Sizing a Generator or Home Battery: kW, kWh and NEC 702

By IES team, Innovative Electrical Services (IES), Auburn, CA · Reviewed November 2026 · 8 min read

Key takeaways

kW is how much you can run at once; kWh is how long you can keep it up. A generator is limited by kW (and fuel), a battery by both.
Motors such as AC compressors and well pumps briefly pull several times their running current when they start, and that surge often decides the size.
Generators make less power up high and in the heat. In the foothills and mountains, check the derate on the spec sheet.
Under the 2023 NEC (the basis of the 2025 CEC), an automatic transfer switch means the backup source must carry everything it picks up, unless a load management system limits it.
PG&E asks to be told about permanent generators, and treats grid-connected batteries as generators for interconnection.

In much of Northern California, losing power for a while is part of life: winter storms, summer heat, and Public Safety Power Shutoffs, which PG&E says may last several hours or several days. If you’re thinking about a generator or a home battery, the most useful thing you can do first is work out two numbers: how much you want to run at once, and for how long. This article walks through both, with the code rules behind them, so the conversation with any installer (us or anyone else) is easier.

Power and energy: kW and kWh

Power, in watts (W) or kilowatts (kW, 1,000 W), is how much you are using right now. Energy, in kilowatt-hours (kWh), is power over time: 1 kW for one hour is 1 kWh. A 1,500 W space heater running for 4 hours uses 6 kWh.

A generator is limited mainly by power. If it is big enough for everything you turn on at once, it keeps going as long as it has fuel.
A battery is limited by both: its continuous output (kW) decides what can run together, and its stored energy (kWh) decides how long.

Think of a battery as a tank. kWh is how much is in the tank (how long you can go); kW is how wide the tap opens (how much you can run at once). A generator has a tap (kW) and runs as long as it has fuel.

Averages matter more than labels for run time. A refrigerator might draw a few hundred watts while its compressor runs, but it cycles on and off. The yellow EnergyGuide label shows its estimated yearly use; a fridge rated at 500 kWh a year averages 500 ÷ 365 ≈ 1.4 kWh a day, or roughly 57 W around the clock. If you have a smart meter, your utility account may also show your home’s usage by the hour, which is a good reality check.

Starting surge: why motors set the size

Lights and electronics draw about the same power from the moment they turn on. Motors don’t. When an induction motor starts, it pulls a large inrush current until it gets up to speed. Generator makers commonly put that at about six times the motor’s full-load current, and higher is not unusual. On the utility, you barely notice. On a generator or a battery inverter, that peak can make the voltage dip, lights flicker, or the motor stall and the unit trip.

When a motor starts (an AC compressor, a well pump, a fridge), it briefly pulls several times its running current, then settles down. The generator or battery has to ride through that peak without the voltage sagging too far.

Look at the nameplate. AC condensers and heat pumps list a starting current, usually as LRA (locked-rotor amps), on the data tag.
Compare it with the source. Generator specs list a surge capability. Battery spec sheets list a short-term peak output, and some also list a motor-starting rating in LRA that you can hold up against the number on your AC’s tag.
Usually only one big motor starts at a time. A common rule of thumb is to add the running watts of everything that will be on, plus the extra starting watts of the single largest motor.
Soft starters help. A soft starter ramps a motor up gently instead of hitting it with full voltage. One maker’s white paper reports cutting inrush by roughly 40 to 65%. For AC units on backup power, this can be the difference between working and not.

Sizing a generator

List what must run together. Fridge, freezer, well pump, furnace fan, lights, internet, medical equipment, and maybe the AC. Use nameplate watts (or volts × amps).
Add the biggest starting surge on top of the running total.
Allow for altitude and heat. Engines make less power in thin, hot air. Generator spec sheets and manuals list a derate: a few percent less output for every 1,000 ft above a set elevation, and a bit less for every 10°F above a set temperature. The numbers vary by model. One home standby manual, for example, lists 3.5% per 1,000 ft and 1% per 10°F, with derating starting anywhere from sea level to several thousand feet depending on the model. In the foothills, that can add up to several percent, so use the figures for the unit you’re considering.
Check the fuel. Output also depends on the fuel. Many home standby units list a lower rating on natural gas than on propane, so look at the rating for the fuel you’ll actually use.
Leave headroom. Standby ratings are meant for loads that come and go during an outage, not for running flat out for days. Check the spec sheet or manual for how much average load it expects over a long run, and don’t plan right up to the nameplate number.

