acalculator

What is the wattage?

Pick what to find, then type the other two values. The wattage calculator multiplies volts by amps (and the power factor for AC), or divides the watts to find the amps or the volts.

Your numbers

Units
Find
Current
Wattage
1,200

120 V at 10 A is 1,200 W.

KilowattskW
1.2
CurrentA
10
VoltageV
120
Apparent powerVA
1,200

Wattage: 1,200. 120 V at 10 A is 1,200 W.

How to calculate

Computes the wattage of a circuit or appliance from volts and amps (P = V × I, times the power factor for AC), or the amps or volts from the wattage, with kilowatts and volt-amperes.

Example with the default inputs (Find Watts, Current DC, Voltage 120 V, Current 10 A): 120 V at 10 A is 1,200 W.

Method: DC: P = V × I. AC (single phase, RMS): P = V × I × power factor. I = P ÷ (V × pf); V = P ÷ (I × pf); VA = V × I.

  • AC values are RMS (the usual outlet and nameplate values) on a single-phase supply.
  • The power factor is 1 for DC and for purely resistive AC loads such as heaters and incandescent bulbs.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Find Watts, Current DC, Voltage 115, Current 20 gives Power 2,300, Kilowatts 2.3.Source: OpenStax University Physics Volume 2, section 9.5, Electrical Energy and Power, Example 9.9: P = IV = (20.00 A)(115 V) = 2,300 W (https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power, retrieved 2026-10-01)
  2. Find Watts, Current AC, Voltage 120, Current 10, Power factor 0.8 gives Power 960, Apparent power 1,200.Source: OpenStax University Physics Volume 2, section 15.4, Power in an AC Circuit: average power = I_rms × V_rms × cos φ, cos φ the power factor (https://openstax.org/books/university-physics-volume-2/pages/15-4-power-in-an-ac-circuit, retrieved 2026-10-01)
  3. Find Amps, Current DC, Voltage 120, Power 1,500 gives Current 12.5.
  4. Find Volts, Current AC, Current 5, Power 1,035, Power factor 0.9 gives Voltage 230.

How it works

Pick what to find (watts, amps or volts) and DC or AC, then fill the other two values. With V the voltage, I the current, P the power and pf the power factor (1 for DC):

  • P = V × I × pf
  • I = P ÷ (V × pf)
  • V = P ÷ (I × pf)

The calculator also shows:

  • Kilowatts = P ÷ 1,000.
  • Apparent power = V × I, in volt-amperes (VA).

Every value shows to 6 significant figures. The maths runs in floating point and each value is rounded half up for display only.

Rules

  • Volts, amps and watts are each from 0.000001 to 1,000,000 in their base unit (V, A, W). Millivolts, kilovolts, milliamps, milliwatts and kilowatts convert exactly (× 0.001 or × 1,000).
  • The power factor is from 0.01 to 1. It applies to AC only; DC always uses 1.
  • AC values are RMS values on a single-phase supply.
  • A value left empty for the chosen mode gives no answer, with a message naming it.

Worked examples by hand

The winch motor (DC). 20 A at 115 V: P = 115 × 20 = 2,300 W = 2.3 kW.

A motor on an outlet (AC). 120 V, 10 A, power factor 0.8: P = 120 × 10 × 0.8 = 960 W; apparent power 120 × 10 = 1,200 VA.

A heater (DC or resistive AC). 1,500 W at 120 V: I = 1,500 ÷ 120 = 12.5 A.

Volts from watts (AC). 1,035 W at 5 A and power factor 0.9: V = 1,035 ÷ (5 × 0.9) = 230 V.

Other questions people ask

How do I calculate wattage?

Multiply volts by amps: P = V × I. A motor drawing 20 A at 115 V uses 20 × 115 = 2,300 W. For an AC load that is not purely resistive, also multiply by the power factor.

How many amps is 1,500 watts?

Divide the watts by the volts: I = P ÷ V. On a 120 V outlet, 1,500 W is 1,500 ÷ 120 = 12.5 A. On 230 V it is about 6.5 A.

What is a power factor?

In an AC circuit the current and voltage can be out of step. The power factor, cos φ, is the share of volts × amps that does real work. It is 1 for heaters and incandescent bulbs and often 0.8 to 0.95 for motors.

What is the difference between watts and volt-amperes?

Volt-amperes (VA) are volts × amps, the apparent power. Watts are the real power: VA × power factor. A 120 V, 10 A motor at power factor 0.8 draws 1,200 VA but uses 960 W.

How do I find the voltage from watts and amps?

Divide the watts by the amps (and by the power factor for AC): V = P ÷ (I × pf). 1,035 W at 5 A and a power factor of 0.9 is 1,035 ÷ 4.5 = 230 V.

How do I get kilowatt-hours from watts?

Multiply the kilowatts by the hours of use. A 1,500 W heater running 3 hours uses 1.5 × 3 = 4.5 kWh. The electricity cost calculator turns that into money.