It’s a question more homeowners are asking, and one that electricians can’t afford to get wrong.
When it comes to whole home backup generators, oversizing might seem like the safe move, but bigger generators are a lot more expensive and you could lose the sale. Undersizing could lead to long term customer dissatisfaction and code compliance issues.
The most objective approach is to size your generator with a proper NEC load calculation. You can then apply the right buffer using reserve margin and voltage dip. With the right process (and the right tools) you can size confidently without overbuilding, and give your customers exactly what they need.
What size generator does a house need?
Size a home standby generator from an NEC load calculation, not the service rating. Calculate the home’s actual load, add a reserve margin, account for maximum voltage dip when large motors start, then convert VA to kW. Kopperfield’s load calculator includes generator sizing with adjustable reserve margin and voltage dip.
In this article, we’ll walk through the fundamentals of generator sizing. Prefer to watch instead? Check out our video demo for a real-world example.
Why generator sizing matters more than ever
More homeowners are thinking ahead about backup power. With hurricane season in full swing and grid reliability worsening, standby generators are no longer a luxury, but a common ask.
The data backs this up: in 2024, the US logged over 1.2 billion hours of power outages, especially in storm-prone states like Florida, Texas, and the Carolinas. And residential generator sales have surged. Generac, for example, reported a 28% year-over-year increase in residential generator revenue.
With generator installs on the rise, homeowners are looking for quick, reliable answers from electricians. Providing generator size estimates adds customer value and could drive future growth.
Don’t size using just the service
One of the most common mistakes in generator sizing is using only the service size (like a 200A main) to choose a generator size. It’s well known that panel size isn’t an accurate reflection of how much power a home actually uses.
The better way to determine generator size is to use a proper NEC load calculation based on NEC Article 220. This provides a true picture of the home’s actual demand. A load calculation based approach used to be slow, but with modern tools it can be done very quickly.
What size generator for a 200 amp service?
The 200 amp rating won’t tell you. Panel size doesn’t reflect how much power a home actually uses, so two homes on identical 200 amp services can need very different generators. Run an NEC Article 220 load calculation and size from the calculated demand.
What does the NEC require for standby generator size?
For an optional standby system with automatic transfer, NEC 702.4 requires the generator to supply the full load that’s automatically connected, calculated under Article 220 or another approved method. Where an energy management system limits the connected load, the generator only has to supply the maximum load that system allows.
Reserve margin
Typical reserve margins range from 10 to 20%, depending on the scenario:
- 5 to 15% for homes with stable loads and low motor loads
- 15 to 25% for homes with lots of motor loads (AC compressors, pumps, etc.) or expected additions like EV chargers or pool equipment
Maximum voltage dip
Generators don’t just need to meet a home’s average load, they also need to handle short-term spikes when large motors start up or multiple appliances turn on at once. That’s where maximum voltage dip comes in.
Voltage dip measures how much the voltage can temporarily drop when a heavy load kicks in. Most residential generator systems are designed to allow up to a 30% voltage dip, which strikes a practical balance between performance and cost. If the home has sensitive electronics or specific appliances that can’t tolerate dips that low, you might need to spec a tighter range (i.e., 15 to 20%), but that usually requires a larger and more expensive generator.
Factoring in voltage dip ensures you’re not just sizing for total demand, but also for how the generator performs under startup conditions, keeping everything running smoothly when it matters most.
Converting to generator size
Once you’ve applied reserve margin and accounted for voltage dip, you’ll arrive at a final VA value for the generator.
To convert VA to kilowatts (kW), simply divide by 1,000. In most residential scenarios, you can assume a power factor of 1, meaning 24,000 VA = 24 kW. This assumption doesn’t always hold, so make sure to check the generator manufacturer’s specs to be sure.
This final kW value is your recommended generator size: based on actual demand, not just guesswork.
Smarter sizing for better installs
Oversizing might seem like the safer move, but it drives up costs, reduces efficiency, and can undercut your credibility with customers. A smarter and more professional approach follows a simple process:
- Start with an NEC load calculation
- Add a reserve margin based on real needs
- Factor in voltage dip for startup performance
- Convert to kW using the manufacturer specs and power factor
This method ensures your generator sizing is accurate, justifiable, and ready for proposals, permits, and installation.
Once the size is set, start the one-line from a template: generator with automatic transfer switch, generator with manual transfer switch and inlet box, or generator inlet with interlock kit.
Our load calculator walks you through each step for a proper generator sizing and produces a clean, professional recommendation in minutes.
Generator sizing FAQ
Should I size a generator by the service size?
No. Panel size doesn’t reflect how much power a home actually uses. Size from an NEC load calculation.
What reserve margin is typical?
Typically 10 to 20%, depending on the home’s motor loads and planned additions.
What voltage dip should I design for?
Most residential systems allow up to 30%. Sensitive equipment may call for 15 to 20%.
Updated September 2026. Originally published August 13, 2025.
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