Power Calculator (Electrical)

Step-by-Step Guide

1. 1Select Calculation Mode: Choose whether to solve for Power, Voltage, or Current. The calculator hides the field you're solving for and asks only for the two values you already know.
2. 2Enter Known Values: Type in the two values you have. Results calculate instantly as you type.
3. 3Select Units: Choose units for voltage (V, mV, kV), current (A, mA), and power (W, mW, kW). The calculator converts internally so units never need to match.
4. 4Apply a Voltage Preset: Optionally apply one of six built-in voltage presets — USB, automotive, industrial, or regional household voltages — to skip manual entry.
5. 5Read, Copy, or Export Results: View the calculated result and its breakdown, then copy it to your clipboard, save it to calculation history, or export it as a text file.

What This Calculator Provides

• ✓Real-time calculation as you type
• ✓Three calculation modes: power, voltage, current
• ✓Full unit support: V, mV, kV, A, mA, W, mW, kW
• ✓Six built-in voltage presets
• ✓Calculation history (up to 20 saved entries)
• ✓Copy result to clipboard
• ✓Export calculation as a text file
• ✓Automatic division-by-zero protection
• ✓Private: your inputs are not collected or stored
• ✓No signup required

Pro Tips

• 💡Match the unit prefix to the value's actual scale before comparing results. 100mW and 0.1W are the same quantity, but misreading milliwatts as watts is a 1,000× error that's easy to miss on a datasheet.
• 💡Use Current mode to check breaker sizing rather than back-calculating from a Power mode result by hand. Breakers are rated in amps, so solving for current directly avoids an extra division step and rounding error.
• 💡When a nameplate lists only volts and amps with no wattage figure, use Power mode — most nameplates already give you the two knowns you need without further conversion.
• 💡Save recurring calculations to history if you're checking the same class of device repeatedly, such as a batch of USB chargers. It's faster than re-entering values or reaching for a separate notepad.
• 💡This calculator assumes resistive, single-phase circuits. If you're working with an AC motor or another inductive load, multiply the result by the load's power factor (P = V × I × PF) — see the FAQ below for details.

Common Mistakes to Avoid

• ✕Entering current in milliamps but leaving the unit dropdown on amps, or vice versa. A 20mA LED entered as "20 A" returns a result 1,000× too high — always double-check the unit selector, not just the number.
• ✕Using the plain P = V × I formula for AC circuits with a power factor below 1.0. Real power in reactive AC circuits is P = V × I × PF, so omitting PF overstates true power for inductive loads like motors and transformers.
• ✕Trying to solve for voltage or current while the other known input is left at zero. Division by zero is mathematically undefined, so the calculator blocks it and shows a validation error rather than a false result.
• ✕Assuming a household voltage preset matches your exact location. Nominal voltage varies by country — 120V in North America, 230V in the UK, Europe and Australia, and 220–240V in many other countries — and even within a region, so measure with a meter for precise calculations.
• ✕Forgetting this is single-phase math. Three-phase power uses a √3 factor and a different formula entirely — applying this tool's result directly to a three-phase load underestimates power by roughly 42%.