Test Method for Calculating the Energy Efficiency of Single-Voltage
External Ac-Dc and Ac-Ac Power Supplies
Chris Calwell, Suzanne Foster, and Travis Reeder, Ecos Consulting
Arshad Mansoor, Power Electronics Application Center (EPRI-PEAC)
August 11, 2004

Funded by the Public Interest Energy Research (PIER) program
California Energy Commission

11. ScopeThis document specifies a test method for calculating the energy efficiency of singlevoltage external ac-dc and ac-ac power supplies, herein referred to collectively as“power supplies.” Power supplies are designed to convert line voltage ac into the low
voltage output (either ac or dc) typically required by laptop computers, cordless andcellular phones, portable stereos, etc. External power supplies are contained in a separate
housing from the product they are powering. These external power supplies are often
referred to as “ac adapters.”
A single voltage power supply provides one dc or ac output that is either at a fixedvoltage or user selectable through a selector switch. Power supplies with multiple,
simultaneous output voltages, whether internal or external, are beyond the scope of thisdocument. Dc-dc voltage conversion equipment such as dc-dc converters are notincluded in the scope of this document, except to the extent that such circuitry may befound within a power supply.
The purpose of this test procedure is to establish a standardized method that can be usedworldwide to measure the efficiency of single voltage external ac-dc and ac-ac power
supplies across a full range of load conditions. Its intent is not to replace IEC 62301,
which focuses closely on the measurement of standby power, but to augment and extend
it downward to the measurement of no load conditions and upward to the measurement ofactive mode conditions. Likewise, its intent is not to replace IEEE 1515-2000, but to add
specificity regarding loading conditions and reporting requirements. A number ofgovernments around the world intend to use this test procedure to assess and compare theefficiency of power supplies.

2. References
The following list includes documents used and/or referenced in the development of this
proposed test specification.
I. IEEE Std 1515-2000, IEEE Recommended Practice for Electronic Power
Subsystems: Parameter Definitions, Test Conditions, and Test Methods
II. IEEE Std 519-1992, IEEE Recommended Practices and Requirements for
Harmonic Control in Electrical Power Systems
III. IEC 62301 Ed 1: Measurement of Standby Power
IV. IEC 60050 International Electrotechnical Vocabulary - Electrical and electronic
measurements and measuring instruments
V. IEEE 100: The Authoritative Dictionary of IEEE Standards Terms

2

3. Definitions
For the purpose of this document the following definitions apply. Terms defined in IEC
60050 and IEEE 100 also apply.
a. Active Mode
Active mode refers to a condition in which the input of a power supply is connected toline voltage ac and the output is connected to a dc or ac load drawing a fraction of thepower supply’s nameplate power output greater than zero.
b. Active Mode Efficiency
Active mode efficiency is the ratio, expressed as a percentage, of the total real output
power (dc or ac) produced by a power supply to the real input power (ac) required toproduce it. See IEEE 1515-2000, 4.3.1.1.
c. No LoadIn this document, no load refers to a condition in which the input of a power supply isconnected to an ac source consistent with the power supply’s nameplate ac input voltage,
but the output is not connected to a product or any other load.
d. No Load PowerNo load efficiency would by definition be 0 when calculated on a percentage basis (see 3babove). In this document, no load power is defined as the wattage of real power (ac)consumed by a power supply operating in the no load condition.
e. UUTUUT is an acronym for “unit under test,” which in this case refers to the power supply
sample being tested.
f. Ambient Temperature
Ambient temperature is the temperature of the ambient air immediately surrounding theUUT.
g. Power Factor (True)
The true power factor is the ratio of the active, or real, power (P) consumed in watts tothe apparent power (S), drawn in volt-amperes (VA).
PF =PS

This definition of power factor includes the effect of both distortion and displacement.

3

h. Total Harmonic Distortion (THD)
THD is the ratio, expressed as a percent, of the rms value of an ac signal after thefundamental component is removed to the rms value of the fundamental component.
THD of current is defined as:
THD I =

I 22 + I 32 + I 42 + I 52 ... + I 132
I1

where I13 = rms value of 13th harmonic of the current signal.
i. Apparent Power (S)The total or apparent power (S) is the product of rms voltage and rms current (VA).
j. Instantaneous PowerThe product of the instantaneous voltage and instantaneous current at a port (the terminal
pair of a load).
k. Active Power (P)The rms value, taken over one period, of the instantaneous power. Most measuring
instruments average active power over a number of periods (ac cycles); readings fromsuch instruments are equally valid for this measurement.
l. Nameplate Input Voltage
Nameplate input voltage is the appropriate ac input voltage of the power supply as
specified by the manufacturer on the label on the housing of the power supply. This isoften expressed as a range, such as 100 to 240 V.
m. Nameplate Input Frequency
Nameplate input frequency is the appropriate ac input frequency of the power supply as
specified by the manufacturer on the label on the housing of the power supply. Many
power supplies are labeled to operate on more than one input frequency.
n. Nameplate Output Voltage
Nameplate output voltage is the voltage output of the power supply as specified by themanufacturer on the label on the housing of the power supply (either dc or ac). Because
unregulated and regulated power supplies both exhibit some voltage deviation from
nameplate output voltage when supplying current, actual output voltage is likely to differ
from nameplate voltage at certain current outputs.

