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.
Open calculatorLast 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.
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.
Running watts vs starting watts
Running watts (rated watts) are what a device draws continuously: a refrigerator's compressor while running, a furnace fan at steady speed, every light bulb. Starting watts (surge watts) are the brief spike when a motor or compressor kicks on — typically 2–3× its running draw, up to 4–5× for hard-starting loads like well pumps and air conditioners. The surge lasts a second or two, but the generator must supply it without stalling or tripping. Sizing therefore always compares two numbers: total running load (with margin) and the single largest starting requirement — the calculator takes the larger of the two, which is the sizing formula in one line: max(running × 1.25, largest surge).
kW, kVA and power factor on a generator nameplate
Real power (kW) does the work; apparent power (kVA) is what the alternator actually delivers, and they relate through the power factor: kW = kVA × PF. Generator nameplates are usually rated in kVA at a stated power factor — commonly 0.8 for standard three-phase sets (so a 10 kVA unit delivers 8 kW) — while small portable inverter generators are usually rated directly in watts for resistive loads. The catch is that your load mix sets the real PF: resistive loads (heaters, incandescent lights) run near PF 1.0, while motors sit around 0.8 at full load and lower at part load. If your loads are motor-heavy, the alternator's kVA ceiling can bind before the engine's kW ceiling does — which is another reason the surge figure, not just running watts, drives the sizing.
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.
What is the generator sizing formula?
Required kW = max(running watts × 1.25, largest motor surge) ÷ 1000. Add the surge of every motor that could start simultaneously rather than just the largest one.