not for power itself. A high-power appliance used briefly can use less total energy than a low-power one left running for hours.
Can power be negative?
In some contexts, yes — power delivered to a system is positive, power extracted from it (as in braking or regeneration) is often described as negative in the same convention. This calculator assumes positive work and time throughout.
