Generator Size Calculator
Check the boxes for what you'd want running in an outage, pick your well pump size, and get a planning range in kilowatts — built from manufacturer spec-sheet wattages and rounded up. This is a planning estimate, not a purchase order: confirm it with a licensed electrician before you buy anything.
How to read this:
Starting watts vs. running watts, in plain English
Running watts are what an appliance draws once it's going — cruising speed. Starting watts are the brief shove needed to get a motor spinning from a dead stop: for two or three seconds, a motor can pull three to eight times its running current. Your generator has to ride through that shove while everything else is already running.
The well pump is usually the villain. It starts under water load, and its starting draw — measured in locked-rotor amps on the motor spec sheet — is the single biggest surge in most rural homes. It's the most common reason an undersized generator trips the moment the pump kicks on.
Not everything surges. Anything that makes heat from electricity — a water heater, a microwave, lights — draws the same watts from the first second. That's why this calculator adds the single largest motor surge to the running total, rather than every surge at once: your motors don't all start in the same two seconds.
Honest limits
- A planning estimate, not a purchase order. Every wattage above is a manufacturer worksheet or spec-sheet default. Your actual equipment has a nameplate — amps × volts is always truer than a default. Confirm the final size with a licensed electrician before buying; show them your load list and your nameplates.
- Central air varies a lot. The 2-ton figure is a worksheet estimate; real condensers vary by tonnage, age, and efficiency, and some need far more to start. An electrician should size from your condenser's nameplate — and a soft-start kit can change the answer substantially.
- What's not in this tool: electric range, dryer, and electric heat strips — deliberately. Those are multi-kilowatt loads that belong in an electrician's sizing conversation, usually with load management. Also not covered: fuel type and runtime, transfer-switch sizing, or which circuits go on backup.
- Portable or standby, the math is the same — but compare running watts to running watts. Portables often advertise their surge rating up front; size from the continuous (running) rating.
Where every wattage comes from
No invented numbers. Every default traces to a manufacturer spec sheet or a published generator sizing worksheet:
| Load | Default (running / starting) | Source |
|---|---|---|
| Well pump 1/2, 3/4, 1 HP | 1,000 / 7,700 · 1,500 / 9,800 · 2,000 / 11,700 W | Franklin Electric AIM Manual, Table 13 (single-phase motor specs): running from max-load watts, starting from locked-rotor amps × 240 V |
| Sump pump, refrigerator, freezer, furnace blower, microwave | 600/1,800 · 1,200/2,400 · 500/500 · 1,200/2,000 · 1,000 W | Champion Power Equipment wattage reference chart (operator's manual) |
| Central AC (2-ton) | 3,800 / 11,400 W | Generator sizing worksheet (24,000 BTU central; worksheet estimate — verify against your condenser nameplate) |
| Electric water heater | 4,500 W, no surge | Generac Generator Sizing Guide, Table 2: hot water 4.5 kW (resistive) |
| Lights, outlets & small electronics | 3 W per sq ft of home | Generac Whole Home Sizing Worksheet (NEC Article 220 general lighting & receptacles) |
| Sizing method | Total running watts + the single largest motor starting surge, rounded up to the next standard generator band — the method used in the Champion and Generac sizing worksheets. | |
Frequently asked questions
Is bigger always better with generators?
No. An oversized generator costs more, burns more fuel, and — on diesel units — running far below its rating for long stretches causes wet-stacking and carbon buildup. Size to your actual load, then confirm with an electrician.
Why does the range show two sizes instead of one?
The lower number is the smallest standard size that covers your calculated load with the standard worksheet method. The upper number is the next band up — the headroom most electricians would actually install, since these are estimates and loads get added over the years. Our judgment, labeled as such.
My well pump nameplate lists amps, not watts. Which do I use?
The nameplate wins. Multiply amps × volts (usually 240 for a well pump) for running watts; the locked-rotor amps on the motor spec sheet give you the starting draw. Compare those to the defaults in this tool and use whichever is higher for planning.
Why is the lights-and-outlets number so high?
It uses the NEC general-lighting figure of 3 watts per square foot — the same method Generac's own sizing worksheet uses — which assumes most lights and outlets are live at once. In a real outage you'd run less, which is exactly why this calculator errs high.
Can I shrink the recommended size with load management?
Yes, and that's a common electrician move: a load-management module sheds big loads (like the water heater or AC) so the generator never sees them all at once. This tool doesn't model that — it's a straight sum — so treat the range as the no-management answer.
What size generator do most rural homes end up with?
For a typical 1,500–2,500 sq ft home on a well with a furnace, fridge, and sump pump, the math in this tool usually lands in the 14–22 kW range. Add central air or an electric water heater and it climbs fast — which is why the tool exists.
Want the full picture behind the numbers? Read how home backup power works. And if your well pump itself is the problem rather than the power, start with pump and pressure tank troubleshooting — or run the no-water triage if the taps are dry right now.