Sizing Guide
Generator sizing is not about square footage — it's about the watts your specific appliances draw simultaneously. Here's how to think through it.
Quick Answer
Most whole-home standby generators for single-family homes fall in the 15–30 kW range. The right size depends on your electrical load — not your square footage. Central air conditioning is usually the largest single load and the primary driver of sizing decisions. A professional load calculation is the only accurate way to confirm the right size for your specific home.
Generator capacity is rated in kilowatts (kW) — 1 kilowatt equals 1,000 watts. A 22 kW generator can supply up to 22,000 watts of power at once. The key is matching that number to the combined running wattage of everything you want to run simultaneously, with headroom for starting surges.
Starting watts (or starting surge) are temporarily higher than running watts when a motor-driven appliance first kicks on — most notably your central AC compressor, which may draw 3–4× its running wattage for a fraction of a second at startup. The generator must handle that surge without dropping, which is why the starting load of your HVAC system is a critical input to any accurate sizing calculation.
Not every standby generator installation has to power the entire house. Some homeowners choose an essentials-tier generator (7–15 kW) wired to cover only a defined set of critical circuits: refrigerator, lighting, furnace blower, well pump, and medical equipment. This is a meaningful and cost-effective approach for homes where running central air during an outage isn't a priority.
Whole-home coverage — powering every circuit in the panel, including central AC — is what most homeowners who research "whole house generator" are looking for. This typically requires 18 kW or more, depending on the home's HVAC load.
| Appliance or System | Typical Running Watts | Starting Surge Note |
|---|---|---|
| Central AC (2-ton) | ~1,500–2,000 W | 3–4× running watts at startup |
| Central AC (3-ton) | ~2,500–3,000 W | 3–4× running watts at startup |
| Central AC (4-ton) | ~3,500–4,500 W | 3–4× running watts at startup |
| Furnace/air handler blower | ~400–1,200 W | Moderate startup surge |
| Electric water heater (50 gal) | ~4,000–5,000 W | No startup surge (resistive) |
| Well pump (1 HP) | ~750–1,000 W | High startup surge (motor) |
| Refrigerator + freezer | ~150–400 W total | Moderate surge at compressor start |
| Electric dryer | ~5,000–6,000 W | Moderate startup surge |
| Lighting (whole home, LED) | ~200–800 W | Minimal |
| EV charger (Level 1, 120V) | ~1,440 W | No startup surge |
These are typical ranges for planning purposes. Actual running wattage and starting surge values vary by equipment manufacturer, model, efficiency, and operating conditions. Use the Electrical Load Worksheet to total your specific appliances.
| Size Tier | Typical Application | HVAC Coverage | Notes |
|---|---|---|---|
| 10–14 kW | Essential circuits, smaller homes | May support smaller window or mini-split units | Not typically sized for central AC; essentials-tier coverage |
| 15–18 kW | Smaller whole-home coverage, careful load management | Smaller central AC systems with load management | Load management required for larger HVAC |
| 20–22 kW | Often considered for many mid-size homes, depending on connected loads and starting current | May support many 2–3 ton central AC systems after a load calculation | Most common residential whole-home size range |
| 24–26 kW | Larger homes with higher load requirements | May accommodate larger HVAC and appliance combinations depending on startup loads and load management | Upper range of air-cooled residential models |
| 30 kW+ | Large homes with high simultaneous electrical demand | Often selected for homes with multiple HVAC systems or higher simultaneous demand, subject to load calculation | Often transitions to liquid-cooled equipment |
| Liquid-cooled 40–100+ kW | Large estate homes, commercial/mixed use | Very high HVAC and appliance combinations | Commercial-grade equipment, substantially higher cost and complexity |
Square footage tells you the floor area of the home — it doesn't tell you whether the home has a 2-ton or 4-ton AC system, whether it uses gas or electric appliances, whether there's a well pump, or how many loads would run simultaneously during an outage. A 2,000 sq ft home with gas heat, gas cooking, and no well pump may need far less generator capacity than a 1,800 sq ft home with a large electric heat pump, an electric water heater, and a deep-well pump.
Sizing rules-of-thumb based on square footage circulate widely, but they can lead to significant over- or under-sizing for a specific home's actual load. The Generator Size Calculator and Electrical Load Worksheet give you a more accurate result based on your actual equipment.
