We're contemplating a backup generator for the house. Most of them have transfer switches that kick in quickly, and automatically, but naturally the generator has to start, spin up, and stabilize before switching over.

Is there a way to make that cutover seamless so the house doesn't notice? Basically, a "whole house UPS" that just needs to last 5-10 minutes. Both for the switch over from line to backup power and vice-a-versa.


We live in California where the power company is shutting the grid down for public safety reasons due to fires. Before, power failures were essentially very rare Acts of God with storms and drunk drivers. Now, during certain parts of the year, they're policy. But even so, the companies issue lots of "might" and "maybe" and "sometime" notices as to when, and where, they might turn the power off. As a consumer, we don't know when it will go out, and when it does go out it can be for several days. And, yes, it can be during some of the hottest parts of the year. A/C is routine in many parts of California.

I'm exploring this simply as I mentioned -- it can be a pain to "reboot" a modern house with it's zillion electronic appliances, devices, etc. If I don't have to, I'd like not to. Since I'm exploring the idea of backup power in the first place, I'm exploring what it might take to go that "last mile" to make it seamless.

We have glanced at the Power Wall, I was simply off-put by it's initial sticker shock compared to a conventional generator. I'm in the early phases of this process.

  • Research into solar power setups. Those often include large battery storage.
    – JPhi1618
    Commented Dec 20, 2019 at 17:51
  • You'll probably pay more for that feature than for the generator.
    – JACK
    Commented Dec 20, 2019 at 17:57
  • 1
    Such products exist, and are used extensively in data centers. The keyword you'll want to search for is 'online' or 'double conversion' UPS. It'll be expensive.
    – Nate S.
    Commented Dec 20, 2019 at 17:57
  • 1
    Hospitals do this - imagine saying to family “the operation on grandpa was going fine, until the power went out for 30 seconds”...
    – Solar Mike
    Commented Dec 20, 2019 at 18:04
  • 1
    @ThreePhaseEel Because he wants to. Commented Mar 12, 2020 at 11:39

1 Answer 1


This already exists COTS

The Tesla PowerWall is the most high visibility version, but there are competitors, and you could even knock one together out of golf cart batteries and inverters. What Tesla brings to the table is positively exquisite battery-management tech, which assures the PowerWall battery will have a very long life.

"But I want a 5-minute solution, this is overkill!"

Actually no, it's a game-changer. Because now, the generator doesn't need to carry your house. Just replenish the PowerWall. So you can get it done with a Honda instead of a CAT.

If you go pure generator, then the generator must have enough capacity for your house's peak loads. You could end up needing a 20kw generator just so it doesn't croak when you put on everything at the same time, the 1 minute in 1000 that you do that.

The difference in generator size may pay for the PowerWall, especially since the generator does not need an ATS and other associated equipment; it only needs to be a generator.

A homes power use is actually quite small, and averages barely 1kw. That's because, while your dryer, water heater, A/C and oven may be going gangbusters on a summer evening, your house also has long periods of time of virtually no loads at all, as litle as 60W for refrigerator, night lights and your vampire loads (looking at you, Comcast). To run 1 KWH the whole month costs about $100 on your electric bill, so you can look at your bill and see for yourself.

I read this negative review from one of those anti-green naysayers, and he said "if your house draws 5kw on average, you'll need 70 PowerWalls for a 30-day outage". Ok, first, 5kw means a $500 electric bill, not likely. And second, a 30-day outage is insane, but if it existed, it'd actually an ideal use case for a small generator to complement a single PowerWall, just as I say.

Off-grid power is 10% generation, 90% conservation

However, only a fool would tarry forth into a power outage and keep using electricity like nothing had happened. Of course you're going to defer those wash loads. Whip up something on the stovetop instead of bake.

Understanding what your big loads are and avoiding heavy appliance usage is instrumental to weathering an outage.

Another option, on the electrical side, is enforce this with multiple panels. Have your main panel for utility-only loads, the ones you never want to run on PowerWall. Sauna, dryer, etc. pThen you have a subpanel for the loads that need to be on generator or PowerWall, and these are the ones that get automatically picked up on-the-fly.

You could even have a third panelboard full of loads that could go either way, like water heater or A/C. That panel gets a $30 Siemens or QO generator interlock, and is normally switched to "Utility" so it dies when the power fails. Once you are sure you have abundant capacity to drive one of those loads, you shut them all off, cut the panel over from utility to PowerWall, and switch on the load you want to run.

Of course, the best way is to make judicious choices about appliances, so you aren't creating a huge electrical load in the first place. For instance instead of baseboard heat, use a heat pump designed to effective at very low-temperature range, so electric emergency heat never runs. My house has an Empire furnace, that is gas and runs without any electricity at all. We also have a gas water heater; being on-demand it needs a small amount of power for controls. A PowerWall is in fact overkill for my house.

  • Thank you, this is quite informative. Commented Dec 21, 2019 at 4:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.