Loan Generator Sizing Calculator
Generators are sized by the hardest second, not the average hour. Tick what has to run in an outage and get running watts, the true starting surge (largest motor, not everything at once), a recommended size with headroom, and the shelf class it lands in.
Wattages are labeled planning figures for typical residential appliances — your nameplates win every argument. The surge math assumes only the single largest motor starts at the worst moment (the standard sizing method); electric resistance loads (dryers 5,400 W, electric water heaters 4,500 W, ranges) are deliberately not on the checklist because putting them on a portable generator is rarely the right outage plan. Whole-home standby systems and transfer switches are electrician territory.
How it works
Running watts = the sum of everything ticked, at its running draw. Starting requirement = running watts + the single largest surge gap (starting − running) among ticked appliances, because motors do not all start in the same second. Recommended size = starting requirement × 1.25 headroom, so the machine cruises at a sane load instead of living at redline. The class is the next standard shelf size at or above the recommendation (2,000 / 3,500 / 5,500 / 7,500 / 9,500 / 12,000 / 14,000 / 17,500 / 22,000 / 26,000 W).
Formula & worked example
Formula: Running watts = Σ running draws of ticked appliances. Starting requirement = running watts + max(starting − running) across ticked appliances. Recommended = starting requirement × 1.25 headroom. Class = next standard size up (2,000 / 3,500 / 5,500 / 7,500 / 9,500 / 12,000 / 14,000 / 17,500 / 22,000 / 26,000 W). Appliance draws are labeled planning figures; nameplates override.
Worked example: Fridge + furnace fan + lights + sump + TV + microwave: 3,200 W running, 4,250 W starting requirement, ≈5,300 W recommended — buy the 5,500 W class.
When to use this
- Storm season is coming and the old unit (or no unit) needs replacing.
- You are choosing between two shelf sizes at the store.
- You need to know whether the well pump or AC makes the list.
- You are speccing a standby system for an electrician conversation.
How to use it, step by step
- Tick only what must run — every tick is fuel and noise.
- Check the nameplates on your two biggest motors against the planning figures.
- Read the starting requirement, not just the running watts.
- Buy the class shown; if two classes bracket your number and a well pump or AC is involved, take the larger.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Appliance checklist | Each item carries planning running/starting watts | Fridge 150/1,200; furnace fan 800/1,200; sump 800/1,300 |
| Largest surge | Computed: the biggest single motor start among ticked items | Fridge surge gap 1,050 W in the example |
| Headroom | Fixed 25% above the starting requirement | Keeps load ≤ ~80% of rating |
Another worked example
Fridge + well pump + window AC + washer + lights + computer: running 3,200 W, largest surge the well pump’s 1,100 W gap, starting requirement 4,300 W — again the 5,500 W class after headroom. Water access plus modest cooling is achievable on a mid-size portable; central air is the line where it stops being true.
Frequently asked questions
What size generator runs a fridge, furnace fan, lights, sump pump, TV and microwave?
Running watts: 150 + 800 + 100 + 800 + 150 + 1,200 = 3,200 W. The largest surge gap is the fridge’s (1,200 − 150 = 1,050 W), so the starting requirement is 4,250 W; with 25% headroom the recommendation is about 5,300 W — a 5,500 W class machine. That is the classic "essentials" answer, and it is why 5,500 W portables sell out before every storm.
Why not just add up all the starting watts?
Because that overbuys dramatically. Motors surge for a second or two at start; the odds of every motor starting in the same second are negligible, and the standard method sizes for running load plus the one largest surge. Adding all surges for the example load would demand nearly 9,000 W for a job a 5,500 W unit handles all weekend.
Can a 5,500 W generator run central air?
A 3-ton central AC (3,500 running / 10,500 starting) breaks the math: swap the microwave for the AC in the example and the starting requirement hits 10,300 W before headroom — a 17,500 W class answer, and really a standby-generator conversation. A soft-start kit on the AC can cut its surge by half or more and changes the answer — that is an HVAC tech install.
Inverter generator or conventional?
For the same watts, inverters run quieter, sip fuel at partial load, and make cleaner power for electronics — at a higher price per watt. Conventional open-frame units are the value play for job-site-style loads. For CPAPs, computers and modern furnace control boards, the inverter’s clean sine wave is the safer default.
How much headroom is right?
25% is the planning standard used here: it keeps the engine in its efficient band, leaves room for the appliance you forgot, and absorbs the reality that rated watts are tested kindly. Running any generator above 80% for hours is loud, thirsty and hard on the machine.
What about a well pump?
A ½ hp well pump (1,000 running / 2,100 starting) is very portable-generator friendly: adding it and a window AC to the essentials list still lands in the 5,500 W class in this model (starting requirement 4,300 W). A 1 hp or larger pump is a different story — check the nameplate surge before promising showers.
How long will it run on a tank?
Roughly: a 5,500 W portable at half load burns about 0.5–0.75 gal/hour, so a 5-gallon tank is a 7–10 hour night. Fuel storage, stabilizer and a rotation plan are part of sizing in practice — the best generator is the one with gas in February. Propane dual-fuel models trade about 10% of wattage for shelf-stable fuel.