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Is it safe and is their any benefit to daisy chaining multiple UPSs together?

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Depending on how the UPSes operate, there is a chance this can work. HOWEVER, with most UPSes and the way they operate, you can end up damaging the devices. In the best case, with most UPSes, you will just end up draining the batteries faster than they normally would.

The reason is that the power a UPS produces can appear as a power surge to the second UPS. The second UPS will then "return" the power back to the first UPS over ground. End result is the power from the first UPS goes to ground and the power from the second UPS powers your equipment until it runs out.

I would warn you that daisy chaining UPSes can void your warranty depending on manufacturer and/or model.

If you still want to do this, then before you do, check with the manufacturer. Their support should be able to tell you if it is safe to chain particular UPSes together or not.

I always recommend people play it safe and split the load between your UPSes. This helps to prevent overloading your UPS as well.

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  • I would like to hear your reasons WHY is is not safe to do this. Surely if you have a two pure sine wave inverters it would not make any difference?
    – Zapnologica
    Aug 26, 2015 at 2:59
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    I didn't actually say it wasn't safe to do this, although in certain conditions it certainly isn't safe to do so. I am not an EE, but I have been around the business long enough to have read at least a half dozen articles from various credible sources that detail why this isn't a good idea. In a very quick search, for example, I turned up this one.
    – YLearn
    Aug 26, 2015 at 5:42
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Not really a good idea with most of them - Most "Computer UPS's" output modified square wave, not sine wave power. But they expect real sine wave power on their input. Daisy-chaining them would probably not be a good thing for that reason, at least.

If they all actually work (do some basic testing with real loads and a timer, don't put too much faith in most "self-tests") you'd be better off to divide up any loads you need to power during an outage among the 4 UPSes, according to their rated power and the loads' draw.

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  • Interesting if its true. Do you have documentation of this? Its not that uncommon for a building/data center to be on UPS and then feed into smaller UPSs.
    – Keltari
    Dec 11, 2013 at 5:25
  • In that scenario, the bigger UPS would have to be true sine wave, or it would destroy a good number of things that are not MSW compatible. I don't know how common that arrangement is - given the considerable expense and annoyance of maintaining UPS batteries, better to have one big one that operates briefly while the backup generator starts, and no small ones - it's quite common for the first notification that a small UPS has once again killed its battery to be the thing it's attached to going out with the power (sometimes on power dips so short that things not connected to a UPS are fine.)
    – Ecnerwal
    Dec 11, 2013 at 18:55

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