Backup Power without Critical Loads Panel


New Member
Oct 27, 2021
I need a hybrid grid-tie/off-grid solar system for backup power because my grid is unreliable. I have 200A service at the main panel and two sub-panels - 125A for the house, 50A for the garage. I could split my 125A house sub-pane into critical and non-critical load panels, and connect the inverter AC output to the critical panel, AC input to the main panel. But I would like to avoid re-wiring my house sub-panel. Is there a way to power the house sub-panel without splitting it into critical and non-critical sub-panels?

From the energy audit spreadsheet, my min continuous inverter size is 2.5KW, min surge inverter size is 7.8KW. Most inverters I see in this size AC output is only 60A, so it can't fill the entire load during normal operation, therefore cannot be the only source of power to the house sub-panel. Is there an inverter that can be connected to the house sub-panel using only AC input, then feed back/sell back/export to the house panel but not the main panel? Would I also need an ATS upstream from the inverter? This thread refers to "Watt Node" but I can't tell which specific product by that company.

I'm in CA and need it to be UL/listed.

Edit: P.S. It would also be nice if there were a single inverter that could output split phase


What, me worry?
Nov 26, 2019
Los Gatos CA
Is there a way to power the house sub-panel without splitting it into critical and non-critical sub-panels?
Not that I am aware least not in the way you are thinking. There are 'smart panels' that combine the traditional breaker box functions with a bunch of power electronics. The idea is that it would replace the main panel and have both grid and back-up power inputs. Through software, a 'virtual' critical load panel can be defined so that the backup power only feeds circuits assigned to virtual panel. Span is a smart panel brand that advertises a lot, but there are other brands available. I am still a bit leery of the smart panels because they don't have a long track record. Will they fail frequently, and when they do fail how hard and how expensive is it to fix?

From the energy audit spreadsheet, my min continuous inverter size is 2.5KW, min surge inverter size is 7.8KW.
I assume you are referring to the audit spreadsheet I put into the resources. The inverter sizing assumes everything is running at one time. This may not be the case. If you can confidently determine the worst case, you might be able to use a smaller unit.

Similarly, the surge size in the spreadsheet assumes everything turns on at one time. If there are just a couple of items that create a surge, this might be a possibility. However if there are several items that add up to the 7.8KW, there is a good chance they will never turn on at once. Perhaps the most likely time everything would turn on at once is if the backup power runs out and the inverter shuts down. When the batteries are charged enough and the inverter turns back on... everything that is waiting for power will turn on at the same time.

If we stick with the 2.5KW and 7.8KW, it sounds like a low-frequency 3K inverter would be a good match. Low-frequency inverters typically have a 3x surge capability but high-frequency inverters are usually limited to 2x surge.


New Member
Jun 26, 2021
That would only require running a cable to a a subpanel adjacent to the main panel and moving the house sub cable to the new crit panel, 1 cable in from ats and 1 cable out to the sub. Then use an auto transfer switch to switch between grid(125A) and backup(60A).


I See Electromagnetic Fields!
Mar 28, 2020
You can have a transfer switch able to pass full power, and switch to battery inverter.

SMA has "Automatic Backup Unit" to do that (200A relay and 120/240V transformer), along with "Sunny Boy Storage".
Not cheap, but it would fit the bill.

If you removed a big load (e.g. electric furnace?) from the 125A house panel, how much would you really need then?
Having inserted an inverter serving as UPS, you could have a fused disconnect feed that one load. Or a suitable small breaker panel for a couple loads.

What I've done is put in four Sunny Island, 2s2p. That allows up to 112A from grid. I could have done just 2s, allowing 56A, and that would probably have taken care of my small house just fine (It was on 70A breaker), but I found a bargain so put in excess capacity.

You're going to need to turn off excess loads, at least at night. If you can add an important loads panel with battery inverter between it and house panel, those loads can be automatically run. An interlocked breaker will let you manually backfeed other panel for balance of house loads, when desired.

Another way to turn off excessive loads is with inverter or charge-controller sending a state of charge signal. With mine, I could control the thermostat wire for heat & AC, disabling if battery below 80% SoC, for instance.


May 20, 2020
Just wire the off-grid inverter to the main panel using an interlocked generator breaker.. It won't be automatic, but it will be easy.

It takes me about 2 minutes to turn off my mains, turn on the generator input breaker, turn on my main battery breaker, boot the BMS, and activate the inverters. Actually, it takes 30 seconds, then I get to wait for another 30 seconds for the inverter to boot up.

This also has the advantage of being isolated from your home's electrical system when not in use.. keeps it safe.