4o. Nameplate Output Current
Nameplate output current is the current output of the power supply as specified by themanufacturer on the label on the housing of the power supply. Loading conditions for thepower supply are determined by multiplying nameplate output current by 100%, 75%,50%, 25%, and 0% respectively.

4. General Conditions for Measurementa. General
Unless otherwise specified, measurements shall be made under test conditions and withequipment specified below.
b. Measuring Equipment
Power measurements shall be made with a suitably calibrated voltmeter and ammeter, orpower analyzer. As is specified in IEC 62301, measurements of active power of 0.5 W orgreater shall be made with an uncertainty of £ 2%. Measurements of active power of lessthan 0.5 W shall be made with an uncertainty of £ 0.01 W. The power measurement
instrument shall have a resolution of 0.01W or better for active power. Measurements ofvoltage and current shall be made with an uncertainty of £ 2%.
c. Test Room
As is specified in IEC 62301, the tests shall be carried out in a room that has an air speed
close to the UUT of £ 0.5 m/s, and the ambient temperature shall be maintained at 23°C –5°C throughout the test. There shall be no intentional cooling of the UUT by use of
separately powered fans, air conditioners, or heat sinks. The UUT shall be tested on athermally non-conductive surface. Products intended for outdoor use may be tested atadditional temperatures, provided those are in addition to the conditions specified above
and are noted in a separate section on the test report.
d. Test Voltage
An ac reference source shall be used to provide input voltage to the UUT. As is specified
in IEC 62301, the input to the UUT shall be the specified voltage ± 1% and the specified
frequency ± 1%. The UUT shall be tested at two voltage and frequency combinations:
115 V at 60 Hz and 230 V at 50 Hz if its nameplate input voltage and frequency indicatethat it can operate safely under both conditions. If testing at both conditions is notpossible, the UUT shall be tested at one of the above voltage and frequency combinations
that is closest to its nameplate input voltage and frequency. If voltage and/or frequency
ranges are not specified by the manufacturer (or the nameplate value is unclear), the UUT
shall not be tested.
e. Input ac reference source
The input voltage source shall be capable of delivering at least 10 times the nameplate
input power of the UUT (as is specified in IEEE 1515-2000). Regardless of the ac source
type, the THD of the supply voltage when supplying the UUT in the specified mode shallnot exceed 2%, up to and including the 13th harmonic (as specified in IEC 62301). The5peak value of the test voltage shall be within 1.34 and 1.49 times its rms value (asspecified in IEC 62301).
f. Test leads
All leads used in the test set-up should be of large gauge and short length in order toavoid the introduction of errors in the testing process. For further guidance, see Table
B.2, “Commonly used values for wire gages and related voltage drops” in IEEE 1515­
2000.

5. Measurement Approacha. Preparing UUT for Test
Any built-in switch in the UUT controlling power flow to the ac input shall be in the “on”
position for this measurement, and the existence of such a switch shall be noted in thefinal test report.
Power supplies that are packaged for consumer use to power a product must be tested
with the output cord supplied by the manufacturer. There are two options for connecting
metering equipment to the output of this type of power supply: cut the cord immediately
adjacent to the output connector, or attach leads and measure the efficiency from theoutput connector itself. If the power supply is attached directly to the product that it ispowering, cut the cord immediately adjacent to the powered product and connect outputmeasurement probes at that point. If the product has more than two output wires, thetests should be conducted on the two output wires that supply the output power. The otheroutput wires (sometimes used for battery monitoring) should be left electricallydisconnected.
It is also possible to utilize this procedure to test the efficiency of a bare circuit board
power supply prior to its incorporation into a finished housing and the attachment of its
dc output cord. For example, a power supply manufacturer or component manufacturer
may wish to assess the efficiency of a design that it intends to provide to an OEM forincorporation into a finished external power supply. However, the efficiency of the barecircuit board power supply may not be used to characterize the efficiency of the final
product (once enclosed in a case and fitted with an output cord). Power supplies must be
tested in their final, completed configuration in order to represent their measured
efficiency on product labels or specification sheets.
b. Load Conditions
All single voltage external power supplies have a nameplate output current, as shown inFigure 1. This is the value used to determine the four active mode load conditions and theno load condition required by this test procedure. The UUT shall be tested at thefollowing load conditions:6

