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Free calculator

Use this calculator to

  • Find the operating voltage from a wattage and a measured current
  • Verify a circuit is genuinely running near 230 V from clamp meter readings
  • Check DC kit such as LED drivers and USB-C chargers from their W and A ratings
  • Spot measurement or power factor errors when W / A lands far from nominal

Watts to Volts Calculator

Convert power (W) to voltage (V) using current (A).

W
A
Result
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Formulas

  • V = W / A

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Working backwards from real power and current to voltage is the check engineers run on motor and pump nameplates before signing off a new installation. A 15kW three-phase motor labelled at 400V should draw around 27A at full load, and the voltage seen at the motor terminals should sit close to 400V. If the calculated voltage drops below 380V, the cable is too long or too thin, the motor will run hotter than its rated insulation class allows, and the warranty position with the manufacturer is at risk.

Voltage drop is the silent cost of long cable runs in warehouses, factories and large commercial sites. BS 7671 limits voltage drop to 3% on lighting and 5% on other circuits. On a 100-metre run feeding a 22kW load, the difference between 6mm squared and 10mm squared cable is the difference between a compliant install and one that fails the next inspection. The watts to volts calculation gives the design engineer a fast way to validate the proposed cable size against the actual demand, before the cable is in the wall.

Purely Energy advises mid-market clients on the interaction between site electrical design and the supply contract. When a site adds 100kW of motor load on a long cable run, the supply transformer sees a different demand pattern, the power factor on the meter changes, and the next contract renewal needs to reflect that. We make sure the operational change shows up in the supply procurement so the client is not surprised by a tariff that no longer fits the operation it was negotiated for.

Common questions

How do I convert watts to volts?

Divide the power by the current: V = W / A. A load consuming 2,990 W while drawing 13 A is running at 2990 / 13, which is 230 V. The formula comes straight from the power law P = V x I, rearranged for voltage. You need a genuine current figure, either from a nameplate, a datasheet, or a clamp meter reading taken while the equipment is running at the stated power.

Can I find volts from watts alone?

No. Power alone does not fix a voltage; the same 60 W can be delivered at 20 V and 3 A or at 230 V and 0.26 A. You need a second quantity. With current, use V = W / A. With resistance, use V = the square root of W x R, which follows from P = V squared / R. If you have neither, measure one of them; guessing the second value defeats the point of the calculation.

What should the calculated voltage be on UK mains?

The UK nominal supply voltage is 230 V single phase, with a statutory tolerance of plus 10 percent and minus 6 percent, so anywhere from 216.2 V to 253 V is within specification. If your W / A result lands well outside that band on a mains circuit, suspect the inputs: the wattage may be a nameplate maximum rather than the actual draw, the current reading may include other loads on the circuit, or power factor may be skewing the comparison.

Does power factor affect a watts to volts calculation?

On AC circuits with reactive loads, yes. The wattage on a nameplate or power meter is real power, but a clamp meter reads the full current including the reactive component. For those loads the relationship is V = W / (A x PF), so dividing watts by measured amps without the power factor understates the voltage. For resistive loads such as heaters, the power factor is effectively 1 and the simple V = W / A holds exactly.

Why does a 60 W USB-C charger run at 20 V?

USB Power Delivery limits the current to 3 A on standard cables (5 A only with electronically marked cables), so higher power has to come from higher voltage rather than more current. At the 3 A ceiling, 60 W requires 60 / 3, which is 20 V, one of the standard USB PD voltage levels alongside 5 V, 9 V and 15 V. The same logic drives mains transmission: raising voltage moves more power through the same conductor cross-section.

Related calculators

The conversions that procurement and engineering use alongside this one.

Watts to Volts Calculator | Purely Energy