What belongs on a critical-load panel

Think about the last time the neighborhood went dark. You probably didn’t miss the dryer. You missed the fridge, the furnace kicking on, maybe the sump, and the little box on the wall so the phone still worked.

A critical-load panel is a small extra breaker box for that short list. It can run from the grid on a normal day, or from a generator or battery through a transfer switch when the street is dead. Everything else stays on the main panel and goes dark.

The switch exists so utility power and backup are never tied together. Feeding power back out to the street is how people kill linemen, which is why this is electrician work in most towns.

What should I put on it?

Move circuits you actually used last time:

• Refrigerator (and a separate freezer if you have one)
• Furnace or boiler controls and the blower. You often don’t need the whole air handler plus electric heat strips.
• Sump pump
• Well pump, if you have one (the startup surge matters more than the quiet running number)
• One lighting circuit per floor you actually walk
• Bathroom receptacle or light you’ll use
• Garage door opener if that’s how you leave
• Modem / router / the internet box on the side of the house. Everyone forgets this one.

Leave off unless the backup machine is huge: electric range, dryer, water heater, hot tub, whole-house AC. Those five are why people outgrow a 6 to 10 circuit switch.

Popular Mechanics notes most manual transfer switches cover a limited set of circuits, often six to ten. Count 240-volt breakers (dryer, range, well) as two slots.

Popular Mechanics: manual transfer switch

How do I add up the watts?

Write running watts and starting watts for the well, the fridge compressor, and anything medical that can’t miss. A 13.5 kWh battery and a 7 kW generator are different jobs. The small panel doesn’t make either one bigger; it just stops you from trying to run the dryer.

Typical running watts (order of magnitude; read the nameplate):

• Fridge: a few hundred running, more at start
• LED lighting circuit: tens of watts
• Modem/router/internet box: tens of watts, and it’s why you still have a phone
• Furnace control plus blower: hundreds to maybe ~1 kW, not including electric heat strips
• Sump: hundreds running, nasty start
• Well pump: this is the one that eats generators; starting watts can be several times running

Add running watts, then look at the two biggest starts (well plus fridge) and whether they can stagger. If the well is 240 volts, that circuit is two slots and a different conversation.

A 13.5 kWh battery at 1 kW of essentials is theoretically 13 hours before you count inverter losses and the well. That math collapses if the heat pump is on the panel.

Walk the house in the last outage you remember and write down what you missed. People say “whole home” and then live in the kitchen and one bathroom.

How do I actually connect the backup?

Three common setups:

1. Select-circuit transfer switch: cheaper, limited slots, circuits stay mapped in the main box. Fine for a portable generator.
2. Dedicated critical-load subpanel: move those breakers once. Pairs cleanly with a battery gateway or a small standby.
3. Whole-house automatic transfer: everything stays up until the generator starts shedding load. That’s a larger invoice.

Don’t invent a way to feed two main panels from one cheap 10-circuit switch. Neutrals and interlocks don’t work that way.

An outdoor inlet plus a sliding plate on the main breaker (an interlock) is cheaper than a listed automatic transfer switch. It’s also easier to get wrong, and some inspectors hate it. A small extra panel fed from a transfer switch is the clean story for batteries and small standbys. Whole-house automatic transfer is the “I don’t want to think” story and the big invoice.

Never backfeed a dryer outlet. That can energize the utility lines and kill a lineman.

The automatic vs manual transfer switch guide is the hardware decision. You already have enough here to put labels on breakers this weekend.

What should the quote include?

1. The circuit list, labeled as they appear in the panel.
2. Transfer equipment type (manual, sliding interlock plus outdoor inlet, or automatic) and amperage.
3. Whether the inlet is outdoors and interlocked so you can’t run utility and generator at once.
4. Permit. Inspection. Who pulls it.
5. What isn’t backed up, in writing.