Siemens Standby Generators, Manual

Generator Sizing Guide

UPS Generator Compatibility 

Passive (also referenced as standby or off-line) and
Line-Interactive

These technologies are most common for personal workstations
and point of sale applications. They are typically single phase
equipment with size ranges of 350 VA – 2000 VA for passive
and 500 VA to 5000 VA for line-interactive.

Passive UPS’s are the simplest type. Under normal conditions
AC power passes straight through to the UPS load. When the
input power supply goes outside of specifications, the UPS
transfers the load from input power to the internal DC to AC
power inverter. Passive UPS’s do not correct for voltage or 
frequency deviations under “normal” operation.  

Line-interactive is similar to the passive technology except 
it has circuitry that attempts to correct for standard voltage
deviations. Frequency deviations under “normal” power 
operation are not corrected.

Equipment Notes:
These devices tend to be electrically / harmonically very noisy.
A single small UPS is not a significant concern, but applications
with multiple UPS’s can be problematic. 

Passive UPS technology typically has normal tolerances of 
10 – 25% on voltage and 3 hertz on frequency. If the input
source goes outside of these tolerances, the UPS will switch
onto the UPS battery source. Some line-interactive units may
have frequency tolerances factory set to .5 hertz. These units
will need to have their frequency tolerance increased to a 
minimum of 2 hertz.    

Generator Sizing Recommendation:
Limit the total UPS loading to 15% – 20% of the generator
capacity.

Double-Conversion

This technology is most common for critical load applications.
Double-conversion UPS’s constantly rectify AC to DC and then
invert the DC back into AC. This configuration results in an 
output that corrects for voltage and frequency deviations.  

There are single and three phase models covering small
through large applications. Most UPS applications larger than
5000 VA use double conversion technology. This approach is
also the preferred technology for generator applications.

Equipment Notes:
Double-conversion UPS’s that are single phase or unfiltered
three phase models tend to create a significant level of 
electrical/ harmonic noise. This is illustrated by harmonic 
current distortions that are greater than 35%. When three
phase models are supplied with harmonic filters (current 
distortion less than 10%), this concern is no longer an issue.  

Generator Sizing Recommendation:
Single phase models:  limit the total UPS loading to 25% 
of the generator capacity. 

Three phase models without filters (current distortion > 30%):
limit the UPS loading to 35% of the generator capacity.

Three phase models with filters (current distortion < 10%):
limit the UPS loading to 80% of the generator capacity.

Supplier(s)

Passive (Standby)

Line-Interactive

Double-Conversion

APC

Back-UPS Series

Smart-UPS Series

Symmetra Series

Liebert

PowerSure PST and PSP

PowerSure PSA and PSI

UPStation and Nfinity Series

Powerware

3000 Series

5000 Series

9000 Series

Note: Ferrups and Delta-Conversion UPS technologies not included in discussion.

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