Based upon the question: Should you plug a laser printer directly into a wall socket?

Given a normal US wall plug with two sockets, it is okay to plug a laser printer directly into one socket (with or without surge protector) and then plug the UPS into the other socket?

Will running the laser printer result in the voltage dropping and the UPS kicking into battery mode? Is there any difference if I use a wall socket somewhere else in the room?

  • 1
    All of the outlets in the room (and adjacent rooms), will typically be wired to the same circuit breaker in your house, so it makes no real difference which outlet you plug it into. Laser printers do have a momentarily high initial current draw, but it probably isn't enough to drop the voltage significantly. Even if it does and the UPS kicks into battery mode for a second, that's why it's there. There is no problem with that.
    – fixer1234
    Feb 17, 2015 at 20:30

3 Answers 3


Should a laser printer be plugged into the same wall outlet as the UPS?

Short answer: that makes no difference. Houses are typically wired so that all of the outlets in adjacent rooms are daisy-chained and connected to the same circuit breaker. Sometimes, several breakers are used to split the outlets so all of them do not go out if a breaker pops. A large draw in one plug could lower the voltage on all the other breaker's outlets, but I think it would also lower the voltage on every outlet in the whole house.

I wouldn't worry about it though. Appliances (in North America) are rated to run at 110-120V, and dropping from 120 down to 110 should be just fine, even lower "briefly". If you've ever seen the old incandescent light bulbs when a dryer/washing machine/electric stove is turned on, they can dim for about a second. It's nothing new and appliances are supposed to be designed to handle average things like that.

If your computer is on a UPS, that will protect it from such temporary "brownouts". The momentary voltage drop from a laser printer typically isn't enough to kick the UPS into battery mode but if it does, that's what the UPS is there for. That doesn't hurt anything.

Should a laser printer be plugged into a surge protector?

A surge protector will protect the printer from any high voltage spikes on the power line, which is valuable. It will have no effect on the printer's momentary high current draw or the effect of that on anything else.

Should a laser printer be plugged into the UPS?

Every UPS I've seen has some non-UPS / surge-only outlets that should be fine. They are not included in the power-rating (VA/watts) of the UPS, which reflects the limits of what the battery can supply when in UPS mode.

However, you don't want to plug a laser printer into one of the UPS-protected outlets. This is discussed in answers from "Why shouldn't I attach laser printers or scanners to a UPS?". Typical UPSs for home use are designed to handle the power requirements for the critical components of your system, to give you enough time to shut down gracefully. Adding the power requirements of a laser printer to that of the other components may exceed the rating of the UPS, in which case it would shut itself down just when you need it. Even if it is within the power rating, it would greatly reduce how long you could operate on the battery, which might not provide enough time to close your work and shut down properly.

  • Just to clarify, in your 2nd paragraph, everything refers to possibly plugging the printer into a surge-only socket, not using a battery backup socket, right? FYI for Eric, the surge-protected sockets will protect the printer from a voltage spike, which is valuable, but it won't do anything related to mitigating the printer's initial current draw affecting anything else.
    – fixer1234
    Feb 17, 2015 at 21:03
  • I think I flipped from "surge only" to "ups protected" half way through that paragraph, but like your comment ( @fixer1234 ) the "ups protected" should be fine too. Your comment should probably be an answer, I'd +1 it. I read it and was just thinking that some newer houses have dozens of different breakers, almost one per outlet, wasn't planning on answering but got involved thinking & reading the linked Qs
    – Xen2050
    Feb 17, 2015 at 21:15
  • I have non-UPS just surge ports on the UPS, but I also have a free socket from the wall. I feel like I had read in the past that you should not connect the UPS to the socket next to the one with the printer for some reason, not sure if that was true. Is the risk only if you plug the printer into a UPS-backed socket?
    – Eric G
    Feb 17, 2015 at 21:31
  • I don't think there is a risk to worry about, except for the total power draw of the printer + computer being within the UPS specs (I'll edit my answer about that actually), and running down your UPS battery if the power ever went out while you were printing. I keep my printer unplugged (power bar "off") when not printing, no parasite power draw or surge risk (short of lightning).
    – Xen2050
    Feb 17, 2015 at 21:39
  • @Xen2050 - re: adding an answer, you already had a good one. I added what I would have put in a separate answer. Feel free to roll back the edit or change it.
    – fixer1234
    Feb 17, 2015 at 22:29

Circuit should be sufficient to support a UPS and printer. Printer should connect directly to a wall receptacle. Also power an incandescent lamp. If a powering printer causes the lamp to dim significantly, then get household wiring fixed. In most cases, dimming only indicates a poorly wired circuit. In rare cases, dimming indicates a serious human safety defect. Printer is not causing a problem. That printer is simply identifying a defect that should be located and fixed.

If a UPS is large enough to provide sufficient power to the printer, it is still a threat to that printer. Power from a UPS in battery backup mode (even when a UPS is many times large enough) is typically so 'dirty' as to be harmful to motorized appliances (ie laser printer). Same 'dirty' UPS power is still ideal for electronics because electronics are so robust. UPS should never power any small motors because a UPS outputs such 'dirty' power.

  • Can you clarify what you mean by "dirty" power from a UPS? Dirty usually refers to either irregular or spiky voltage or a lot of electrical noise. UPS's have very uniform, noise-free voltage, although many do not output a sine wave. Can you point us to further reading on hazards of UPS output on small motors? Regarding the dimming light, I suspect that relates to whether the wiring complies with rigorous building codes.
    – fixer1234
    Feb 21, 2015 at 21:30

Peak power consumption of a laser is typically around 1000W. Average consumption is far less, but that peak (added to whatever else is plugged into it) may exceed the UPS's maximum power capacity. For example, my home printer a DocuPrint CM305 df uses 410W during continuous printing, but up to 1100W peak during warm-up - and 8W in deep sleep mode. Buying a UPS that will handle that peak power is overkill.

When a computer experiences a brown-out or black-out, it may end up with a corrupt disk, which will seriously inconvenience you. On the other hand, a printer at worst will stop in the middle of a print, causing a paper jam. After power comes back on and you've cleared the jam, the print spooler on the PC will usually re-print the job automatically.

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