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B. Marion, J. Adelstein, and K. Boyle
National Renewable Energy Laboratory

H. Hayden, B. Hammond, T. Fletcher, B. Canada,
and D. Narang
Arizona Public Service Co.

D. Shugar, H. Wenger, A. Kimber, and L. Mitchell
PowerLight Corporation

G. Rich and T. Townsend
First Solar

Prepared for the 31
st
IEEE Photovoltaics Specialists
Conference and Exhibition
Lake Buena Vista, Florida
January 3-7, 2005
February 2005 NREL/CP-520-37358
Performance Parameters for
Grid-Connected PV Systems

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Performance Parameters for Grid-Connected PV Systems


B. Marion,
1
J. Adelstein,
1
K. Boyle,
1
H. Hayden,
2
B. Hammond,
2
T. Fletcher,
2
B. Canada,
2
D. Narang,
2

D. Shugar,
3
H. Wenger,
3
A. Kimber,
3
L. Mitchell,
3
G. Rich,
4
and T. Townsend
4

1
National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401
2
Arizona Public Service Co., 1500 E. University Dr., Tempe, AZ 85281
3
PowerLight Corporation, 2954 San Pablo Ave., Berkeley, CA 94702
4
First Solar, 4050 E. Cotton Center Blvd. #6-68, Phoenix. AZ 85040


ABSTRACT

The use of appropriate performance parameters
facilitates the comparison of grid-connected photovoltaic
(PV) systems that may differ with respect to design,
technology, or geographic location. Four performance
parameters that define the overall system performance
with respect to the energy production, solar resource, and
overall effect of system losses are the following: final PV
system yield, reference yield, performance ratio, and
PVUSA rating. These performance parameters are
discussed for their suitability in providing desired
information for PV system design and performance
evaluation and are demonstrated for a variety of
technologies, designs, and geographic locations. Also
discussed are methodologies for determining system a.c.
power ratings in the design phase using multipliers
developed from measured performance parameters.

INTRODUCTION

Accurate and consistent evaluations of photovoltaic
(PV) system performance are critical for the continuing
development of the PV industry. For component
manufacturers, performance evaluations are benchmarks
of quality for existing products. For research and
development teams, they are a key metric for helping to
identify future needs. For systems integrators and end
customers, they are vital tools for evaluating products and
product quality to guide future decision-making.

As the industry has grown, a clear need has arisen for
greater use of and education about appropriate industry-
standard performance parameters for PV systems. These
performance parameters allow the detection of operational
problems; facilitate the comparison of systems that may
differ with respect to design, technology, or geographic
location; and validate models for system performance
estimation during the design phase. Industry-wide use of
standard performance parameters and system ratings will
assist investors in evaluating different proposals and
technologies, giving them greater confidence in their own
ability to procure and maintain reliable, high-quality
systems. Standard methods of evaluation and rating will
also help to set appropriate expectations for performance
with educated customers, ultimately leading to increased
credibility for the PV industry and positioning it for further
growth.

Parameters describing energy quantities for the PV
system and its components have been established by the
International Energy Agency (IEA) Photovoltaic Power
Systems Program and are described in the IEC standard
61724 [1]. (IEA task members have used these
performance parameters to develop a database of
operational and reliability performance [2]. The database
contains information for several hundred PV systems and
may be viewed at
www.task2.org
.)

Three of the IEC standard 61724 performance
parameters may be used to define the overall system
performance with respect to the energy production, solar
resource, and overall effect of system losses. These
parameters are the final PV system yield, reference yield,
and performance ratio.

The final PV system yield Y
f
is the net energy output
E divided by the nameplate d.c. power P
0
of the installed
PV array. It represents the number of hours that the PV
array would need to operate at its rated power to provide
the same energy. The units are hours or kWh/kW, with the
latter preferred by the authors because it describes the
quantities used to derive the parameter. The Y
f
normalizes
the energy produced with respect to the system size;
consequently, it is a convenient way to compare the
energy produced by PV systems of differing size:

0
f
P
E
Y
=
(kWh/kW) or (hours) (1)

The reference yield Y
r
is the total in-plane irradiance
H divided by the PVs reference irradiance G. It represents
an equivalent number of hours at the reference irradiance.
If G equals 1 kW/m
2
, then Y
r
is the number of peak sun-
hours or the solar radiation in units of kWh/m
2
. The Y
r

defines the solar radiation resource for the PV system. It is
a function of the location, orientation of the PV array, and
month-to-month and year-to-year weather variability:

G
H
Y
r
=

(hours)
(2)


1 The performance ratio PR is the Y
f
divided by the Y
r
.
By normalizing with respect to irradiance, it quantifies the
overall effect of losses on the rated output due to: inverter
inefficiency, and wiring, mismatch, and other losses when
converting from d.c. to a.c. power; PV module
temperature; incomplete use of irradiance by reflection
from the module front surface; soiling or snow; system
down-time; and component failures:

r
f
Y
Y
PR
=

(dimensionless) (3)

PR values are typically reported on a monthly or
yearly basis. Values calculated for smaller intervals, such
as weekly or daily, may be useful for identifying
occurrences of component failures. Because of losses due
to PV module temperature, PR values are greater in the
winter than in the summer and normally fall within the
range of 0.6 to 0.8. If PV module soiling is seasonal, it
may also impact differences in PR from summer to winter.
Decreasing yearly values may indicate a permanent loss
in performance.

The PVUSA rating method [3] uses a regression
model and system performance and meteorological data
to calculate power at PVUSA Test Conditions (PTC),
where PTC are defined as 1000 W/m
2
plane-of-array
irradiance, 20
°
C ambient temperature, and 1 m/s wind
speed. PTC differs from standard test conditions (STC) in
that its test conditions of ambient temperature and wind
speed will result in a cell temperature of about 50°C,
instead of the 25°C for STC. This is for a rack-mounted PV
module with relatively good cooling on both sides of the
module. For PV modules mounted close to the roof or
integrated into the building with the airflow restricted, PTC
will yield greater cell temperatures. Nordmann and
Clavadetscher [4] report that PV module temperatures rise
above ambient for fielded system ranging from 20°C to
52°C at 1000 W/m
2
, with the largest temperature rise for
an integrated façade. The difference between the
nameplate d.c. p