What a critical load panel is, and why you need one

Why a house cannot simply run on a battery

The intuition is that a battery holds energy and a house uses energy, so a big enough battery runs the house. That is true about energy and false about the thing that actually binds.

The constraint is current, not capacity. A house draws power through a service rated in amps, and during an outage everything must pass through the battery's controller instead. What that controller passes continuously is a published specification, and it is smaller than most people assume.

Two manufacturers we read rate their backup controller at 160 amps continuous. Both sheets also carry a 200 amp figure, which is the busbar and overcurrent protection rating rather than the continuous throughput. Those are different specifications and only one of them describes what your house can draw.

So a critical load panel is not a compromise imposed by a small battery. It is how the architecture works. The circuits you want during an outage move into a separate panel that the system can energise on its own, and the rest of the house stays on the main panel, which is dead until the grid returns.

The manufacturers are explicit about the limit. One installation guide forbids whole-home backup outright for one of its configurations. Another manufacturer offers a whole-home option and still excludes the circuits it designates as sheddable, which are the large 240-volt loads. In at least one case, "whole home" means the whole home minus the things that use the most power.

Where the transfer switch actually lives now

Older backup arrangements used a separate automatic transfer switch: a box whose only job was to disconnect the house from the grid and connect it to the backup source. Modern battery systems have absorbed it.

One manufacturer describes its controller as containing an "inbuilt neutral forming transformer, microgrid interconnection device, automatic transfer switch, and a panel board". Four functions in one enclosure, one of which is the transfer switch and another of which is the small panel the backed-up circuits live in.

Another manufacturer names three separate islanding devices depending on the configuration.

Two practical consequences follow. First, when you are quoted a battery you may also be being quoted a controller, a subpanel and the labour to move circuits between panels, and those are real line items rather than padding. Second, the controller is a single point through which all backup power flows, which is why its continuous rating matters more than the battery's capacity for deciding what can run.

The islanding function is the safety half of this and it is not optional. A system that kept exporting during an outage would energise lines that utility crews may be working on. That is why a grid-tied system without this equipment shuts down entirely when the grid fails, which surprises homeowners during their first outage and is the system working correctly.

Choosing the circuits, which is the actual decision

Everything above is architecture. The decision you make is which circuits go in the panel, and it is worth doing deliberately because moving them later means an electrician and a permit.

The obvious candidates are cheap in current and expensive to lose. A refrigerator or freezer, some lighting, the router and modem, phone charging, and a furnace fan if you heat with gas, since a gas furnace that cannot run its blower heats nothing.

The hard ones are the large 240-volt loads: electric water heating, electric ranges, air conditioning, electric vehicle charging. These are what a whole-home configuration typically sheds first, and each is a candidate for a deliberate decision rather than an assumption.

The special cases are the ones nobody thinks of until they matter, and they are worth listing because each one has caught somebody out. A well pump, without which there is no water at all. A sump pump, whose failure during the storm that caused the outage is the classic sequence. Medical equipment. A septic pump. Any of these can move the whole decision, and none of them appears on a standard list.

There is also a sizing trap in the opposite direction. A panel with too few circuits is the obvious mistake and a panel with too many is the quieter one, because every backed-up circuit is a route by which something can be switched on during an outage and drain the battery faster than you intended. A household that backs up its entire kitchen will discover this the first time somebody uses the kettle out of habit. The circuits you choose are also the temptations you leave available, and there is an argument for keeping the list genuinely short and using an extension lead for anything occasional.

And the constraint is surge as well as continuous draw. Motors demand far more current starting than running, so a well pump or an air conditioning compressor can be within the panel's continuous rating and still fail to start. We cover the surge question in more detail alongside the generator comparison.

One structural point is worth understanding before you start choosing. Circuits, not appliances, are what move. A single kitchen circuit may carry the refrigerator and four countertop outlets; a lighting circuit may cover two rooms and a hallway. So backing up the fridge may mean backing up whatever else shares its circuit, and separating them means an electrician rewiring rather than simply relabelling a breaker. This is why the exercise is cheaper at design stage than afterwards, and why an installer walking your panel with you is worth the hour it takes.

The way to do this properly is a list before a design. Write down what must work, find the nameplate figures for anything with a motor, and give that to the installer. A system sized against your list is a different product from one sized against a capacity number.

What the code says about the equipment itself

Battery equipment brings its own code provisions, and the residential code text is readable where the electrical code is not.

There is a threshold. The residential provisions on energy storage apply above one kilowatt-hour, so anything a homeowner would call a home battery is inside them.

Location is restricted. The provisions specify where storage may be installed rather than leaving it open, which is why the answer to "can it go in the basement" is a code question rather than a preference.

Spacing and clearances are specified. Three feet between units, and three feet of clearance from doors and windows. Multiple units on a wall are not simply a matter of fitting them.

And fire separation may be required, with the provisions specifying five-eighths inch Type X gypsum board where separation is called for.

Manufacturer restrictions sit on top of these, and can be stricter. One manufacturer's installation guide bans habitable-space installation outright with its own clearance figures. The code sets a floor; the manufacturer's instructions are also part of the requirement, and your local official applies both.

One thing we are not giving you is electrical code article text. The relevant articles on optional standby and stand-alone systems are paywalled and we did not read them. Where this page describes what equipment does, it comes from manufacturers' installation guides, which state what the manufacturer requires rather than what the code does.