Generator sizing goes wrong in one of two directions: undersizing, which leaves you scrambling during the exact moment you need power most, or oversizing, which costs hundreds to thousands of dollars more than necessary. Getting it right comes down to understanding two numbers — running watts and starting watts — and doing honest math about what you'll actually run, both today and over the generator's realistic multi-year service life.
Running watts vs. starting watts
Running watts (sometimes called rated watts) is the continuous power draw once an appliance is up and running. Starting watts (or surge/peak watts) is a brief spike, usually lasting only a few seconds, that motor-driven appliances need to overcome initial resistance when they first switch on. A refrigerator that runs at 150 watts continuously might briefly draw 600–800 watts the instant its compressor kicks on.
Your generator's running watts rating must exceed the sum of everything running simultaneously. Its starting watts rating must cover that continuous total plus the single largest startup surge happening on top of it — not every motor's surge at once, since they don't all start at the exact same instant in practice, but the worst-case single surge.
Appliance load reference chart
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 150–400 | 600–1,200 |
| Sump pump (1/2 HP) | 800–1,050 | 1,300–2,150 |
| Well pump (1/2 HP) | 1,000–2,000 | 2,000–3,000 |
| Central AC (3 ton) | 3,000–3,500 | 7,000–10,500 |
| Window AC unit | 1,000–1,500 | 2,200–3,000 |
| Electric water heater | 3,000–4,500 | — |
| Electric range (one element) | 1,500–3,000 | — |
| Furnace fan/blower | 600–1,000 | 1,200–2,300 |
| Microwave | 600–1,200 | — |
| Lighting (per typical circuit) | 600–1,800 | — |
Motor-driven appliances (compressors, pumps, blowers) have a starting surge; resistive appliances (heating elements, incandescent-style lighting, most microwaves) generally don't. This distinction matters most when your load includes central AC or a well pump, since those surges dominate the sizing math.
Worked example: a typical essential-circuits load
Say you want to run a refrigerator, a sump pump, a window AC unit, and basic lighting during an outage:
- Refrigerator running: 200W · Sump pump running: 900W · Window AC running: 1,200W · Lighting: 800W
- Total continuous running load: 3,100W
- Largest single surge (window AC starting): 2,800W on top of the AC's own running watts already counted, so add roughly 1,600W of additional surge above its running contribution
- Minimum starting watts needed: roughly 4,700W
A generator rated for at least 3,500 running / 4,700+ starting watts comfortably covers this load with some margin. Undersizing here — say, a 3,000W unit — would trip an overload or fail to start the AC at all.
For whole-house standby sizing
If you're sizing a standby generator rather than a portable, the math shifts to kW instead of watts, and most homes fall into predictable bands: 14–18kW for essential circuits, 22–24kW for most homes including one central AC, and 26–36kW+ for larger homes with two AC units and heavier appliance loads. See our full whole-house backup comparison for real installed pricing at each tier. A licensed electrician running a proper load calculation is always worth the cost before committing to a standby purchase — it's a one-time expense that prevents a five-figure sizing mistake.
Sizing for future needs, not just today
A generator purchase, especially a standby installation, is a long-term commitment — typically 15-20+ years of service life. It's worth sizing with some margin for changes that might come during that window: an EV charger addition, a home addition or finished basement adding circuits, or a shift toward more electric appliances (induction range, heat pump) that draw more than the gas or combustion equipment they might replace. Building in headroom during initial sizing is far cheaper than upgrading a standby system's capacity later, which often means a full unit replacement rather than an incremental add-on.
A note on generator derating at altitude and temperature
Generator output ratings are typically based on standard conditions near sea level at moderate temperature. Real-world output drops at high altitude (roughly 3-4% per 1,000 feet above sea level for gasoline engines) and in very high ambient temperatures, since engines rely on air density for combustion efficiency. If you're in a mountain or high-desert region, or regularly run a generator in extreme summer heat, factor in a derating buffer beyond the baseline sizing math — a generator rated exactly to your calculated need at sea level in mild weather may fall short under your actual operating conditions.
Frequently Asked Questions
What's the difference between running watts and starting watts?
Running watts (also called rated watts) is the continuous power a generator can sustain. Starting watts (or surge/peak watts) is a brief higher output the generator can provide for a few seconds when a motor-driven appliance first kicks on. Your generator's running watts must exceed your total continuous load, and its starting watts must cover the single largest startup surge on top of everything else already running.
How many starting watts does a central AC unit need?
A typical central AC compressor draws 2-3 times its running wattage on startup. A unit that runs at 3,500 watts continuous can surge to 7,000-10,500 watts for a few seconds when the compressor kicks on — this surge, not the running wattage, is usually what determines your minimum generator size.
Should I size my generator for every appliance in my house?
No — size it for what you'll realistically run simultaneously during an outage, not everything you own. Most households prioritize refrigeration, a well or sump pump, some lighting, and either AC or heat, then decide the rest based on budget and generator class. A licensed electrician can help you build a realistic priority list.