I have a 350 Watt appliance that I need to run pretty much continuously. An electrical outage of a few seconds isn't a problem but more than a minute or two becomes an issue. So I need a back-up system in case of any prolonged power outage, ideally for up to 24 hours.

I've looked into a UPS but most of these seem tailored for the computing sector, providing high current to large numbers of devices for just long enough for them to power down without data loss. I need the opposite of that, relatively low power for just one device but for up to a day.

Do they make UPSes for this sort of application? Or would an auto-start generator be more effective?

Specs: 120 Volt AC, 60 Hz, 8.3 Amps max (typically much less).

  • So if it has the capacity kWh then how that is used ie high A short time or low A long time is not an issue.
    – Solar Mike
    Jul 28, 2021 at 14:43
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    Well, a battery-based backup has the issue of degrading battery performance. A gas-based solution has the issue of stale gas. A natural gas or propane solution is likely too expensive for your budget. Making either gas solution hands-free is also expensive. You essentially need a big UPS and monitor the battery health. What is this 350 watt appliance anyways? Is it a sump-pump?
    – MonkeyZeus
    Jul 28, 2021 at 14:47
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    For a large battery backup, I'd start with the total power needed = 350W x 24 hours = 8,400 Wh. Plus allowance for startup/surge to 8.3A (=1 kW) and conversion losses, this is easily a 10 kWh system. Compare that to a Tesla or other electric car in the 60 kWh - 100 kWh range. So this is a very much non-trivial amount to provide by battery. A Tesla PowerWall - designed to provide intermittent backup for an entire house is 14 kWh. Another way to look at it - 350W - round to 360W = 30A continuous from a 12V battery - that's a lot. Jul 28, 2021 at 14:52
  • Given the amount of stored energy needed, might be time to consider a generator capable of providing whole-house levels of output. One example: a Generac 10kW model is around $2800 - plan on doubling that for electrician labor and automatic switchover equipment. But then you'd be able to "live in comfort" during any power outage Jul 28, 2021 at 17:10
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    Or, for a small fraction of that $2800x2, buy two modules out of a Tesla Model S (14KWH) and the necessary BMS, power supply and inverter. I can't believe anyone buys generators anymore. Jul 28, 2021 at 17:32

1 Answer 1


Lithium battery inverters are very good for this - if you have the budget. They are not inexpensive. Other battery chemistries are more problematic from a lifetime or percent of supposed capacity that's actually usable point of view, and become not-inexpensive themselves when you account for the oversize needed to deal with those factors.

350 watts for 24 hours is 8.4 kWh, so aim to have 10kWh capacity if this is the only thing it will run.

Adding some solar input ups the cost but also increases the usable time, or decreases the battery needed for a usable time, to a variable extent depending on insolation. Solar+battery sized to get 24 hours in gray November might get you through weeks of sunny summertime.

Fuel powered generators have their own reliability issues, and are often better used to recharge a battery solution as they are annoying and inefficient to run all the time.

  • And Tesla Model S modules are 24V @ 7KWH, and sell for about $1100 each pulled from wrecks. Jul 28, 2021 at 18:25

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