Volts to Watts Calculator
Calculate power from voltage with either current or resistance.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Volts to Watts Calculator does
Power from voltage needs one more quantity. With current known it is a simple product; with resistance known, power scales with the square of voltage, which is why small voltage increases cause disproportionate heating.
Formula
P = V × IP = V² ÷ RI = V ÷ R
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Second known value | one of 2 options | Yes | — |
| Voltage | V | Yes | Accepts more than 0. |
| Current | A | In some modes | Accepts more than 0. Shown Second known value is Current (amps). |
| Resistance | Ω | In some modes | Accepts more than 0. Shown Second known value is Resistance (ohms). |
| Power factor | number | Optional | Accepts 0.01 or more, up to 1. |
How to use it
- Choose Second known value.
- Enter Voltage.
- Fill in the remaining inputs the form shows for your choice.
- Optionally add Power factor.
- Select Calculate.
Worked example
A 230 V supply across a 23 Ω heating element.
- Known
- Resistance
- Voltage
- 230
- Resistance
- 23
I = 10 A, P = 230² ÷ 23 = 2,300 W.
Frequently asked questions
Why does power scale with voltage squared?
Because raising voltage across a fixed resistance also raises current proportionally. Power is the product, so it grows with the square.
Method and sources
Method. Power as the product of voltage and current, adjusted for power factor and phase configuration.
Applies to. None — a circuit relationship, not a regulated procedure
Assumptions
- The values entered are RMS for an AC circuit.
Limitations
- Using peak rather than RMS values overstates power substantially.
- Applies to a steady sinusoidal supply; distorted waveforms need true-RMS measurement.
Sources
- Standard AC circuit theory — Established electrical engineering fundamentals. The power relationship and the role of power factor.