Utility July 13, 2026 · 8 Min Read

Wattage Calculator – Guide & Formulas

Calculate electrical power in Watts from voltage and current, or estimate appliance wattages for generator sizing.

Compute electrical power in Watts. Solve for power using Voltage, Current, or Resistance inputs, and refer to standard appliance profiles for sizing backup generators or inverters.

Key Takeaway

Use the free Wattage Calculator to calculate electrical power in watts from voltage and current, or estimate appliance wattages for generator sizing. Get instant results with step-by-step explanations.

How to Use the Wattage Calculator

  1. Select your calculation mode (e.g., Watts from Volts & Amps, or Watts from Volts & Ohms).
  2. Enter the corresponding electrical values in their respective boxes.
  3. If applicable, select a typical home appliance from the reference dropdown to load its average power profile instantly.
  4. Read the calculated active electrical power in Watts (W) and Kilowatts (kW).

The Formula

Power (Watts) = Voltage (Volts) * Current (Amps) | Power (Watts) = Current^2 * Resistance (Ohms)

Variable Definitions

  • P (Watts): Electrical power representing the rate of energy consumption or work performed
  • V (Volts): Electric potential difference or electromotive force
  • I (Amps): Electrical current or rate of electrical charge flow

Determining Microwave Power Draw

Find the wattage of a microwave plugged into a standard US 120-volt outlet that draws 10 Amperes of current.

  1. Step 1: Identify variables: Voltage V = 120V, Current I = 10A.
  2. Step 2: Choose formula: P = V * I.
  3. Step 3: Calculate: 120 Volts × 10 Amps = 1,200 Watts.
  4. Step 4: The microwave operates at 1.2 kW.

Frequently Asked Questions

What is the difference between Watts, Volts, and Amps?

Think of water in a pipe: Volts is the water pressure pushing the fluid, Amps is the volume of water flowing per second, and Watts is the total power delivered by the stream (Volts × Amps).

What is "surge wattage" or "starting wattage"?

Many motorized appliances (refrigerators, AC units, pumps) require 2 to 3 times their normal running power for a brief split-second to kickstart the motor. This is called surge wattage.

Is a kilowatt-hour (kWh) different from a Kilowatt (kW)?

Yes. A kilowatt (kW) measures power rate, while a kilowatt-hour (kWh) measures the cumulative amount of energy consumed over time. Using a 1 kW appliance for 1 hour consumes exactly 1 kWh of energy.