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 AC-output UPSs that operate in a single input dependency (i.e., VFD, VI or VFI). Additional criteria for UPSs that can operate in multi-mode (e.g., VFD and VFI) and DC-output 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 Ac-output UPSs

EffAVG = t25% x Eff25% + t50% x Eff50%+ t75% x Eff75% + t100% x Eff100%


  • EffAVG is the average loading-adjusted efficiency,
  • tn% 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
  • Effn% is the efficiency at the particular n% of the Reference Test Load, as measured according to the ENERGY STAR Test Method.

Table 1: Ac-output UPS Loading Assumptions for Calculating Average Efficiency
Rated Output Power, P, in watts (W) Input Dependency Characteristic* Proportion of Time Spent at Specified Proportion of Reference Test Load, tn%
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

Table 2: Ac-output UPS Minimum Average Efficiency Requirement
Minimum Average Efficiency Requirement (EffAVG_MIN), where:
  • P is the Rated Output Power in watts (W),
  • EMOD is an allowance of 0.004 for Modular UPSs applicable in the commercial 1500-10,000 W range,
  • ln is the natural logarithm; and
  • The requirement shall be rounded to the third decimal place for certification and reporting
Rated Output Power Input Dependency Characteristic
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 - EMOD 0.980 - EMOD 0.0145 X ln(P) + 0.800 - EMOD
P > 10,000 W 0.97 0.94 0.0058 X ln(P) + 0.886

In addition to these criteria, all AC-output 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 "line-interactive" 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 multi-tap variable-voltage 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.