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Short version:
When a UPS is plugged in to the mains, is it possible to plug the UPS output back to the same mains?
Will that work as an emergency power backup for the mains circuit?

Longer version:
The question makes no sense at first, since a UPS is supposed to do that by itself. No need to connect it in paralel. But here is my issue.
I need to plug in high consumption devices occasionally to the same circuit that the UPS is supplying to. Devices like a vacuum cleaner or an air heater which surpass the UPS's output power.
So what if I put the UPS in parallel with the mains so that I get emergency power and I can still plug in high power devices??

Full version:
Where I live there are frequent power outages for short periods of time. And I have too many electronic devices that are reset with every outage and they lose any configuration they had.
It's not just my PC, it's also my router, cable TV decoder, stereo system, TV set, clock radio and so on.
I can't afford buying several UPSs to put in each room. Plus, it would be too messy with all those cords and cables going from the UPS to every device.
So it occurred to me if I could power the entire mains circuit (the one that goes to the bedrooms' plugs) with one UPS so all the devices would have emergency power when an outage happens.
And when I need to use the vacuum cleaner or some other high load, it would bypass the UPS and get the power directly from the mains.
Would this idea work?
If not, is there a way I can achieve this in an inexpensive way?

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    \$\begingroup\$ Connecting a private power source to the mains wiring requires a "transfer switch" that reliably disconnects the incoming mains in order to safeguard linemen who may be working on the connections. It also would keep your equipment from being damaged if power were to be restored. \$\endgroup\$ – Spehro Pefhany Jan 15 '15 at 4:11
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UPS-powered wall sockets certainly exist in some buildings. At one place I worked at, we had red-colored sockets that were on the building's (giant) UPS (batteries, generators, etc.) and normal/white color for the regular stuff, with the convention that employees would not plug non-critical equipment in the UPS-backed sockets.

You could do something similar to the above at your place if you can access the house's wiring and disconnect certain sockets from the mains and then route them through an UPS.

But what you propose doesn't seem workable firstly because you don't seem to have a plan on how a big consumer like a vacuum cleaner would actually bypass the UPS. If mains power fails while the vacuum cleaner is on, you'd still overload the UPS and cause it to shut down turning everything else off too. (Unless you want to place bets on the mains never failing while you vacuum clean... but I won't delve into that.)

And (if I'm not mistaken on this second part) if you use your UPS to power the mains, when the mains power fails, your UPS will basically try to power the entire neighborhood! So it will probably not work for long... This is one of the reasons why you need an additional device like a transfer switch that Spehro Pefhany mentioned in his comment. In a sense, your UPS contains one of those too because when it starts to power the load from its battery (in case of mains failure) it won't try to power the mains as well. So if you do plug the UPS' output to your house's mains, then you need achieve a similar disconnection effect further upstream in order to prevent your UPS from powering the entire neighborhood. I'm not really familiar with stand-alone transfer switches, so if someone else wants to propose/outline a solution based on that... by all means do so. However the problem discussed in my previous paragraph (potentially overloading the UPS with big internal consumer) would not be automagically solved by an upstream transfer switch, so you'd [still] have to take additional measures to segregate the internal loads.

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    \$\begingroup\$ Actually, it's unlikely that a UPS would be designed to synchronize to the AC waveform from the mains. Connecting the UPS output to the mains would would cause blown fuses/breakers at the best and fireworks at the worst. \$\endgroup\$ – DoxyLover Jan 15 '15 at 6:22
  • \$\begingroup\$ I see. Then I guess I'll have to go for a simpler solution. That is, to isolate the mains circuit in question from the rest of the house and power it with the UPS. And for high consumption machines... I guess I'll just have to use an extension cord from the other mains circuit. \$\endgroup\$ – GetFree Jan 15 '15 at 6:55
  • \$\begingroup\$ @DoxyLover: most/cheap UPSes simply pass the mains unmolested to the load unless they detect over/under voltage. But you're right that if the UPS detects, say, undervoltage but the mains are not completely off, then a UPS that outputs to the mains may indeed activate while the mains are still (partially) working and there would be no phase synchronization no doubt... so a bang is quite likely. I'm guessing this is what you had in mind. \$\endgroup\$ – Fizz Jan 15 '15 at 7:19
  • \$\begingroup\$ @Fizz I have sort of a simliar question. Appreciate if you can answer that too. Thank you. diy.stackexchange.com/questions/95428/… \$\endgroup\$ – Ε Г И І И О Jul 30 '16 at 13:10

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