A critical-loads panel is not a software setting or a list you hand the installer. It is a separate load center fed through the storage system, and every circuit you want to keep alive during an outage has to be physically moved into it. That means pulling conductors, possibly extending them, finding room for a new enclosure, and accepting that some circuits cannot practically be relocated at all.
Why the protected circuits are physically separated
During an outage the storage system disconnects from the utility and energizes its own island. Whatever is connected downstream of that transfer point gets power; everything else does not. Building that boundary means installing a second panel fed from the storage inverter, then relocating the breakers and conductors for the chosen circuits from the main panel into it. Conductors that were cut to reach the main panel may not reach the new one, which means splices in a junction box or pulling new cable. In a finished house with a cramped service area, that work can be more involved than installing the battery itself, and it is frequently the item that surprises people when the quote arrives.
Choosing circuits against a power budget, not a wish list
The limiting factor is usually not stored energy but the inverter’s continuous and surge output. A refrigerator, some lights, outlets for phones and a modem, a furnace blower and a sump pump form a typical set that fits comfortably. Electric water heating, an electric range, a clothes dryer, central air conditioning and an electric vehicle charger are large loads that will either exceed the island’s capability or drain the battery quickly. Motor loads deserve particular care, because starting current is far higher than running current, and a well inside pump or a compressor can trip an island that would run it happily once started. Ask the installer to compare the connected load and the starting requirements of each candidate circuit against the inverter’s published limits.
The circuits people forget
Several circuits only reveal their importance during an outage. A gas furnace needs its blower and controls powered, and that circuit is often in an awkward place. A septic pump, a sewage ejector, a sump pump and a well pump are all worth thinking about before the water rises. Garage door openers matter if that is the only way out. Network equipment, including the router and any cellular gateway, keeps communication and system monitoring alive. Medical equipment obviously takes priority and should be discussed explicitly. Meanwhile, circuits that seem essential, such as every lighting circuit in the house, often are not, because a few well-chosen lighting circuits cover the need at a fraction of the load.
Alternatives to a separate panel
Some systems back up the entire main panel through a whole-home transfer arrangement, relying on load management to shed large circuits automatically when the island cannot support them. That avoids relocating circuits and is elegant, but it requires equipment capable of the whole-panel current and a control scheme that sheds loads reliably. Other systems use smart breakers or a managed panel that can switch individual circuits under software control, which offers flexibility at a higher equipment cost. Each approach has different code requirements, different costs and different failure modes, and which is appropriate depends on the existing service, the panel’s condition and what the storage equipment supports.
Documentation and what happens later
After installation, two panels exist where there was one, and the circuit directories in both must be accurate. An out-of-date directory turns a simple future repair into an afternoon of tracing, and in an emergency it can cost real time. Ask for both panels to be labeled legibly, for a one-line diagram to be left with the equipment, and for a plain-language list of which rooms and appliances are backed up. Give a copy to anyone who might need it. Systems get sold with houses, and the next owner will otherwise discover the boundary during their first outage.
What to check before you act
- Ask for the inverter’s continuous and surge limits and compare them to your chosen circuits.
- Include heating controls, pumps and network equipment in the discussion.
- Confirm whether conductors will reach the new panel or need splicing.
- Require accurate directories in both panels and a written backed-up circuit list.
Common questions
Can I add a circuit to the backup panel later?
Usually yes, if there is space in the panel and the island can support the extra load, but it is electrical work by a qualified person and may require a permit.
Why can I not just back up the whole panel?
Some systems can, given sufficient equipment ratings and load management. Whether yours can depends on the service size, the panel condition and what the storage product supports.
The practical takeaway
Budget for the rewiring, choose circuits against the inverter’s power limits rather than by preference, and do not forget heating controls, pumps and network gear. Insist on accurate labeling in both panels, and leave the work itself to a qualified electrician.
References and further reading
- NFPA: the National Electrical Code and how panel and circuit requirements are applied
- CPSC: a home electrical safety checklist covering panels and circuits
- Department of Energy: consumer resources on home energy systems
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
