What belongs on a critical-load panel

A critical-load panel (essential-loads subpanel) is a small panel that can be fed from the grid or from a generator/battery through a transfer switch. Everything else stays on the main panel and goes dark in an outage.

The switch exists so utility and backup are never tied together. Backfeeding a lineman is how people get killed. This is electrician work in most jurisdictions.

We have not opened your panel.

The list that is usually worth the slot

Move circuits, not vibes:

• Refrigerator (and a separate freezer if you have one)
• Furnace / boiler controls and the blower — not always the whole air handler plus electric strips
• Sump pump
• Well pump, if you have one (starting watts matter more than nameplate)
• One lighting circuit per floor you actually walk
• Bathroom receptacle or light you will use
• Garage door opener if that is how you leave
• Modem / router / ONT — the circuit everyone forgets

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

Popular Mechanics notes most manual transfer switches cover a limited set of circuits — often six to ten. Count 240-volt breakers as two slots.

Popular Mechanics: manual transfer switch

Watts, not wishes

Write running watts and starting watts for the well, the fridge compressor, and any medical device that cannot miss. A 13.5 kWh battery and a 7 kW generator are different jobs. The panel does not make either one bigger; it just stops you from trying to run the dryer.

Three ways to get there

1. Select-circuit transfer switch — cheap, limited slots, circuits stay mapped in the main. 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-service automatic transfer — everything stays up until the generator load-sheds. That is a different (larger) invoice.

Do not invent a way to feed two main panels from one cheap 10-circuit switch. Neutrals and interlocks do not work that way.

What a serious quote includes

1. The circuit list, labeled as they appear in the panel.
2. Transfer equipment type (manual, interlock + inlet, ATS) and amperage.
3. Whether the inlet is outdoors and interlocked.
4. Permit. Inspection. Who pulls it.
5. What is not backed up, in writing.

How to build the watt list without a spreadsheet religion

Walk the house in the last outage you remember. What did you actually miss? People say “whole home” and then live in the kitchen and one bathroom.

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/ONT: tens of watts, and it is the whole point of “I still have a phone”
• Furnace control + 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 + fridge) and whether they can stagger. If the well is 240 V, 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. It is not 13 hours if the heat pump is on the panel.

Interlock vs transfer switch, in one paragraph

An inlet plus a sliding interlock on the main breaker is cheaper than a listed automatic transfer switch. It is also easier to get wrong, and some inspectors hate it. A critical-load subpanel fed from a transfer switch is the clean story for batteries and small standbys. Whole-service ATS is the “I do not want to think” story and the big invoice.

Do not backfeed a dryer outlet. That sentence is the whole safety briefing.

Next

Automatic vs manual transfer switch is still queued. You already have enough to label breakers this weekend.