What Size Generator Do I Need? — Sizing Guide with Real Model Picks
Sizing & selection · Updated 2026-06-29
What Size Generator Do I Need?
Short answer: add up the running watts of everything you'll run at once, then add the starting (surge) watts of the single largest motor — a well pump, AC compressor, or air compressor can pull 3–7× its running watts for a second or two at startup. Take that total and add 20–25% headroom so the generator never runs flat-out. That number, in watts (or kVA × 0.8 for kW), is the size you need. Most homes land at 10–22 kW; a 50-amp RV needs about 10–15 kW.
Once you have a target kW, you can jump straight to real models: our generator directory lets you filter every model by kW band, and the generators-by-size pages group them for you. The size-band table near the bottom shows exactly how many real models exist at each rating.
Running watts vs. starting watts — what's the difference?
Direct answer: running (rated) watts is the steady draw once a device is on; starting (surge) watts is the brief spike when an electric motor spins up. Resistive loads (lights, heaters, kettles) have no surge — their starting and running watts are equal. Motors are what blow your sizing budget.
| Load type | Starting surge | Why |
|---|---|---|
| Lights, heaters, chargers (resistive) | ~1× running | No motor inrush |
| Refrigerator / freezer | ~2–3× | Compressor motor |
| Well pump, AC compressor, air compressor | ~3–5× | Induction-motor locked-rotor inrush |
| Deep-well submersible / hard-start motors | up to ~6–7× | High torque demand at start |
Sizing rule: size for running watts of everything on at once plus the largest single surge, since only one big motor typically starts at a time. Then add 20–25%.
What's the running and starting wattage of common loads?
Direct answer: use these typical figures to build your total. Values are approximate — check the nameplate on your own equipment, then start the largest motor first so its surge has room.
| Load | Typical running watts | Typical starting watts |
|---|---|---|
| RV rooftop AC, 13,500 BTU | 1,300–1,800 | 2,800–3,500 |
| RV rooftop AC, 15,000 BTU | 1,500–2,000 | 3,300–4,000 |
| Refrigerator / freezer | 600–800 | 1,800–2,400 |
| Window AC, 1 ton (12,000 BTU) | 1,200–1,500 | 3,000–4,000 |
| Well pump, 1 HP | 1,000–1,200 | 2,300–3,000 |
| Well pump, 1.5 HP | 1,600–2,000 | 4,000–5,000 |
| Air compressor, 4 HP single-phase | 2,800–3,200 | 8,000–12,000 |
| Microwave / electric kettle | 1,000–1,500 | same (resistive) |
| Sump pump, 1/3 HP | 800 | 1,300–2,100 |
| Furnace blower, 1/2 HP | 600–900 | 1,500–2,400 |
These motor-driven loads are exactly the equipment we track in the corpus: it holds 1,823 compressor, 296 welder, and 58 pump power-equipment applications, every one an induction-motor load with a real start-up surge.
What size generator do I need for a 50-amp RV?
Direct answer: a 50-amp RV service is 50 A × 240 V = 12,000 VA, so a 10–15 kW generator covers it — enough to run two roof ACs plus the microwave, fridge, and converter. A 30-amp RV (3,600 VA) is happy on a 3–4 kW unit.
That 10–15 kW window is one of the deepest in our catalog: 1,111 generator models fall in the 10–15 kW standby band. Examples you can spec right now: Kohler 10RESV (10 kW), Kohler 14RES (10 kW), and Kohler 20RES (16 kW). Browse the full set on the Under-20-kW generators page.
How do I size a generator for a water pump or air compressor?
Direct answer: size for the surge, not the running load. A 7.5 HP single-phase pump motor draws ~5.6 kW running but can spike past 20 kW at start — practically you want a 15–20 kW generator (or a 3-phase supply) so the inrush doesn't stall it. An 80-gallon, 4 HP air compressor runs around 3 kW but surges to 8–12 kW, so a 7–10 kW generator with good motor-start capability is the floor.
The math: take the motor's running kW, multiply by the surge factor for its type (3–5× typical, up to 7× for hard-start), and make sure your generator's surge/starting watt rating clears that number. Then confirm the running watts still leave 20% headroom. For motors above ~5 HP, a soft-starter or a 3-phase generator dramatically lowers the inrush and lets you use a smaller set.
How many air conditioners can a 15, 20, or 30 kVA generator run?
Direct answer: convert kVA to usable kW at 0.8 power factor, budget ~3 kVA per 1-ton AC (running plus surge headroom), and stagger the starts so only one compressor inrushes at a time. On that basis:
| Generator | Usable kW (0.8 PF) | 1-ton (1 HP) ACs, staggered start | Notes |
|---|---|---|---|
| 15 kVA | ~12 kW | 3–4 | A single 2 HP AC (~6 kVA surge) leaves room for little else |
| 20 kVA | ~16 kW | 4–5 | Comfortable for a small office / clinic |
| 30 kVA | ~24 kW | 6–8 | Start the largest unit first; hold ~20% spare |
A 1 HP AC (≈1.2 kW running, ~3 kVA surge) sits comfortably on a 3 kVA generator with nothing else loading it. The same staggering logic applies to a 9 kW 3-phase oven (mostly resistive — size for ~11 kW with headroom) or a 6.5 kVA generator feeding a 7.5 kVA inverter (undersized — the inverter can demand more than the genset can deliver under load, so match or exceed the inverter rating).
Real generator models by size band
Direct answer: here's how our 15,424 rated models spread across the standard kW bands, with real examples in each. Pick your target kW from the sizing rules above, then click through to live spec pages.
| Size band | Models in our catalog | Example models (click to spec) |
|---|---|---|
| Under 10 kW | 1,013 | FG Wilson K6-3S · FG Wilson P7.5-4S |
| 10–25 kW (RV / home) | 2,328 | Kohler 10RESV · Kohler 20RES · Perkins 403D-11 |
| 25–60 kW (light commercial) | 2,909 | Kohler 25REZG · Kohler 30REYG · FG Wilson P50-1 |
| 60–150 kW (commercial) | 2,873 | Kohler 60REZG · Perkins 1104C-44TG1 · FG Wilson P110 |
| 150–500 kW (industrial) | 2,857 | FG Wilson P200H · Caterpillar 3406 · Caterpillar SR4 |
| 500 kW and up (prime power) | 3,444 | Kohler 500EOZCS · Caterpillar 3512 · Caterpillar 3516 |
Browse any band directly: under 20 kW · 20–60 kW · 60–150 kW · 150–500 kW · 500 kW and up. Or compare makers on the brands page.
Key takeaways
- Size = running watts (all loads on at once) + the single largest motor's starting surge + 20–25% headroom.
- Motors are the trap: pumps, ACs, and air compressors surge 3–7× their running watts at startup. Resistive loads (heaters, ovens, lights) don't surge.
- Start the biggest motor first so its inrush has the whole generator behind it; staggering lets a smaller set handle more total load.
- 50-amp RV → 10–15 kW (1,111 real models in that band). 30-amp RV → 3–4 kW.
- kVA → kW: multiply by 0.8 power factor. A 30 kVA set = ~24 usable kW.
- For motors above ~5 HP, a soft-starter or 3-phase supply cuts inrush and lets you use a smaller generator.