Calculate the total load wattage of your appliances, including running and starting watts, and estimate required running and surge capacity with a user-selected margin.

Starting watts are total watts during startup, not extra watts. Blank starting watts are unknown; blank quantity means 1.

Estimate only. Check appliance nameplates and both generator running and surge ratings. Single-device AC calculations require true power factor.

20% is an editable planning allowance, not a universal requirement.

Related Calculators

Load Wattage Formula

The following equation applies to DC or a unity-power-factor single-phase load. For single-phase AC real wattage, use W = V ร— I ร— PF with RMS voltage/current and measured true power factor.

W = V ร— I
  • Where W is the wattage (W)
  • V is the voltage (V)
  • I is the current (A)

To calculate the missing measurement, multiply voltage by current to find wattage or use division when voltage or current is missing.

What is Load Wattage?

Definition:

Load wattage is real electrical power consumed by a device, measured in watts. It equals voltage ร— current for DC; single-phase AC also requires power factor.

How to Calculate Load Wattage?

Example Problem:

The following example outlines the steps and information needed to calculate the load wattage.

First, determine the voltage of the circuit. In this example, the voltage is 120 V.

Next, determine the current of the circuit. Here, the current is 5 A.

Finally, calculate the wattage using the formula above:

W = V \times I

W = 120 ร— 5

W = 600 W

FAQ

What factors can affect the load wattage calculation?

The load wattage calculation can be influenced by the accuracy of voltage and current measurements, fluctuations in the power supply, and additional circuit losses that are not accounted for by the simple multiplication of voltage and current.

Why should one leave one field empty in the calculator?

Choose the value to solve for. In appliance mode, leave unused rows blank, use total starting watts for each device, and choose whether devices start one at a time or together.

Can this calculator be used for both AC and DC circuits?

Choose DC/unity power factor or single-phase AC. The AC mode requires true power factor and RMS voltage/current; it does not calculate three-phase loads.