For a portable generator with a checklist of common loads, try our generator sizing estimate, and see the inlet and interlock guide for the safe way to connect one.

Sizing a battery

Battery specs give two numbers worth writing down for each unit:

Usable energy (kWh). How much the unit can actually deliver, not just its total capacity. Home batteries come in a range of sizes, so compare usable kWh, not the model name.
Continuous output (kW). How much it can run at once, all day. Most spec sheets also list a higher peak output for a few seconds, which matters for motor starting.

Most systems let you add units for more of either. Run time is usable kWh divided by your average load, less the reserve you keep for the next outage. Big loads empty a battery fast: an EV charging at 32 A on 240 V (about 7.7 kW) would use 10 kWh in well under two hours. That is why many battery systems back up a selected group of circuits, or use smart load control, instead of the whole house.

With solar, a battery can recharge during the day, which changes the math for multi-day shutoffs. How much depends on the season, the weather and the size of the array. Batteries also have their own rules for where they can go in a home; see Solar and batteries in the code.

What the code asks for: Article 702

A backup system for a home is usually an optional standby system under NEC Article 702. California’s current code is the 2025 California Electrical Code, in effect since January 1, 2026 and based on the 2023 NEC. In plain English:

Manual transfer (an interlock or a manual transfer switch, where someone throws the switch): the system needs enough capacity for the loads you choose to run at one time, and you pick them.
Automatic transfer (an automatic transfer switch): the source has to be able to carry the full load it picks up automatically, worked out with the Article 220 load calculation or another approved method. The alternative is an energy management system that automatically limits the load (Article 750). Then the source only has to cover the most that system will ever allow on.
North Carolina’s fire marshal’s office, explaining the same 2023 rule, notes that an app alert asking you to turn things off yourself doesn’t count as automatic management. If you’re unsure how your local building department reads it, ask them before you choose equipment.
Transfer equipment has to be listed and installed so the backup source and the utility can never be tied together by accident.
Generators other than plug-in portables need a disconnect that locks open (445.18), and at one- and two-family homes an emergency shutdown device outside the house, labeled “Generator Emergency Shutdown” (445.19(C)). It may be the one on the generator’s own enclosure.

Our article on load calculations (Article 220) explains how the full-load number is worked out.

The utility side

Never back-feed. PG&E’s guidance is clear: power from a generator must never flow onto its lines. That is what interlocks and transfer switches are for. Please never plug a generator into a wall outlet.
Permanent generators: PG&E suggests a professional installer, an inspection by your city or county building department, and contacting PG&E to let them know about your backup system once it’s installed.
Batteries: PG&E treats storage as a generator for its Rule 21 interconnection process, so a grid-connected battery goes through interconnection and should wait for permission to operate. Our guide to solar permits and interconnection covers how that usually goes.

A simple way to start

Write down what you truly need during an outage, and what would just be nice to have.
Note the nameplates of anything with a motor, especially the AC and any well pump.
Look at your hourly usage on a typical day and on a hot day.
Decide whether you want to flip a switch yourself (manual) or have it happen on its own (automatic).

If you’d like help, we’re glad to look at your panel and talk it through, with no pressure. See generators and battery backup, solar and battery, or our homeowner guide standby generator vs home battery.

Sources and further reading

Local rules decide. These articles explain general ideas and point to the code sections we use. They are not legal or engineering advice and don’t replace the code itself. California adopts the NEC with its own amendments, cities and counties can add more, and the local inspector (the AHJ, or authority having jurisdiction) has the final say on any job. Read the code free at NFPA’s free access page.