APC UPS configuration and Generator Compatibility

A client recenlty was looking to install a generator for their small datacenter. The server room housed roughly 40 physical servers as well as a combination of network switches, routers, and a few other smaller electrical devices.  The environment was a small shop with a handful of mixed and matched APC 2200XL UPS’s carrying load on a handful of servers each as well as couple of larger APC UPS’s – a 5000XL and a 10000XLT which between them service a couple of server enclosures as well as a couple of storage arrays.  This is a small client working on a limited budget so we wanted to make sure we could use as much of the existing equipment as possible.  In researching the compatibility of the already provisioned UPS’s, we found many blog and tech posts about many who had problems with certain types of APC products or other UPS products after the installation of a generator.  Many said they had issues with generators and UPS’s either not recognizing the “dirty” power of the generator and remaining on battery power when the generator power kicks in or other miscellaneous spikes and drops in voltage from the UPS to the equipment.

In a call to APC support, the technician basically told us that the “VA” line of UPS’s from APC are generator supported and that the only UPS model that our client had in house that was supported for generator usage was the 10000XLT which not coincidently is also the newest UPS although several years old.  However, the technician told us that on the Smart 5000 UPS as well as the Smart 2200’s to adjust the sensitivity down to the lowest levels which would allow a more generous threshold for the less than standard utility power that happens with generators. 

On the Smart 2200’s this is done in a rather non traditional but easy to perform method of pressing a sensitivity button on the back of the UPS and determining by the brightness level, what setting the UPS is on.  Below is a snippet directly from the 2200 manual.  Sensitivity can be adjusted to high, medium, and low levels.


On the Smart 5000, this can easily be done from the web management interface as seen below:


This is found under:

  • UPS tab
  • Configuration Menu on Left
  • “Power” submenu 

The Generator:

For the generator a local electrical company was contracted who installed a Generac Guardian Series 20KW.  This model is actually a residential model that was selected to reduce cost but should adequately cover the load of the room as well as an auxilliary air unit to make sure the servers stay cool in an extended outage.  The 20KW size is about as large as you can go with air cooled generators and then you have to step up to liquid cooled.  From there the price increases dramatically.

Cost is certainly a factor these days with everyone, so companies have to find the sweet spot between what they want and what they can afford.

In addition to the transfer switch, a “buck and boost” transformer was installed in between to make adjustments to voltages between the Generator and the power panel which helps to compensate for any line voltage discrepancies.

Testing, Final Thoughts

In testing the generator and transfer switch, we found that the UPS’s with the adjustments made to the sensitivity settings, had no problems running on the generator power.  Everything transferred from utility voltage, to UPS battery power, and then to generator power without a hitch.  Once utility power was restored, everything swapped back over correctly.  Certainly, the best MO would be to buy all new generator compatible UPS’s from vendors such as APC, but smaller companies on a limited budget simply may not be able to do this.  By utilizing the sensitivity settings on the older UPS’s however and by using a good quality generator, most will find they shouldn’t have any problems with a generator installation in their environment.

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Brandon Lee

Brandon Lee is the Senior Writer, Engineer and owner at and has over two decades of experience in Information Technology. Having worked for numerous Fortune 500 companies as well as in various industries, Brandon has extensive experience in various IT segments and is a strong advocate for open source technologies. Brandon holds many industry certifications, loves the outdoors and spending time with family.

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