Whole-home or partial backup: which one is on this job?
Backup scope is a design decision with hardware consequences, and it is a load question rather than a battery question.
Answers the Backup scope question during setup.
Everything in this article can be read with the enclosure door open and every metal cover left in place. Do not remove a cover, and do not open a meter enclosure. If a number is not visible that way, it is a question for a licensed electrician, not something to go looking for.
“The customer bought batteries, so the house stays on. Backup is backup.”
The common misreading. Here is what is actually true.
Two systems with the same batteries can back up completely different things. Whole-home backup puts a gateway ahead of the main panel so everything downstream rides through an outage. Partial backup builds a protected-loads subpanel, moves selected circuits into it, and only those circuits stay alive.
It is easy to hear "batteries" and picture the house running normally. What gets installed is one of these two arrangements, and the drawing has to match the one that was sold.
It is a load question
The limit is not how much energy the batteries hold. It is how much the system can supply at any one instant, and whether that covers what turns on at once. Two Powerwalls on a 200A service sounds like whole-home until the air conditioning, the well pump, and the oven start together.
So "2 batteries, whole home" is a claim the design has to survive, not a setting to select. If the calculated load exceeds what the system can carry, either the scope shrinks to a protected-loads subpanel or the equipment grows.
What was actually promised
Get the backup scope in writing, as a list of circuits. A system promised as "the fridge, some lights, and the internet" is partial backup, whatever the proposal called it. One where the air conditioning is expected to run is whole-home, and that has to be checked against the load.
Settling this before the drawing is done costs a conversation. Settling it after installation costs a change order.
How to get the answer
On an existing system the arrangement is visible from outside the enclosures. On a new one it is a question for whoever sold it.
- 1A gateway or transfer switch installed ahead of the main panel, with the main panel fed from it: whole-home.
- 2A second smaller panel near the batteries with a handful of circuits listed on its directory: partial, and that panel is the protected-loads subpanel.
- 3For a job not yet built, get the list of circuits that will stay on, in writing. If you are buying the system, ask for that list on the proposal. If you are drawing someone else's sale, ask whoever sold it.
- 4Where the answer is whole-home, compare the system's continuous output in kW, off the battery datasheet, against the calculated demand of the circuits being backed up. Energy in kWh is not the number that answers this.
What to photograph
- The gateway or transfer switch and how it is fed.
- Any protected-loads subpanel, door open, with its circuit directory readable.
This is guidance for reading equipment that is already visible and for describing it on a drawing. It is not instruction for electrical work. Any work on service equipment, panels, or conductors is for a licensed electrician, and local code and utility rules govern.
Every one of these questions is asked once, in setup.
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