Watt Calculator

Calculate power in watts from energy and time, voltage and current, or force and velocity, with kilowatt, horsepower, and BTU/h equivalents. Calculates in your browser.

Power-
Kilowatts-
Horsepower (mech.)-
BTU / hour-

Show the math

Power is the rate of energy use, one watt is one joule per second:

P = E ÷ t  ·  P = V × I  ·  P = F × v

Equivalents use 1 kW = 1000 W, 1 hp ≈ 745.7 W, and 1 W ≈ 3.412 BTU/h. All math runs in your browser.

What this does

A watt calculator finds power — the rate of energy use — in watts. Power can come from energy divided by time, from voltage times current, or from force times velocity, and the result is also shown in kilowatts, horsepower, and BTU per hour.

How to use it

  1. Choose how you want to find power.
  2. Enter the two known values.
  3. Read the power in watts and its equivalents.
  4. Switch methods to use whichever values you have.

How it works

One watt is one joule per second. Electrically, power is voltage times current; mechanically, it is force times velocity. Each result is converted to kilowatts (÷1000), mechanical horsepower (÷745.7), and BTU per hour (×3.412).

P = E ÷ t = V × I = F × v

Understanding your result

The watt ties together energy and time: a 100 W appliance running for an hour uses 100 watt-hours. Horsepower is just a larger power unit — one mechanical horsepower is about 746 watts — while BTU/h is common for heating and cooling equipment.

Example

A 120 V circuit drawing 0.5 A delivers 60 W, and 6,000 J spread over 60 s is 100 W.

Sources & methodology

Last updated .

Frequently asked questions

How many watts are in one horsepower?

One mechanical horsepower is about 745.7 watts, so a 1 hp motor draws roughly 746 W of mechanical power.

How is this different from Ohm’s law?

Ohm’s law focuses on voltage, current, and resistance. This calculator adds the energy÷time and force×velocity routes to power and shows kW, hp, and BTU/h equivalents.

Is my data stored?

No. Everything is calculated locally in your browser.