I have an FSP EP1000 UPS.
The User's Manual says:
CAUTION: NEVER connect a laser printer or scanner to the UPS unit. This may cause the damage of the unit.
Why is this restriction?
Why you shouldn't connect a printer to a UPS
(Unless you accurately size the UPS for peak power requirements)
When turned on, Laser printers draw a high current to heat up their fuser roller. A typical UPS cannot cope with such a spike.
Descriptions of the problem by UPS manufacturers do not go into details.
The problem may be one or other of
Fuser temperature is up to 200 °C (392 °F).
From HP specification for LaserJet 3200
If we disregard power factors and other AC complications,
The inrush current is an order of magnitude larger than average operating currents.
In conjunction with other equipment, the 700W max during printing could overload a small UPS.
For example Belkin say
Anecdotal reports say
Laser printer usage is sometimes reported as causing voltage sags in power circuits such that lights noticeably dim in the building for a brief moment.
Here is an example of power used vs time.
Laser printers use a lot of power. At peak loads, laser printers may draw more power than the UPS can provide. As for scanners... I dont believe modern scanners use a lot of power, perhaps some higher-end multi feed scanners do...
Take a Canon imageCLASS LBP6300DN for example. The spec sheet (linked) says a maximum power consumption of 1120W. Let's try to find an UPS for just the printer, no other load. So maybe we look for 1200W.
You might first think, no big deal, grab a 1500VA UPS, more than enough. But VA != watts, and it turns out that (for example) an APC Back-UPS 1500 has a max of 865 watts. They assumed a power factor of .57. Finding a "consumer" ups above 1500VA is hard. So instead (as APC recommends) you look at the SmartUPS line. The smallest one you could use is a SmartUPS 2200, which will set you back $1000, more or less. I can't find anything in its manuals saying not to connect a laser printer, so APC is probably OK with this.
You could go with a Cyberpower UPS, they're usually cheaper, appears you'd need a PP2200SW, which you could have for around $500. Indeed, cheaper, but still pretty expensive. The manual says you "should" not connect a laser printer (or vacuum cleaner, wonder who tried that?).
You can connect a laser printer to an UPS. You just need to buy a really big (and expensive) UPS.
Over and above the inrush issue (discussed above) there is also the consumption under load...
Unlike servers and desktops, printers are very rarely business critical, and users can usually do without them for the duration of a power-outage (maybe at a slight inconvenience).
On the other hand, UPS infrastructure is often spec-ed at the lowest that can be got away with, before adding extra hardware - so printers sucking out battery-reserves will limit the up-time of the rest of the network.