Running watts vs. starting watts

Why motors can be harder to start

An induction motor can draw substantial inrush current before it reaches operating speed. The amount and duration depend on motor characteristics, the mechanical load and the starting method. Starting is therefore a transient event, rather than simply another continuous appliance load. [2]

A generator’s response includes changes in voltage and frequency as the load is applied. Sizing a motor load may require starting kVA, starting power factor, acceptable voltage dip and acceleration time. A running-watt total alone does not describe that response. [2][3]

Total starting watts and extra starting watts

Published tables use different conventions. “Starting watts” may mean the device’s total demand during startup, while “additional starting watts” means only the increase above its running demand. The column heading and documentation determine which interpretation applies. [1][4]

Illustration: a device using 500 W when running and 1,800 W total at startup has 1,300 W of additional starting demand. Adding the full 1,800 W to a running-load total that already includes its 500 W would count the running portion twice. Conversely, subtracting running watts from a value already labeled additional would undercount it.

Starting sequence and simultaneous loads

Consider the loads that are operating while a motor starts. For each relevant event, combine those existing loads with the starting requirement of the motor or motors starting at that time. Compare the most demanding credible events, as well as the steady running condition. [3][5]

A worksheet using total running watts plus the largest single additional starting value assumes that only one motor starts at a time. That assumption needs verification. Automatic restart after an outage, cycling compressors or pumps, and controls that bring several loads back together can produce a different requirement. Adding every appliance’s surge value also ignores the actual sequence.

Using equipment data

Use manufacturer documentation or appropriately measured data for the equipment involved. Approximate appliance charts are starting points, not universal ratings. The generator’s continuous output and transient capability require separate checks, including the fuel-specific rating, voltage, phase and connection limits. [3][4]