How to Size a Whole House Generator: kW Chart

Buying a whole house generator without doing the math first is one of the costliest mistakes homeowners make. Undersize it, and your unit trips every time the AC compressor and well pump kick on together. Oversize it, and you're paying thousands extra for capacity you'll never use. Getting the kW rating right is the single most important decision in the entire purchase—more important than brand, fuel type, or price.

This guide walks through the actual math, gives you a sizing chart by home size, and shows you how to calculate your own load so you can shop with confidence.

Why Generator Sizing Matters More Than You Think

A whole house generator isn't rated in "size" the way a suitcase is—it's rated in kilowatts (kW), which measures how much electrical load it can supply continuously. Every appliance and system in your home draws a certain number of watts to run, and some draw a much larger surge of watts just to start.

Get the sizing wrong in either direction and you'll feel it:

·         Undersized: The generator's automatic transfer switch may shed circuits, or the whole unit can stall and shut down when big loads (AC, well pump, electric range) start simultaneously.

·         Oversized: You pay a higher upfront cost, higher fuel consumption at idle, and in some cases inefficient operation because the generator rarely runs near its rated capacity.

Running Watts vs. Starting Watts (The Math Everyone Gets Wrong)

Every appliance has two numbers that matter:

·         Running watts — what it draws continuously once it's on

·         Starting (surge) watts — the extra jolt it needs for the first 2–3 seconds to start, especially for anything with a motor or compressor

A refrigerator might run at 150 watts but spike to 1,200 watts on startup. Central air conditioners are the biggest surge offenders in most homes. Whole house generators handle this with a rated "starting capacity" that's typically 1.5–2x the running capacity, but you still need to size for your largest single starting load plus everything else running.

Whole House Generator Sizing Chart

Use this as a starting benchmark based on home size and typical circuit load. Actual needs vary based on whether you're running central AC, electric heat, or a well pump.

These figures assume a natural gas or propane standby unit with an automatic transfer switch. If you're shopping portable or dual-fuel options for partial-home backup instead of whole-house coverage, sizing needs are lower—browse Generator Vault's standby generators collection to compare units already matched to these kW tiers.

How to Calculate Your Exact Wattage Needs (Step-by-Step)

A chart gets you close. A real calculation gets you exact. Here's the process:

1. List every circuit you want backed up. Walk your panel and note fridge, freezer, furnace/AC, well pump, sump pump, garage door, key outlets, and any medical equipment.
2. Find the running watts for each. Check the appliance nameplate or manual—most list wattage or amps (watts = amps × volts).
3. Add up all running watts. This is your baseline continuous load.
4. Identify your single largest starting load. Usually central AC or a well pump. Add its starting watts (not running watts) on top of the running total of everything else.
5. Add a 20% buffer. This accounts for voltage fluctuation, aging appliances drawing more than rated, and future additions like an EV charger or hot tub.
6. Round up to the nearest standard kW size (typically sold in 10, 12, 14, 16, 18, 20, 22, 24, and 26 kW increments, then larger jumps above that).

Example: A 2,200 sq ft home with a 3-ton AC unit (starting load ~5,000W), fridge, furnace fan, and typical outlets totaling 4,500W running load, plus the AC's 5,000W surge = roughly 9,500W minimum, pushed to about 11,500W with the 20% buffer. That lands squarely in the 12–14 kW range—consistent with the chart above.

If you'd rather work backward from your outage priorities than your panel, our home backup power planning guide walks through prioritizing essential circuits, which is especially useful if you're deciding between whole-home and partial-home coverage.

Fuel Type and Its Effect on Sizing

Fuel choice doesn't change your wattage math, but it does affect real-world output:

• Natural gas units run slightly lower output than their propane-rated equivalent due to lower BTU content, so check the natural gas rating specifically, not just the propane number.
• Propane generally delivers closer to full rated capacity and is easier to store for extended outages.
• Dual-fuel and tri-fuel portables are a good fit for partial-home backup (essential circuits only) rather than true whole-house coverage, since their max output is typically lower than standby units.

Common Sizing Mistakes to Avoid

• Sizing off square footage alone. Two 2,000 sq ft homes can have very different loads depending on AC tonnage, electric vs. gas appliances, and well pump presence.
• Ignoring the transfer switch capacity. Your automatic transfer switch has its own amperage rating that needs to match or exceed your generator and panel.
• Forgetting future loads. If an EV charger, pool pump, or workshop is on the horizon, size up now rather than replacing the unit in three years.
• Using running watts for startup loads. This is the single most common calculation error and the main cause of "my generator can't handle my AC" complaints.

Generator Vault

Phone: 9042068588
Email: sales@generatorvault.com
Address: 6501 Arlington Expressway, Jacksonville, Florida 32211
Website: https://generatorvault.com/

FAQs

What size generator do I need to run a whole house?

Most homes between 2,000–3,000 sq ft need 16–24 kW to run essential systems plus central AC. Larger homes with electric heat or multiple AC zones often need 30 kW or more.

Can a 22kW generator run a whole house?

Yes, for most homes up to roughly 3,000–3,500 sq ft with one central AC unit and standard appliances. Homes with electric heat, large well pumps, or multiple AC zones may need more.

How many watts does a house need in a power outage?

Essential-only backup (fridge, some lights, furnace fan, a few outlets) typically needs 5,000–7,500 watts. Full whole-house coverage with AC usually needs 15,000–25,000+ watts depending on home size.

Do I need a professional to size my generator?

A licensed electrician should confirm your final calculation and handle installation and transfer switch sizing, but you can absolutely narrow down the right kW range yourself using the steps above before you get a quote.

Is it better to oversize or undersize a whole house generator?

Slightly oversizing is safer than undersizing. An undersized unit can stall or shed circuits under peak load, while a modestly oversized unit simply runs below its max capacity with no functional downside.

Conclusion

Sizing a whole house generator comes down to one core discipline: know your running watts, know your largest starting watts, add a buffer, and round up to a standard kW tier. The chart above gets you in the right neighborhood fast, and the step-by-step calculation gets you to an exact number you can shop against with confidence. When in doubt, size slightly larger rather than smaller—a generator running comfortably under its max capacity lasts longer and performs better than one working at its limit every outage.