Uninterruptible Power Supplies Key Product Criteria
The ENERGY STAR specification for UPSs establishes minimum average efficiencies for UPSs of different:
 Input dependency characteristics  Voltage and Frequency Dependent (VFD), Voltage Independent (VI), and Voltage and Frequency Independent (VFI).
 Rated Output Power  from less than 1500 kVA to greater than 10,000 kVA.
The key criteria are shown below for ACoutput UPSs that operate in a single input dependency (i.e., VFD, VI or VFI). Additional criteria for UPSs that can operate in multimode (e.g., VFD and VFI) and DCoutput UPSs/Rectifiers used in telecommunications can be found in Sections 3.2.2  3.2.3 and Section 3.3 of the Version 2.0 of the UPS Program Requirements (PDF, 979 KB), respectively.
Calculation of Average Efficiency for Acoutput UPSs
Eff_{AVG} = t_{25%} x Eff_{25%} + t_{50%} x Eff_{50%}+ t_{75%} x Eff_{75%} + t_{100%} x Eff_{100%}
Where:
 Eff_{AVG} is the average loadingadjusted efficiency,
 t_{n%} is the proportion of time spent at the particular n% of the Reference Test Load, as specified in the loading assumptions in Table 1, and
 Eff_{n%} is the efficiency at the particular n% of the Reference Test Load, as measured according to the ENERGY STAR Test Method.
Rated Output Power, P, in watts (W)  Input Dependency Characteristic*  Proportion of Time Spent at Specified Proportion of Reference Test Load, t_{n%}  

25%  50%  75%  100%  
P ≤ 1500 W  VFD  0.2  0.2  0.3  0.3 
VI or VFI  0  0.3  0.4  0.3  
1500 W <P ≤ 10,000 W  VFD, VI, or VFI  0  0.3  0.4  0.3 
P > 10,000 W  VFD, VI, or VFI  0.25  0.5  0.25  0 
*See information below about input dependency characteristic
Minimum Average Efficiency Requirement (Eff_{AVG_MIN}), where:



Rated Output Power  Input Dependency Characteristic  
VFD  VI  VFI  
P ≤ 350 W  5.71 x 10^{5} x P + 0.962  5.71 x 10^{5} x P + 0.964  0.011 X ln(P) + 0.824 
350 W < P ≤ 1500 W  0.982  0.984  
1500 W < P ≤ 10,000 W  0.981  E_{MOD}  0.980  E_{MOD}  0.0145 X ln(P) + 0.800  E_{MOD} 
P > 10,000 W  0.97  0.94  0.0058 X ln(P) + 0.886 
In addition to these criteria, all ACoutput VI and VFI UPSs at 100 % load must have a minimum power factor of 90%.
Information About Input Dependency Characteristics
Also known as "passive" or "offline" or "standby" UPSs, Voltage and Frequency Dependent (VFD) UPSs are capable of protecting the load from power outages. Typically in sizes up 1500 VA, they are used for small offices, personal home computers and other less critical applications to provide surge protection and battery backup. During normal use, the mains feed the load directly and also power a rectifier (AC to DC) to charge the battery. During a power outage, battery power through an inverter (DC to AC) feeds the load. This type of UPS cannot correct for voltage fluctuations which may cause switches to battery power during low main voltage and drain the battery.
Also known as "lineinteractive" UPSs, Voltage Independent (VI) UPSs are capable of protecting the load from power outages and provide additional corrective voltage functions. Typically in sizes up to 5,000 VA, they are used commonly for small business, Web, and departmental servers. They also normally draw directly from the mains and switch to battery during an outage. They are equipped with multitap variablevoltage autotransformers that keep output voltages within an allowed tolerance during low and high main voltages and eliminate battery drain during low main voltage.
Also known as "online," "continuous" or "double conversion" UPSs, Voltage and Frequency Independent (VFI) UPSs are capable of protecting the load against adverse effects of voltage and frequency variations without depleting the stored energy source. They typically come in sizes up to 1,000 kVA and are used in data centers. Under normal operation, the line power is run through a rectifier (AC to DC power conversion) to the batteries, which then powers equipment through an inverter (DC to AC power)  which is why VFI UPSs are sometimes called "double conversion" UPSs. Filters and fast regulating circuits in the converters isolate the load from voltage and frequency variation from the mains. With this configuration, there is no transfer time in the event of a power failure as the power is supplied continuously to the equipment through the battery.