Watt Calculator

Quick answer

What is a watt?

A watt (symbol: W) is the SI unit of power — the rate at which electrical energy is transferred or converted. One watt equals one joule of energy per second. Power tells you how fast energy is being used right now, not how much total energy has been consumed.

A 100 W light bulb, for instance, converts electrical energy to light and heat at a rate of 100 joules every second while it's switched on. That rate doesn't change with time, which is the key distinction between watts (power) and watt-hours (energy) — a topic covered in detail further down this page.

Watts scale up and down using standard SI prefixes: milliwatts (mW) for very small loads like sensors, kilowatts (kW) for household appliances, and megawatts (MW) for industrial or utility-scale systems. The calculator above converts your result across all four units automatically.

Watt formula

The basic watt formula for a DC circuit, or for the "apparent-to-real" simplification often taught first, is:

Where:

• P — Power, measured in watts (W)
• V — Voltage, measured in volts (V)
• I — Current, measured in amps (A)

Example: a 12 V battery supplying 5 A delivers 12 × 5 = 60 W. This formula is exact for DC circuits and for purely resistive AC loads. For most AC circuits, a power factor term is added — see the AC section below.

How to calculate watts

1. Find the voltage. Check the equipment nameplate, a multimeter reading, or your supply specification (e.g., 12 V battery, 230 V mains).
2. Find the current. Read it from a nameplate, a clamp meter, or a circuit's rated draw.
3. Multiply voltage by current. For AC loads, also multiply by the power factor.
4. Express the answer in watts. The result of V × I (× PF) is your power in watts.
5. Convert to kilowatts when useful. Divide by 1,000 for larger loads, such as whole-house consumption.

Calculate watts from volts and amps

Watts = Volts × Amps is the relationship you'll use most often. A few worked examples across common voltages:

• 12 V × 2 A = 24 W
• 12 V × 10 A = 120 W
• 24 V × 5 A = 120 W
• 48 V × 10 A = 480 W
• 120 V × 5 A = 600 W
• 230 V × 5 A = 1,150 W
• 240 V × 10 A = 2,400 W

Watts from amps alone

Amps alone can't tell you the power draw — current is only half the equation. A 10 A load could be 120 W at 12 V or 2,300 W at 230 V. You always need voltage as well: W = V × A.

Watts from volts alone

The same applies in reverse. Voltage describes electrical potential, not the rate of energy flow. A 230 V outlet delivers 0 W with nothing plugged in and thousands of watts under a heavy load — current is what changes.

AC watt calculation vs DC watt calculation

DC (direct current) systems, like batteries and solar panels, use the simple relationship P = V × I because voltage and current stay in phase. AC (alternating current) systems, like household mains, often need an additional factor.

DC power

Single-phase AC power

PF, the power factor, accounts for the phase difference between voltage and current caused by inductive or capacitive loads (motors, transformers, some electronics). Purely resistive loads — heaters, incandescent bulbs — have a power factor at or near 1.0, so P = V × I applies directly. Motors, pumps, and compressors commonly run at 0.7–0.9.

Example: 230 V × 5 A × 0.8 = 920 W.

Power factor matters because ignoring it on inductive loads overstates real power and can lead to undersized wiring or an underpowered inverter. It's less important for simple resistive appliances, where PF is already close to 1.

Three-phase power calculation

Three-phase systems are common in industrial and commercial settings. The formula introduces √3 (≈1.732) to account for the phase relationship between the three conductors:

Where V is the line-to-line voltage — the voltage measured between any two of the three phase conductors, not the phase-to-neutral voltage. Mixing these up is one of the most common three-phase calculation errors.

Example: 400 V × 10 A × 0.8 × √3 ≈ 5,543 W.

Watts vs volts vs amps

A common conceptual aid compares electricity to water flowing through a pipe: voltage is like water pressure, current is like flow rate, and power is the combined result of both. Like any analogy, it's a simplification meant to build intuition, not an exact physical equivalence — use the formulas above for real calculations.

Watts vs kilowatts

A kilowatt is simply 1,000 watts: 1 kW = 1,000 W. Kilowatts are typically used for larger loads — whole appliances, inverter capacity, or household demand — where expressing everything in watts would be unwieldy.

• 500 W = 0.5 kW
• 1,500 W = 1.5 kW
• 2,500 W = 2.5 kW

Watts vs watt-hours

This distinction trips up more people than any other on this page. Watts measure power — the instantaneous rate of energy use. Watt-hours measure energy — power sustained over a period of time.

A 100 W appliance running for 5 hours consumes 100 × 5 = 500 Wh, assuming the stated power draw stays constant over that time. This is the calculation behind battery capacity planning, solar sizing, and utility bills — all of which bill or measure energy (Wh/kWh), not instantaneous power (W).

Real-world watt examples

The figures below are illustrative examples for calculation practice, not universal appliance ratings — always check your own equipment's nameplate for its actual rated power.

Inverter load example

Watt calculations are the starting point for estimating an inverter's continuous load. Adding up individual appliance ratings gives a running total:

Real inverter sizing goes further than this running total — it also depends on surge (starting) power, inverter efficiency, battery voltage, battery capacity, power factor, and how long the continuous load runs. Use the Inverter Size Calculator to size a system properly.

Solar example

A calculated load in watts is a useful input for solar planning, but watts alone don't determine panel or battery size. Solar sizing also depends on daily energy consumption, peak sun hours at your location, system losses, battery requirements, and inverter efficiency. Run your numbers through the Solar Panel Calculator for a complete estimate.

Battery example

Battery power follows the same P = V × I relationship, while stored energy is calculated as watt-hours: Wh = V × Ah. Actual runtime varies with efficiency, discharge rate, battery chemistry, temperature, and depth of discharge — model it with the Battery Runtime Calculator.

Watt calculation table

Quick-reference values using the basic relationship P = V × I. These do not include an AC power factor unless stated — for real AC loads, apply the power factor formula above.

Common watt calculation mistakes

• Confusing volts with watts. Voltage alone says nothing about power draw.
• Assuming amps alone determine watts. Current without voltage is an incomplete picture.
• Ignoring power factor in AC systems. This overstates real power for inductive loads like motors.
• Confusing watts with watt-hours. Power (rate) and energy (total consumed) are different quantities.
• Forgetting inverter and system losses. Real-world efficiency is never 100%.
• Ignoring starting or surge loads. Motors and compressors often draw several times their running power at startup.
• Assuming nameplate power is always actual consumption. Nameplate values are typically maximums, not continuous averages.
• Using the wrong voltage in three-phase calculations. Line-to-line and line-to-neutral voltages are not interchangeable in the formula.

Frequently asked questions