How It Works

The relationship between volts, amps, and watts is straightforward. Power (in watts) equals voltage (in volts) multiplied by current (in amps). For DC circuits, it’s a simple multiplication. For AC circuits, you need to account for the power factor, which represents how efficiently the circuit converts electrical power into useful work.

The formulas:

Where P is power in watts (W), V is voltage in volts (V), I is current in amps (A), and PF is the power factor (a number between 0 and 1).

Most household appliances run on AC power with power factors between 0.8 and 1.0. Resistive loads like heaters and incandescent bulbs have a power factor of 1.0. Motors, transformers, and devices with switching power supplies typically have lower power factors, usually around 0.8 to 0.95.

What If I Only Know Volts and Watts?

You can rearrange the formula to find amps: divide watts by volts. For example, a 1,200-watt microwave on a 120-volt circuit draws 10 amps (1,200 ÷ 120 = 10). This works for DC circuits directly. For AC circuits, you’d also need to know the power factor to get the exact current draw.

Why Does Power Factor Matter?

Power factor only applies to AC circuits. It measures how much of the current actually does useful work. A power factor of 1.0 means all the current is being used efficiently. A power factor of 0.8 means only 80% of the current does useful work, while the rest just bounces back and forth in the circuit.

Pure resistive loads (space heaters, toasters, old-style light bulbs) have a power factor of 1.0. Inductive loads (motors, fans, air conditioners) and capacitive loads (switching power supplies) have lower power factors. If you’re not sure, 0.9 is a safe estimate for most modern appliances.

Can I Use This for Solar Panels?

Yes. Solar panels produce DC power, so use the DC calculation. For example, a solar panel rated at 18 volts and 5.5 amps produces about 99 watts (18 × 5.5 = 99). This is the panel’s output under ideal conditions. Real-world output varies with sunlight intensity, temperature, and angle.

If you’re calculating power after an inverter (which converts DC to AC), use the AC formula and account for inverter efficiency, typically 90% to 95%.

What About Three-Phase Power?

Three-phase AC systems are common in industrial settings. The formula changes slightly: P = √3 × V × I × PF, where √3 is approximately 1.732. For a 480-volt three-phase system drawing 20 amps with a power factor of 0.85, you get about 14,150 watts (1.732 × 480 × 20 × 0.85 ≈ 14,150).

This calculator focuses on single-phase and DC systems, which cover most residential and light commercial applications.

Common Examples

When Should I Care About This?

Knowing watts helps you understand your electrical costs and capacity. Every circuit in your home has a limit (usually 15 or 20 amps at 120 volts, which is 1,800 or 2,400 watts). If you plug in too many high-wattage devices, you’ll trip the breaker.

For solar systems, watts tell you how much energy you’re producing. For batteries, watts combined with time give you watt-hours, which tells you how long your power will last. A 100-watt device running for 10 hours uses 1,000 watt-hours (1 kilowatt-hour).

For DIY projects and repairs, knowing the power draw helps you choose the right wire gauge, fuse, and power supply. Undersized components overheat and fail. Oversized components waste money.