Real starting watts, from nameplate data
Almost every wattage chart online estimates startup surge by multiplying running watts by two or three. For a fridge that's close enough. For a submersible well pump it's off by thousands of watts — and that gap is the single most common reason a generator won't start something it looks big enough to run.
The figures below come from published or professionally-cited motor nameplates — Franklin Electric submersible motors, Copeland scroll compressors, Goulds effluent pumps — rather than a multiplier. Where we couldn't find real nameplate data, we say so instead of filling the gap with an estimate dressed up as a measurement.
Measured vs. the rule of thumb
Starting watts derived from real locked-rotor amps (amps × volts), next to what a 3× running-watts estimate would have predicted for the same appliance.
| Appliance | Running | Real starting | 3× estimate | Understated by |
|---|---|---|---|---|
| Central AC (3-ton / 36,000 BTU) | 3,500 W | 25,750 W | 10,500 W | +15,250 W |
| Sump Pump (1/2 HP) | 1,050 W | 2,640 W | 3,150 W | −510 W |
| Sump Pump (1 HP) | 2,000 W | 8,325 W | 6,000 W | +2,325 W |
| Well Pump (1/2 HP) | 1,000 W | 7,400 W | 3,000 W | +4,400 W |
| Well Pump (1 HP) | 2,000 W | 11,200 W | 6,000 W | +5,200 W |
A positive number means the rule of thumb would have told you to buy too little generator. It isn't wrong every time — the 1/2 HP sump pump starts at about 2.5× its running watts, so a 3× estimate covers it with room to spare. It's the pumps and compressors at the top of the table, running 5.6× to 7.4×, where the shortcut quietly fails. Every appliance above is a motor or compressor load; resistive loads (ovens, dryers, kettles, heaters) have no meaningful inrush and aren't shown, because for those the running figure genuinely is the whole story.
What locked rotor amps actually is
The instant a motor is energized, the rotor hasn't started turning yet. A spinning motor generates a back-EMF that opposes the incoming current and limits it; a stationary one generates none, so for a fraction of a second the winding behaves close to a short circuit. That inrush is locked rotor amps — the highest current the motor will ever draw, and the number a generator has to survive to get the thing spinning at all.
Locked rotor current typically runs five to eight times a motor's running current. That's the root of the discrepancy: consumer sizing charts apply their multiplier to watts and land near 2-3×, while the underlying electrical reality is a multiple of currentat 5-8×. For small motors the difference is absorbed by the headroom everyone builds in anyway. For a 1 HP submersible pump it's the difference between a generator that starts it and one that trips.
Finding the number for your own appliance
Check the nameplate — the metal plate or sticker on the motor, compressor, or control box. Two things to look for, in order:
- An explicit "LRA" or "Locked Rotor Amps" figure. Multiply it by the motor's voltage and you have starting watts directly. A pump rated 48.7 LRA at 230V starts at roughly 11,200W.
- A "kVA Code" or "Code Letter" — a single letter, A through V. NEC Table 430.7(B) maps that letter to locked-rotor kVA per horsepower. Multiply by the motor's horsepower for total locked-rotor kVA, then divide by voltage for amps. Code letter G — the most common on standard NEMA Design B motors — is 5.6 to 6.29 kVA per horsepower.
locked-rotor kVA = 1 HP × 6.0 kVA/HP = 6.0 kVA
LRA = 6,000 VA ÷ 230V ≈ 26 A
starting watts ≈ 26 A × 230V ≈ 6,000W
(a submersible pump motor of the same horsepower publishes 48.7 LRA — code-letter math gives you a floor, not the whole story)
Where we don't have real data
These are motor loads a soft starter would apply to, but we couldn't find published nameplate LRA for the appliance class. The figures we show are estimates, and we'd rather label them than quietly present them alongside measured data:
| Appliance | Running | Starting (estimated) |
|---|---|---|
| Window AC (5,000 BTU) | 600 W | 1,800 W |
| Window AC (10,000 BTU) | 1,200 W | 3,600 W |
| Dehumidifier | 500 W | 1,500 W |
| Sump Pump (1/3 HP) | 800 W | 1,300 W |
| Sewage Ejector Pump (1/2 HP) | 1,050 W | 2,150 W |
| Pool Pump (1.5 HP) | 1,500 W | 4,500 W |
Cord-connected room air conditioners are a specific case: they're exempt from locked-rotor labeling under UL 484, so no true nameplate figure exists for that class at all. Inverter-driven mini-splits are another — they ramp the compressor electronically rather than hard-starting it, so there's no locked-rotor event to publish, and a soft starter has nothing to act on.
Frequently asked questions
What are locked rotor amps (LRA)?
Locked rotor amps is the current a motor draws in the instant it starts, while the rotor is still stationary and generating no back-EMF to oppose the incoming current. It's the highest current the motor will ever pull, it lasts a fraction of a second, and it's what a generator has to survive to get the motor spinning. Multiply LRA by the motor's voltage to get starting watts.
Why is my well pump's real starting wattage so much higher than wattage charts say?
Because most charts estimate startup surge as roughly two to three times running watts, which is a reasonable approximation for many appliances but badly wrong for submersible pump motors. Locked rotor current typically runs five to eight times a motor's running current, and submersible well pump motors sit at the high end. A 1 HP Franklin Electric submersible motor publishes 48.7 locked rotor amps at 230V — 11,200W of startup surge against 2,000W running, a ratio closer to 5.6x than 3x.
How do I find the LRA for my own appliance?
Check the nameplate — the metal or sticker label on the motor, compressor, or control box. Look for 'LRA' or 'Locked Rotor Amps' directly. If it isn't listed, look for a 'kVA Code' or 'Code Letter' (a single letter, A through V): NEC Table 430.7(B) maps that letter to locked-rotor kVA per horsepower, which you multiply by the motor's horsepower and divide by voltage to get amps. Code letter G, the most common for standard NEMA Design B motors, is 5.6 to 6.29 kVA per horsepower.
Does a soft starter reduce locked rotor amps?
Yes, substantially — a soft-start device ramps voltage to the motor instead of applying it all at once, typically cutting the inrush by around two thirds. That's why a soft starter can let an air conditioner or well pump run on a generator that otherwise couldn't start it. It only applies to single-phase motor and compressor loads; it does nothing for resistive loads like ovens and dryers, which have no inrush to reduce.
Why do generators fail to start motors they seem big enough for?
Because the generator is sized against running watts, or against an underestimated surge figure. When the motor's real locked-rotor draw exceeds what the generator can momentarily deliver, voltage sags, the motor stalls instead of spinning up, and the overload breaker trips — usually in the dark, during the outage you bought the generator for. Sizing against the real nameplate surge is the fix.
Sources
- Franklin Electric — AIM Manual, submersible motor specifications (LRA tables)
- ABB — How to read a NEMA motor nameplate
- Engineering ToolBox — Electrical motors, locked rotor design code letters