Generator Size Calculator
Running watts, ONE motor surge, the 20% comfort margin — and the honest class it maps to, from 3.5k inverter to 22k standby.
Generator Size Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Class ladder only — not a per-model recommendation.
- No fuel consumption or runtime model.
- No ATS/transfer-switch sizing.
In short: 1,900 W of essentials plus a 1,550-W start-up surge: 3,450 W peak — the max() law with the 1.2× margin picks 3,450 → the 3,500-W inverter class. The sump-pump variant, 3,000 running + 2,900 surge = 5,900 → the 6,500 portable class.
Formula
peak = running + ONE largest start-up extra
required = max(running × 1.2, peak)
class = next of 3.5k / 6.5k / 12k / 22k ≥ required
[('2-3×', 'motor start draw vs running, for a second'), ('ONE', 'surges stack one at a time, never summed'), ('1.2', 'the comfort margin — 1.25 if adding loads'), ('3.5k/6.5k/12k/22k', 'the class ladder')]
Worked Example
- List essentials' RUNNING watts — sum them.
- Find the biggest motor; take its start-up extra.
- Required = max(running × 1.2, running + surge).
- Round UP to the class ladder.
- Plan starts: stage motors one by one.
1,900 W running, 1,550 surge: peak 3,450 beats margin 2,280 → 3,450 W → 3,500 class. Sump house: 3,000 + 2,900 = 5,900 → 6,500 class.
Strengths & Limits Of This Model
Where this engine is strong
- The one-surge law stated as arithmetic, not folklore
- max() makes margin vs surge visible, not invisible
- Refuses beyond-class results honestly
Where it stops
- Four classes, not a continuous curve
- No diesel/RV subcategories
Practical Use Cases
Outage essentials
Fridge, furnace, lights, phones.
Sump & well homes
The pump-surge cases.
Camping / job site
Inverter class picking.
Transfer switch prep
The wattage the switch must carry.
Upsize checks
When the margin beats the surge.
Methodology & Editorial Standards
Required watts = max(running × 1.2, running + ONE largest start-up extra), then the smallest catalog class (3,500 / 6,500 / 12,000 / 22,000 W) at or above it. Results beyond the top class are refused with the engineered-standby guidance. Nonpositive running watts, negative surge, running past 25,000 W refused.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Generator Size Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What size generator for a house?
Add the RUNNING watts of what must survive an outage, add the largest motor's start-up extra, then apply max(running × 1.2, that peak). Essentials land near 3,500 W; pumps plus kitchen near 6,500; most of a house 12,000; everything 22,000 standby.
Why not add every appliance's surge watts?
Because they never start together — you stage starts. One surge rides on the running total at a time; summing every compressor's start-up draw triples the generator for no physics.
What is the 20% margin for?
A generator at 100% load runs hot, loud, and burns prime. The 1.2× headroom keeps it near 80% — and leaves room for the load you forgot.
Will a 3,500-watt generator run a refrigerator?
Yes, with room to spare — a fridge runs near 600 W with a start-up extra of roughly 2,100. The essentials bundle (fridge, freezer, furnace fan, lights, phones) fits inside the 3,500 class.
What size for a sump pump?
A ½-hp sump runs near 3,000 W with the well pump counted in — 3,000 running + 2,900 surge = 5,900 W, the 6,500 portable class. Basements size generators around pumps, not TVs.
What happens if I overload a generator?
The breaker trips — or worse, voltage sags and the motors you are trying to run overheat on thin power. The max() law plus the 20% margin exists so the trip never happens on the coldest night of the year.
Inverter or portable?
Inverters (3.5k class) run clean power for electronics and sip fuel at variable load; portables (6.5k+) give more watts per dollar but rougher power. Sensitive electronics want inverter or an in-line conditioner.
Can I plug the generator into a dryer outlet?
No — backfeeding without a transfer switch or interlock energizes the utility line outside and can kill a line worker. The transfer switch is not an accessory.