In order to provide graceful degradation of function in case of total blackouts, I would like to have my smaller UPS plugged into the larger one so my main computer lasts the longest.

Question: Is it safe to plug a UPS into another UPS like this?

migrated from electronics.stackexchange.com Mar 11 '17 at 4:45

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

  • In the end I did chain an Opti 1500VA unit for most stuff, including a small inkjet. When it gives up, then everything will be off (one laptop will have a bit more time due to its own internal UPS) including the printer. The smaller APC will buy just a little more time - finally, the router and most-important computer and one monitor will be last - and it runs on a SSD so when it gets the "now" signal it does a full clean shutdown. I posted this 5 months ago, and have been using this setup for 3 months, with one involuntary full-on test, and it seems to be fine. – SDsolar Aug 16 '17 at 22:02

I'm assuming that you're talking about typical, cheap UPSes. These devices only convert DC to AC when they have no input AC or their input AC is out of tolerance.

First, the problem you won't have: You will never have double conversion. If one UPS is on, the other will not be. The UPS connected to the wall, if converting, will keep the second UPS from converting since it will have AC input. The UPS connected to the other UPS, if converting, must not have an AC input and hence the other UPS is not converting.

Since you will never have double conversion, there will be no degradation. Each UPS will produce the output it produces, regardless of what the other UPS is doing.

With the power on, both UPSes will charge from the AC that passes straight through them. The load will run on AC as well.

When the AC fails, the first UPS will start converting. The second UPS will see AC and its battery is already charged. So it will just pass the first UPSes output AC through itself to the load.

When the first UPS dies, the second UPS will stop getting AC. It will run normally, converting DC to AC just as it would if the first UPS wasn't there.

Really there are only two problems you'll have.

First, the UPS plugged into the wall might need to supply power to charge the second UPS's battery some of the time. This will almost never happen, but you do need to make sure the first UPS can handle it. If it shuts down under that load, you'll be in trouble.

Second, the second UPS might not be happy with the output waveform of the first UPS. Some UPSes allow you to adjust their tolerance for crappy input waveforms. If the first UPS is at least a stepped square wave, you'll probably be okay. But you may find that the second UPS refuses to see the output from the first UPS as acceptable. (This only rarely actually happens, but it's hard to guarantee that it won't.)

  • It should be said that doing this will not work if both UPS are outputting stepped sine waves. Any UPS supplying another UPS with power will (I think) need to be outputting pure sine waves. So under this configuration, all UPS in the chain will need to be pure sine except for the last going to devices. See the warnings here: power-solutions.com/pressroom/newsletter/2013/09/… – xendi Dec 14 '18 at 8:31
  • @xendi That page is mostly garbage for the reasons that I explained in my answer above. The only legitimate point is that it will interfere with emergency power off, but in home applications, there is no such thing. – David Schwartz Dec 14 '18 at 18:39

I have been researching this today because I was considering this. The short of it, don't do it. UPS into a UPS might work, but it can also cause damage to both UPSs or even a fire.

There are several reasons. The first has to do with how the UPS outputs power. When a UPS goes on battery, it doesn't output the same kind of power as what passes through. The second UPS will usually either also go on battery at the same time, or interpret the bad power as a spike and try to ground it. That means the 2nd UPS will send power down it's ground to the first UPS. That could very bad.

There are a number of other reasons, but it isn't something you should do. I am a chemical engineer by education, but I have been doing electrical engineering work in my role at an industrial plant and did the research today and stumbled across this thread so thought I would contribute.


Simpler overall answer regarding UPS or surge protectors or plain old power strips : Don't plug them into each other. There's a risk from fire, data loss, and equipment damage from someone in the future overloading either device, the outlet, or the breaker. That's before we get into the power efficiency loss, or damaging downstream units from lack of active power factor correction.


Can you? Sure. Should you? No. Quality of the voltage will drop, and the back and forth Ac to dc to Ac to dc to Ac to dc will be inefficient. Get a laptop or a bigger UPS.

  • 1
    I assume he's talking about typical standby devices and not an online UPS, so there would be no back and forth except when the power fails. – David Schwartz Mar 11 '17 at 4:54
  • And why do you think the "quality of the voltage" will drop? – David Schwartz Mar 11 '17 at 4:57
  • @David ups do not produce a very clean Ac waveform – cde Mar 11 '17 at 5:26
  • 1
    I don't see any quality problems. Tested with Kill-a-Watt units in various places to be sure. – SDsolar Aug 16 '17 at 22:08
  • 1
    @Arun It sounds like you encountered a situation where the second UPS is not happy with the first UPS's waveform. If that happens, the chained UPSes will not work. You can try swapping them or seeing if the second UPS has an adjustable tolerance. – David Schwartz Dec 14 '18 at 18:40

I don't get the point. If you plug the smaller one into the bigger one it will just drain the bigger one faster and wont come on till the big one peters out.

If you plug the big one into the small one.. same thing but the small one wont last long.

In all that there is an efficiency loss... A typical UPS is only between about 92 - 95% efficient when on line. While the first UPS is still up you are wasting 4-8% of it's capacity just keeping the second UPS alive.

Ultimately, you end up getting less time out of them and risk blowing up one or both UPS units.

Better to be smart about what you really need powered up through the outage. Only plug things into the backup power plugs you really need kept alive.

Also, you should remember the UPS is really there to give you sufficient time to get to your system and shut it down in a controlled fashion. It is not really intended to let you keep running for a long time. That's what backup generators are for.

  • 2
    In theory, you get the combined total backup time when they are in series. If you use either one alone, you will have less backup time. This is what the OP is thinking, anyway. – mkeith Mar 11 '17 at 4:41
  • 1
    Yes I know, but you wouldn't. There is an overhead because the UPS itself consumes power from the mains too in addition to supplying the load. – Trevor Mar 11 '17 at 4:44
  • 1
    @mkeith theory assumes perfect conversion. They don't have superconductor ups yet. – cde Mar 11 '17 at 4:52
  • Why would there be an efficiency loss? There is no case where both UPSs would be converting DC to AC at the same time. The one plugged into the wall would feed into the second, which would stay bypassed, until the one plugged into the wall died, at which point the second one would feed the load directly. – David Schwartz Mar 11 '17 at 4:55
  • @cde It doesn't assume perfect conversion at all. Each one's backup time takes into account the conversion of that UPS. There will never be double conversion because each UPS will only be converting when the other isn't. – David Schwartz Mar 11 '17 at 4:56

Most UPS systems output a stepped sine wave when on battery power. Almost all UPS equipment interprets a stepped sine wave as bad power. Therefore, as soon as the first UPS goes on battery and outputs a stepped sine wave, the second UPS in the series will read that input as bad power and convert to battery also. With both UPS systems running on battery at the same time, there is no increase in battery capacity.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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