Power Calculator (Electrical)

Step-by-Step Guide

1. 1Select calculation mode: Choose whether you want 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 your known values: Type in the two values you have. Results calculate instantly as you type — there's no button to press and no page reload.
3. 3Choose units for each value: Pick from V, mV, or kV for voltage; A or mA for current; and W, mW, or kW for power. The calculator converts everything internally, so the units you pick never need to match each other.
4. 4Apply a voltage preset (optional): Use one of the six built-in presets — USB 5V, 12V automotive, 24V industrial, 110V US, 220V EU, or 240V UK — to skip manual entry for common voltage standards.
5. 5Read the result and breakdown: The highlighted result box shows your answer plus the two input values used, so you can verify the math without leaving the page.
6. 6Copy, save, or export: Copy the result to your clipboard, save it to your local calculation history (up to 20 entries), or export it as a downloadable 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
• ✓100% browser-based — no data sent to server
• ✓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 — 110-120V in North America, 220-240V across Europe, the UK, and most of Asia — 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%.