# Watt Calculator

Calculate watts from volts and amps using P = V × I. Convert between watts, kilowatts, horsepower, and BTU/hr. Free electrical power calculator.

## What this calculates

Watts measure the rate at which electrical energy is used or produced. The formula is simple: multiply volts by amps to get watts. A standard US outlet at 120V supplying 10 amps delivers 1,200 watts. This calculator also converts your result to kilowatts, horsepower, and BTU per hour.

## Inputs

- **Voltage** (V) — min 0 — Electrical potential difference in volts.
- **Current** (A) — min 0 — Electrical current in amperes.

## Outputs

- **Power** (W) — Electrical power in watts.
- **Power** (kW) — Power in kilowatts.
- **Power** (HP) — Power in mechanical horsepower.
- **Power** (BTU/hr) — Power in BTU per hour.

## Details

**The formula:**

P (watts) = V (volts) × I (amps)

This is the most fundamental equation in electrical work. If you know the voltage of your power source and the current your device draws, you can find exactly how much power it consumes.

**Common examples:**
- A 60W light bulb on a 120V circuit draws 0.5 amps
- A 1,500W space heater on 120V draws 12.5 amps
- A 240V dryer pulling 25 amps uses 6,000 watts (6 kW)

**Unit conversions:**
- 1 kilowatt (kW) = 1,000 watts
- 1 mechanical horsepower (HP) = 745.7 watts
- 1 watt = 3.412 BTU/hr

Kilowatts are what your electric utility uses for billing. Horsepower shows up on motors and engines. BTU per hour is common in HVAC sizing.

## Frequently Asked Questions

**Q: How do you calculate watts from volts and amps?**

A: Multiply volts by amps. For example, 120 volts times 15 amps equals 1,800 watts. This works for DC circuits and single-phase AC circuits with a power factor of 1.

**Q: What is the difference between watts and kilowatts?**

A: One kilowatt equals 1,000 watts. Your electric bill is measured in kilowatt-hours (kWh), which is 1,000 watts used for one hour. A 100-watt light bulb running for 10 hours uses 1 kWh.

**Q: How many watts is one horsepower?**

A: One mechanical horsepower equals 745.7 watts. So a 5 HP motor draws roughly 3,728 watts at full load. Electrical horsepower (used for electric motors) is slightly different at 746 watts.

**Q: Does this formula work for AC circuits?**

A: For simple resistive loads like heaters and incandescent bulbs, yes. For inductive or capacitive loads (motors, transformers), you need to multiply by the power factor: P = V × I × pf. The power factor is typically between 0.8 and 0.95 for motors.

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Source: https://vastcalc.com/calculators/physics/watt
Category: Physics
Last updated: 2026-04-08