Amps to Watts Calculator

Turn an amp reading from a PDU, clamp meter or device label into the real power it represents, in watts.

Inputs

Result

Enter the current in amps to see the power in watts.

Related tools

How the calculation works

Real power in watts comes from the current, the voltage, the circuit type and the power factor (PF):

Single-phase: watts = volts × amps × PF
Three-phase: watts = √3 × volts × amps × PF, where amps is the current on one phase, volts is the line-to-line voltage and √3 ≈ 1.732

Volts × amps on its own (× √3 on three-phase) is the apparent power in volt-amperes (VA). The calculator shows both: watts are what the equipment uses, VA is what the circuit carries. The power factor links them, and the default of 1.0 makes the two equal. IT equipment with power-factor-corrected supplies runs close to 1.0 under load; if your PDU or meter reports a power factor, enter it.

The default is a 208 V single-phase circuit, a common feed for a single server or a small PDU in North American datacenters.

Which voltage is your reading on? In a 208Y/120 V panel, a branch circuit wired from one phase to neutral runs at 120 V, and one wired between two phases runs at 208 V. A clamp meter shows the same kind of amps on both, so the voltage you enter decides the answer: 10 A is 1,200 W on the 120 V branch and 2,080 W on the 208 V one (PF 1.0). Use three-phase only when the reading comes from a three-phase feed, and then enter the line-to-line voltage (208 V here, not the 120 V line-to-neutral voltage, which is 208 ÷ √3).

Worked example

A server PDU input on a 208 V single-phase circuit reads 16 A, and its display shows a power factor of 0.95:

watts = 208 × 16 × 0.95 = 3,161.6 W
apparent power = 208 × 16 = 3,328 VA

A 120 V branch circuit feeding a network switch and a console server reads 12 A. At a power factor of 1.0:

watts = 120 × 12 = 1,440 W, which is exactly 80% of a 15 A, 120 V circuit, the usual planning limit for a continuous load

A small three-phase PDU on a 208 V feed reads 10 A per phase. At a power factor of 1.0:

watts = 1.732 × 208 × 10 = 3,602.7 W

Frequently Asked Questions

How many watts is 15 amps at 120 volts?

At a power factor of 1.0, 15 A × 120 V = 1,800 W. That is the full rating of a common 15 A, 120 V circuit. For a load that runs for hours, the usual North American practice is to stay at or below 80% of the rating, which is 12 A or 1,440 W.

Why is my watts figure lower than volts times amps?

Volts × amps gives volt-amperes (VA), the apparent power. Watts are VA × power factor. A device drawing 16 A at 208 V is 3,328 VA; at a power factor of 0.95 that is 3,161.6 W of real power. A meter that shows both W and VA is also telling you the power factor: W ÷ VA.

My PDU shows amps per outlet and per input. Which do I use?

For one device, use its outlet reading. For everything on the PDU, use the input current. On a three-phase PDU each input line has its own reading; the three-phase formula assumes the lines carry about the same current, so if they differ, the average of the three gives an estimate. If the PDU also reports kW, that reading is more accurate than any conversion.

Does the amps figure on a device label tell me what it really draws?

Usually not. A label or power supply rating is a maximum, often well above what the device draws in normal use. A reading taken under typical load gives a more realistic figure for capacity planning. Providers and electricians may size circuits from nameplate or stated maximum values, so ask which one your provider limits against.