Generator Size Calculator
Add up your running loads and the biggest motor surge to get the minimum generator rating — with the sizing logic that prevents nuisance shutdowns.
Last updated: 2026-09-15
How the calculation works
- Running loads are multiplied by 1.25 to keep the generator out of continuous overload.
- The larger motor's starting surge is compared directly — induction motors draw 2–3× running power at startup.
- The generator must satisfy whichever requirement is larger.
Formula
Required = max(running × 1.25, surge)
| Symbol | Meaning | Unit |
|---|---|---|
W_run | Simultaneous running load | W |
W_surge | Motor starting requirement | W |
Worked example
Interpreting the result
Undersized generators stall on motor starts and run hot at continuous load; oversized units waste fuel and suffer carbon buildup at light load (wet stacking). The sweet spot is running load at 50–75% of rating. Inverter generators handle electronics cleanly; conventional units are fine for resistive and motor loads.
Assumptions
- Single largest surge dominates — multiple motors starting simultaneously need their sum.
- Resistive and electronic loads have no surge; only motors and compressors do.
Limitations
- Air conditioners and well pumps can surge 4–5× — check nameplate LRA.
- Transfer switch and grounding requirements are code matters, not sizing ones.
Frequently asked questions
What size generator for a house?
For essentials (fridge, furnace, lights, phone charging): 3–5 kW. Adding a well pump or AC pushes it to 7–10 kW. Whole-house with central AC: 14–20 kW standby units.
Why 1.25 times the running load?
Generators shouldn't run above ~80% of rating continuously — the margin also absorbs measurement error and future loads.