Watts Explained: Running vs. Starting vs. Surge

Generators list two wattage numbers, and they mean different things. Running watts is what the generator can supply continuously. Starting (or surge) watts is a short burst it can supply for a few seconds. Knowing the gap between them keeps you from buying a unit that trips off the first time your fridge kicks on.

Running watts

Running watts, also called rated or continuous watts, is the power a generator can deliver for hours. Every device you plug in draws some of it. If the total of what you plug in goes over the rated number, the generator overloads and shuts down or trips its breaker.

Starting and surge watts

Devices with motors or compressors, such as refrigerators, air conditioners, well pumps and power tools, pull extra power for a moment when they start. This surge is often two to three times their running watts. A generator's starting or peak rating tells you how much of that burst it can cover. The burst lasts only a few seconds, so it never counts toward the running total.

An example

A refrigerator might run at about 150 W but need 600 W or more for a second or two at startup. A generator rated for 2,000 running watts and 2,400 starting watts handles that easily. A generator rated 500 running and 600 starting does not.

Watts, volts, amps and watt-hours

These four units come up in every spec sheet:

• Watts (W) measure how fast power is used right now
• Volts (V) measure electrical pressure; US outlets are 120 V
• Amps (A) measure current; a standard outlet is typically rated for 15 A
• Watt-hours (Wh) measure how much energy is stored or used over time

Gas generators are rated in watts. Power stations are rated in both watts (how much they can run at once) and watt-hours (how long they can run it).

How to use this when shopping

1. Add up the running watts of everything you want to power at the same time
2. Find the single device with the biggest startup surge and add its extra surge watts
3. Choose a generator whose running rating covers step 1 and whose starting rating covers step 2
4. Leave about 20 percent headroom so the unit is not working at its limit

Our sizing guide walks through this with a wattage chart for common appliances.