UPS (Uninterruptible Power Supply) and Laser Printers

UPS (Uninterruptible Power Supply) and Laser Printers

I found out an interesting bit of information today about UPS devices.  UPS is Uninterruptible Power Supply.


I installed a 850Watt Eaton device at a client and connected a HP Desktop all-in-one  and a Multi-function Laser Printer.  That’s all.  Well after charging the unit for the prescribed 6 hours, I turned off the computer, and then plugged it and the Printer into the UPS.  I turned on the UPS and after a short while, a loud continuous beep sounded and did not stop.  I checked the manual (2 page document) and the only mention was the beep could be caused by a fault in the UPS.  I went back to my supplier who stated that he would loan me another unit to test to see if it was the UPS at fault.  It also had the same issue.

After calling the Australian Supplier of these units, I was asked if I was connecting a Laser Printer.  I told them that I was.  He said that I should try not to connect the printer, but to only connect the PC.  I returned and did this, and the unit did not beep.    Very strange!  Well I researched this a little further and found the following article in

In summary it states that one should never connect a laser printer to a UPS.  I guess that should be in the manual in bold lettering!!

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

  • The initial inrush current at start-up, This can be seven or more times the average operating current of the printer.
  • Initial power-on when the fuser is heated to the temperature needed to fuse toner. Subsequent re-heating of the fuser may be periodic or may occur when the printer switches from and idle state to full-power to satisfy a print request after a period of inactivity.

Fuser temperature is up to 200 °C (392 °F).

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