Table 1 – Load Conditions for UUT
Percentage of Nameplate Output Current
Load Condition 1
100 % ± 2%
Load Condition 2
75% ± 2%
Load Condition 3
50% ± 2%
Load Condition 4
25% ± 2%
Load Condition 50%
The 2% allowance is of nameplate output current, not of the calculated current value. Forexample, a UUT at Load Condition 3 may be tested in a range from 48% to 52% of rated
output current.
Additional load conditions may be selected at the technician’s discretion, as described inIEEE 1515-2000, but are not required by this test procedure.
Figure 1– Example of Ac-Dc Power Supply Nameplate Output Voltage and Current

c. Loading Guideline
In order to load the power supply to produce all four active mode load conditions, a set ofvariable resistive or electronic loads shall be used. While these loads may have different
characteristics than the electronic loads power supplies are intended to power, they
provide standardized and readily repeatable references for testing and productcomparison.
Note that resistive loads need not be measured precisely with an ohmmeter. A variable
resistor is simply adjusted to the point where the ammeter confirms that the desired
percentage of nameplate output current is flowing. Figure 2 shows a simplified schematicof an external power supply test set-up using variable resistance as the load. For7

electronic loads, the desired output current should be adjusted in constant current (CC)mode rather than adjusting the required output power in constant power (CP) mode.
Figure 2 - Generic Test Set-up Using a Variable Resistance LoadOutput PowerMeterd. Testing Sequence
As noted in IEC 62301, instantaneous measurements are appropriate when power
readings are stable in a particular load condition. The UUT shall be operated at 100% of
nameplate current output for at least 30 minutes immediately prior to conducting
efficiency measurements.
After this warm-up period, the technician shall monitor ac input power for a period of 5
minutes to assess the stability of the UUT. If the power level does not drift by more than
5% from the maximum value observed, the UUT can be considered stable and themeasurements can be recorded at the end of the 5 minute period. Subsequent loadconditions (see below) can then be measured under the same 5 minute stability
guidelines. Note that only one warm-up period of 30 minutes is required for each UUT at
the beginning of the test procedure.
If ac input power is not stable over a 5 minute period, the technician shall follow theguidelines established by IEC 62301 for measuring average power or accumulated energy
over time for both ac input and dc output.
Efficiency measurements shall be conducted in sequence from Load Condition 1 to LoadCondition 5 as indicated in Table 1 above. If testing of additional, optional load8

conditions is desired, that testing should be conducted in accordance with this test
procedure and subsequent to completing the sequence described above.
e. Efficiency Calculation
Efficiency shall be calculated by dividing the UUT’s measured active output power at agiven load condition by the active ac input power measured at that load condition.
Average efficiency shall also be calculated and reported as the arithmetic mean of theefficiency values calculated at Test Conditions 1, 2, 3, and 4 in Table 1. This is a simple
arithmetic average of active mode efficiency values, and is not intended to represent
weighted average efficiency, which would vary according to the duty cycle of the productpowered by the UUT.
f. Power Consumption Calculation
Power consumption of the UUT at each Load Condition 1 – 4 is the difference between
the active output power (W) at that Load Condition and the ac active input power (W) atthat Load Condition. The power consumption of Load Condition 5 (no load) is equal tothe ac active input power (W) at that Load Condition.

6. Test Report
The following information shall be reported once for each UUT: UUT manufacturer,
UUT model number, UUT dc cord length (± 1 cm), whether a built-in switch is present
on the UUT, product powered by the UUT if known, photo of UUT that a) clearly shows
nameplate information and b) displays the size of the entire UUT with a centimeter rule
for scale, UUT country of manufacture, name of test lab, name of technician performing
the test, ambient temperature immediately surrounding the UUT (± 1 �C), date andlocation of test, and a description of test equipment used with most recent calibration
date.
The key data (measured and calculated) to report for each input voltage and frequency
combination at which was test conducted are found in Table 2 below.
Table 2 – Required Reported Data (Measured and Calculated)

Reported Quantity
Rms Output Current (mA)Rms Output Voltage (V)Active Output Power (W)Rms Input Voltage (V)Rms Input Power (W)Total Harmonic Distortion (THD)
True Power Factor
Power Consumed by UUT (W)Efficiency
Average Efficiency

Description
Measured at Load Conditions 1 – 4

Measured at Load Conditions 1 – 5
Calculated at Load Condition 1 – 4, Measured at Load Condition 5
Calculated at Load Conditions 1 – 4
Arithmetic Average of Efficiency at Load Conditions 1 – 49The test data are most usable and readily compared to other results if also presented ingraphical form, as shown in the sample test report in Annex A. Note that the efficiency
curve shown in Annex A is similar in format to that shown in IEEE 1515-2000, 4.3.1.2,
Figure 10.
The “Input vs. Output Power” chart in Annex A provides an additional, useful means ofconveying a power supply’s relative efficiency that includes ac active input power andactive output power at 100%, 75%, 50%, 25%, and 0% of nameplate current output on asingle chart. The shaded area between the input and output power curves will be quite
small with highly efficient power supplies. Note also that the output power curve will beclose to a straight line in a regulated power supply, but may deviate from a straight line
significantly in unregulated units where voltage is not stable across a range of loadconditions.

10Annex A(Final report form generated by Microsoft Access and provided in separate document)

11