Critical loads subpanel for a battery or generator

Why a second panel helps

Take a typical outage. A home battery or a 7,000-watt generator is feeding a house that has an electric range, a dryer, an air conditioner, a water heater and an EV charger. If everything stays connected, the first big motor kicks on and the backup source shuts down. The fix is to decide which circuits matter and keep only those connected.

A critical loads subpanel does that. The electrician moves the selected circuits off the main panel and onto a new small panel. In normal times the subpanel is fed by the main panel and nothing changes. In an outage, a battery gateway or a transfer device feeds the subpanel instead. The rest of the house stays off.

Cost range

This range is a desk estimate, not a published price. As of October 2026, the typical SoCal range is $1,200 to $2,800, about the same as the subpanel cost for a garage or addition. A bigger job costs more.

What changes the number:

• How many circuits move. Each one needs wire length, junction work and sometimes drywall repair.
• How far the subpanel sits from the main panel.
• Whether the main panel has room for a feeder breaker.
• Panel amp size and wire size for the feeder.
• Whether the battery installer supplies a ready-made backup panel, which would lower the electrical labor.

Choosing circuits

Make a list, then add the watts. A simple pass:

Group	Examples
Food and water	Fridge, freezer, well pump
Safety and comfort	Lights in main rooms, furnace fan, smoke alarm circuits
Communication	Internet, router, phone charging
Health	Medical device outlets, CPAP, oxygen concentrator
Access	Garage door opener

If someone relies on medical equipment, ask your utility about the Medical Baseline program, which provides extra notice before a shutoff. A backup panel is part of a plan for that case, not the whole plan.

Small outlets often matter most. A bedroom circuit with a CPAP outlet matters more than a living room lamp.

How it fits with a battery

Home battery installation often includes a backup or critical loads panel. Some batteries ship with a gateway that works as one. Others expect the electrician to build the subpanel. Get this in writing. If you plan to add a battery later, a subpanel installed now can be used then. The wire size and breaker space should leave room for growth.

How it fits with a generator

A subpanel also works with a portable generator. The inlet feeds the subpanel through a transfer switch or an interlock arrangement, and the generator only sees the chosen load. That makes a smaller generator workable. A load calculation gives the number.

Permit and inspection

Cities normally treat a subpanel with moved circuits as an electrical alteration that needs a permit. The inspector looks at breaker and wire sizes, grounding and bonding at the new panel, and labels. For general panel questions, see subpanel installation.

Plan around PSPS season

Fire-weather shutoffs cluster in dry, windy periods. The CPUC explains that utilities may switch power off in specific areas to reduce the risk of fires from their equipment. Have the subpanel work done before the dry months and test the backup setup once.

What the electrician needs from you

A list of the circuits you want, a rough idea of the battery or generator size, and photos of the main panel interior. If you can, flip each breaker and note what goes dark. This takes twenty minutes and saves labor on the day, because the electrician no longer has to trace each circuit.

Also mention appliances you plan to add later, such as an EV charger or a heat pump, so the feeder and the main panel are sized with room to spare.

Common mistakes

• Putting too much on the panel, which defeats the purpose.
• Leaving out one outlet for a medical device or a router.
• Forgetting that a well pump may need a 240 volt circuit and a larger breaker.
• Skipping labels. The next person to open the panel will need them.