Watts to Volts Calculator - Voltage from Power and Current

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Standard electrical engineering formulas

Watts do not tell voltage unless you also know current or resistance. With current, V = W / A. With resistance, V = sqrt(W x R).

Formula at a glance

  • from current: V = W / A
  • from resistance: V = sqrt(W x R)
  • AC single-phase: V = W / (A x PF)

Field note: Reverse calculations are good for checking a label or meter reading. If the circuit is live, measuring voltage directly is better and faster.

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Calculator Tool

Convert power to voltage

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A
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Formula

V = W / A

How to use the Watts to Volts Calculator

Use this as a fast electrical check, then compare the result with the nameplate, measured voltage and power factor. The formula is clean. Real panels, motors and UPS loads usually have one extra wrinkle.

Worked example

Example: 600 W at 5 A is 120 V. A 100 W load through 25 ohms needs 50 V.

Practical checks before you trust the number

  • Use PF when watts and amps come from an AC load.
  • For three-phase, divide watts by 1.732 x amps x PF.
  • Measured current must be actual load current, not fuse size.

Common mistake

Reverse calculations are good for checking a label or meter reading. If the circuit is live, measuring voltage directly is better and faster.

Sources and references

Related calculators

Frequently Asked Questions

V = W ÷ A for DC or single-phase resistive. For single-phase inductive: V = W ÷ (A × PF). For three-phase: V = W ÷ (√3 × A × PF). Example: 1000 W at 8 A on a single-phase resistive load → V = 1000 ÷ 8 = 125 V. This formula helps when you know the load specs and need to verify supply voltage compatibility. We use it on imported equipment quotations all the time.

Single-phase: V = W ÷ (A × PF). Three-phase: V = W ÷ (√3 × A × PF). Example: 2000 W single-phase, 9 A, PF 0.85 → V = 2000 ÷ (9 × 0.85) = 261 V. Keep PF in the formula whenever the load is inductive — leaving it out gives a voltage that's too low and misleads about supply requirements.

V = W ÷ A for DC or PF = 1: V = 500 ÷ 5 = 100 V. With PF 0.8: V = 500 ÷ (5 × 0.8) = 125 V. So the answer depends on whether the load is resistive or inductive. For most heaters and incandescent bulbs assume PF = 1, for fans and small motors use PF 0.85 to 0.95. Always cross-check with the device nameplate before specifying a power supply.

Use V = √(P × R) when current is unknown. Example: 100 W dissipated in a 50 Ω resistor → V = √(100 × 50) = √5000 = 70.7 V. This is purely resistive analysis, so it works for heaters, lamps, and DC loads. For AC reactive loads, replace R with impedance Z. The formula is the rearranged P = V² ÷ R.

PF affects voltage when you back-calculate from real power. Single-phase: V = W ÷ (A × PF). At lower PF, the same wattage requires either more current or higher voltage. Example: 1000 W, 5 A, PF 1 → V = 200 V. Same load at PF 0.7 → V = 286 V. So always use the actual load PF, especially for motors, otherwise the calculated supply voltage will be wrong.

Not without current. You need at least two known quantities. With watts and resistance: V = √(P × R). With watts and current: V = P ÷ I (for DC). For AC, you also need PF. Without current, resistance, or PF, voltage cannot be uniquely determined from watts alone. So always ask for at least one extra parameter when sizing supplies from a power rating.

Yes, especially for DC supplies and adapters. If a device is rated for 24 W at 2 A, you need a 12 V supply (24 ÷ 2 = 12). Once voltage is known, pick a supply rated 1.25 to 1.5 times the current to allow startup surge. We use this every day for selecting LED driver and CCTV adapter ratings on residential security installations.