What an essential loads panel (backup subpanel) is

An essential loads panel is a second, smaller breaker panel. It holds only the circuits you've decided matter most in an outage. Other names for it: backup loads panel, critical loads panel, backup subpanel. They mean the same thing in most conversations.

Main panel and subpanel

Your main panel is the box where utility power enters the house and gets split into circuits. Each circuit has a breaker, a switch that trips if too much current flows.

A subpanel is another breaker box fed from the main one. One breaker in a panel can feed a subpanel.¹ "Larger installations will often use subsidiary distribution boards."¹ So subpanels aren't exotic.

A backup loads panel is a subpanel with a particular job. Instead of just adding space for more breakers, it separates the circuits you want to keep on from the rest.

How it's used in a backup setup

In a typical layout, a few existing circuits are moved from the main panel into the backup panel. The backup panel is then fed through a transfer switch or an equivalent device that can choose between the grid and the backup source.

The switch can be set up to supply "only critical circuits or entire electrical subpanels," as Wikipedia puts it.² That's the whole trick. The backup source never has to carry the full house, only what's in the subpanel.

When the grid is on, the backup panel is fed from the grid like everything else. When the grid fails and the system switches over, only that panel stays live.

What goes in it

Whatever you chose in your loads list. Common picks are a fridge circuit, some lighting, internet equipment and a medical device outlet. Your list is yours. The electrician checks that each circuit can really be moved and that the wiring on it suits the new panel.

How it differs from a plug-in solution

A backup panel is permanent wiring. Everything on it is hard-wired behind the walls, so you don't need to move cords around when the lights go out. That convenience is also why it needs a licensed electrician, not a weekend.

What to ask about the panel

• How many circuits it can hold, and how many you actually need today.
• Which breakers are moving, and whether any are two-pole (a double-width breaker for large appliances).
• How the panel will be labeled, so anyone in the house can tell at a glance what's on it.
• Where the shutoff is, and whether everyone in the house knows about it.

The electrician question list goes through these in more detail.