The critical-loads (backup) panel explained
When the grid goes down, most solar-plus-battery systems do not power the entire home — they power a chosen set of circuits. The critical-loads panel is where those circuits live. Here is how partial-home backup works and how to design it well.
What it is
A critical-loads panel (sometimes called a backup or protected-loads subpanel) is a smaller electrical panel fed by your battery inverter. During normal operation it runs off the grid like any subpanel. During an outage, the inverter isolates it from the grid and keeps only those circuits energized from the battery.
Why partial backup makes sense
Backing up the whole house means your battery and inverter must be able to start and run everything at once — central air, electric heat, a well pump, an oven. That drives up cost fast and drains a battery quickly. By moving just the essentials onto a critical-loads panel, a modest battery can keep the important things running much longer.
| Good backup candidates | Usually excluded |
|---|---|
| Refrigerator / freezer | Central AC |
| Lights, outlets | Electric range / oven |
| Internet / phone charging | Electric water heater |
| Furnace blower | EV charging |
| Medical equipment | Dryer |
How it is wired
There are two common approaches:
- Dedicated critical-loads subpanel. The essential circuits are physically moved into a new subpanel downstream of the battery inverter. Clean and clear, but it means relocating breakers.
- Whole-panel backup with management. Some hybrid inverters can back up the entire main panel and use smart controls or load-shedding to avoid overloading the battery. This can avoid a separate panel but relies on the system managing big loads.
Which fits depends on the inverter and battery you choose and how many circuits you want protected.
Sizing the backup
Match the panel's likely load to what the battery and inverter can deliver. Add up the running watts of the circuits you plan to back up, account for motor-starting surges (a fridge or furnace fan draws extra at startup), and confirm the inverter's continuous and surge ratings cover them. Oversubscribing the panel is the most common design mistake.
Solar, battery, and the panel during an outage
It helps to picture how power flows when the grid is down. The battery inverter disconnects from the utility (so it cannot back-feed the lines), then energizes only the critical-loads panel. During daylight the solar array can recharge the battery and run those loads at the same time, stretching backup for days if the sun cooperates. At night the panel runs on stored energy alone. This is why solar plus a battery outlasts a battery by itself — but only for the circuits on the protected panel. Loads left on the main panel stay dark until the grid returns, which is exactly the trade you make to keep the battery small and the backup long.
Where it sits in the permit set
The critical-loads panel appears on your one-line diagram between the battery inverter and the loads, and its interconnection still has to satisfy the service busbar rules. Backup wiring, transfer equipment, and labeling are all things a plan reviewer checks, so they belong in the permit drawings.
Confirm backup and transfer requirements with your AHJ and utility, and have a licensed electrician or PE review and stamp the final design.
FAQ
What is a critical-loads panel?
It is a subpanel that holds only the circuits you want to keep running during an outage. The battery and inverter feed this panel, so backup power goes to essentials like the fridge, lights, and internet instead of the whole house.
Why not just back up the whole house?
Whole-home backup needs a battery and inverter large enough to carry every load, including big draws like AC and electric heat, which is expensive. A critical-loads panel backs up only what matters, so a smaller, cheaper battery lasts far longer.
How do I choose which circuits to back up?
Pick low-power essentials that matter most during an outage — refrigeration, some lights, phone and internet, medical devices, a furnace fan. Avoid large loads like electric ranges, EV charging, and central AC unless the system is sized for them.
Related: Design & sizing
Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.
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