Also in: Disaster Response and Recovery
Generator Sizing
Continuous and surge wattage from running totals and the largest motor's starting draw.
Example
You enter
- Items name Refrigerator, running watts 700, starting watts 2200, and 2 more
You get
- Continuous 1900
- Surge 3400
- Running (kW) 1.9
- Surge (kW) 3.4
Checked against published worked examples from: Lee County Electric Cooperative (LCEC).
Details, formula, and sources
Formula or tableRunning kW = sum of the running watts of every load; surge kW = running kW + the largest single (starting − running) excess, i.e. the biggest motor starts while everything else runs. No efficiency, power-factor, or voltage-dip model; the generator's rated surge and the manufacturer's motor-starting data govern.
Source and editionNEC 2023 (NFPA 70). Article 700 / 701 / 702; NEMA MG 1 by name.
Free accessFree read-only access at nfpa.org/freeaccess.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesitems, running_W, surge_W, running_kW, surge_kW
- Motor starting one motor starts at a time, the one with the largest starting-over-running excess; starting watts are the TOTAL draw at start (nameplate or manufacturer figures); a wattage chart that lists 'additional starting watts' must have the running watts added back firstengineering practice