Introduction
The generator sizing formula looks almost too simple — add running watts, add the biggest surge, add headroom. But each term exists to solve a specific real-world failure, and misunderstanding any one of them is how people end up with generators that stall, trip, or cost twice what they needed. This guide takes the formula apart piece by piece.
The Generator Size Calculator applies this exact logic automatically from your appliance list. Read on to understand what it’s doing under the hood — and to sanity-check its answer before you spend money.
The Formula at a Glance
Minimum generator running watts = (sum of all running watts) + (largest starting watts − that appliance’s running watts) + 20–25% headroom
Expressed more compactly: total running watts, plus the extra surge of the single largest motor load, plus a safety margin. Divide by 1,000 for kW. That’s the whole thing — now the why behind each piece.
Term 1: Sum of Running Watts
This is your steady-state load — everything operating normally at once. The reason we sum everything rather than just the big loads: small loads add up. Ten 60-watt lights are 600 watts, nearly a whole refrigerator. Undercounting the small stuff is one of the quietest ways to undersize, covered in the common mistakes guide.
Where the numbers come from: appliance nameplates list watts directly, or list amps and volts, in which case watts = volts × amps. A furnace fan drawing 4.2 A at 120 V is 4.2 × 120 = 504 W running. For 240 V loads, multiply by 240. The estimating guide walks through reading nameplates and building the list.
Term 2: The Largest Single Surge (the Extra, Not the Whole)
Motor-driven appliances — refrigerators, AC units, furnace fans, well and sump pumps — draw 2–3× their running watts for a fraction of a second at startup while the motor overcomes inertia. Resistive loads (heaters, lights, microwaves) and most electronics draw ~1×, so they contribute nothing extra here.
Two subtleties people get wrong:
Why only the largest surge? In practice, you plan for the worst single startup moment, not every motor starting simultaneously. Motors cycling at exactly the same instant is unlikely, and planning for all-at-once surges produces comically oversized answers. (If two large motors genuinely will start together — say a well pump and a central AC on the same thermostat call — size for the combined surge of those two; that’s a judgment call the formula can’t make for you.)
Why subtract that appliance’s running watts? Because they’re already in Term 1. The “starting watts” figure on a nameplate or in a table is the total draw during startup, which already includes the appliance’s own running draw. Adding the full starting number on top of the running total double-counts the appliance. The correct addition is the extra — starting minus running. Mathematically identical and easier to see: (total running − appliance running) + appliance starting.
Term 3: 20–25% Headroom
The margin covers three realities: loads you forgot (phone chargers, the garage freezer), nameplate numbers that run optimistic, and the fact that engines perform worse hot, old, and at altitude. It also keeps the generator running below its continuous rating, where voltage regulation is steadier, fuel efficiency is better, and the engine lasts longer. Headroom isn’t padding for its own sake — it’s the difference between a generator that handles a surprise load gracefully and one that trips.
Full Worked Example
A homeowner’s essentials list:
| Appliance | Running W | Starting W | Extra surge |
|---|---|---|---|
| Refrigerator | 700 | 2,200 | 1,500 |
| Furnace fan | 500 | 1,200 | 700 |
| 10 lights | 600 | 600 | 0 |
| Microwave | 1,200 | 1,200 | 0 |
| Sump pump | 1,000 | 2,000 | 1,000 |
Term 1: 700 + 500 + 600 + 1,200 + 1,000 = 4,000 W running.
Term 2: the largest starting figure is the refrigerator’s 2,200 W. Extra surge = 2,200 − 700 = 1,500 W. Subtotal = 4,000 + 1,500 = 5,500 W.
Term 3: 5,500 × 1.20 = 6,600 W → 6.6 kW.
Answer: a generator with at least ~6.6 kW continuous rating — in practice, a 7–7.5 kW portable. Watch how the traps play out: skipping surge gives 4,800 W (a generator that stalls on fridge startup); adding every surge gives over 8,400 W (paying for capacity that will never be needed at once).
Second Example: Minimal Loads
Suppose you only need a fridge (150 W running / 450 W starting), LED lighting (100 W), phone/laptop charging (150 W), and a Wi-Fi router (30 W). Running total = 430 W. Largest surge = fridge: 450 − 150 = 300 W extra. Subtotal = 730 W. With 25% headroom: ~915 W. A 1 kW unit covers it — a case where a small inverter generator beats a big portable on fuel, noise, and cost. The worked case study shows how these decisions play out for a full household.
Unit Conversions You’ll Need
- Watts ↔ kilowatts: 1 kW = 1,000 W. Generator ratings are usually sold in kW; your math is usually easier in watts.
- Amps ↔ watts: W = V × A. At 120 V, a 15-amp circuit’s theoretical max is 1,800 W; at 240 V, a 20-amp load is 4,800 W.
- Running vs. peak ratings: a “7,500 W” portable is often 7,500 W peak / 6,000 W continuous. The formula’s answer must fit under the continuous rating, not the peak number on the box.
Related Guides
- Complete Guide — the full sizing walkthrough with appliance tables.
- Estimating Guide — building an accurate appliance list from nameplates.
- Common Mistakes — where the formula’s inputs usually go wrong.
- Case Study — a hypothetical household sized end to end.
- FAQ — quick answers on watts, surges, and sizing.
Get Your Number
The formula is simple; getting honest inputs is the real work. Inventory your loads carefully, respect the surge, keep your headroom, and let the Generator Size Calculator do the arithmetic — then verify the answer against the continuous (not peak) rating of any generator you’re considering.
Disclaimer: Calculator results are planning estimates based on the values you enter. Always verify wattage against your equipment nameplates; actual loads vary by model, age, and condition. Generator connections to home wiring and any transfer equipment must be installed by a licensed electrician and must comply with the National Electrical Code and your local code. Never operate a generator indoors or in enclosed spaces.
