Performance Verification Guide for Grid-Tied PV Systems
Adapted from NABCEP PV Installation Professional Resource Guide, Section 7.2
Verifying system operation
Performance data can be used to verify PV system output expectations and identify problems that may
require service or maintenance. Most inverters provide some indication of performance and operating
status, such as power output or energy production, and fault or error indicators. This information is
extremely helpful in verifying proper system operation. Also ensure that the system can be disconnected
and shut down safely and that it starts properly. Knowledge of the specific equipment used and the
product installation and operation instructions are crucial to verifying their safe and proper operation.
For simple interactive (grid-tied) PV systems without energy storage, the key indicators for system
performance are:
 AC power output (kW)
 AC energy production (kWh)
AC power output (kW)
The AC power output (kW) for an interactive system is determined by the rated DC power output of the
solar array, the inverter efficiency and system losses, and is proportional to solar irradiance on the array.
Measurement of AC power output is usually given on inverter output displays, or can be recorded over
time and accessed remotely. Power measurements may be an instantaneous (snap-shot) measurement,
or averaged over a certain interval.
The AC power output of an interactive PV system at any moment can be compared with expectations,
using the basic translation formulas for solar irradiance and temperature. The AC power output can be
read from inverter displays or by additional power meters, and the array temperatures and solar
radiation in the plane of the array can be measured with simple handheld meters without working on
energized equipment. Power verification can be done any time when the system is operating under
steady sunlight conditions, preferably at higher irradiance levels.
Generally, the AC power output for interactive systems can be related to the rated maximum DC power
rating of the array and adjusted by a number of derating factors. The factors include several types of DC
and AC system losses and power conversion efficiencies, which in combination result in AC power
output typically between 70% to 85% of the PV array DC rating at Standard Test Conditions (STC).
AC energy production (kWh)
The AC energy production (kWh) for grid-tied PV systems is measured over periods of months and years
to compare with sizing and long-term performance expectations. The AC energy production for grid-tied
PV systems can be estimated using popular tools such as PVWATTS: http://pvwatts.nrel.gov/. PVWATTS
performs an hour-by-hour simulation for a typical year to estimate average power output for each hour
and totals energy production for the entire year. PVWATTS uses an overall DC to AC derate factor to
determine the rated AC power at STC. Power corrections for PV array operating temperature are
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Performance Verification Guide for Grid-Tied PV Systems
performed for each hour of the year as PVWATTS reads the meteorological data for the location and
computes the performance. A power correction of -0.5%/°C for crystalline silicon PV modules is used.
The actual PV power array power output will be based on the system design and environmental
conditions. System adjustment factors for module mismatch and DC and AC wire losses vary based on
the actual installation. Another factor that can limit the irradiance is soiling on the array. This is
particularly a concern in climates in the western U.S. that can go for several months without rain.
*See http://www.solarworld-usa.com/~/media/www/files/technical-bulletins/pv-watts-estimatedperformance-data.pdf for more information on SolarWorld modules and associated derate factors. Note
the listed 84% average derate factor for SolarWorld designs assumes the solar modules are new and
free of shade.
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Performance Verification Guide for Grid-Tied PV Systems
Example, expected inverter output power (string inverter)
How much power should a 5,500 (STC) DC-rated PV array using SolarWorld SW 275 monocrystalline PV
modules and an SMA Sunny Boy 5000TL-US inverter produce when the array temperature is 46°C
(115°F) and the irradiance is 800 W/m2? *Note that an array temperature of 46°C is expected under
environmental conditions of 800 W/m2 irradiance and 20°C (68°F) ambient air temperature. The inverter
efficiency is 97%; module mismatch and the DC/AC wiring losses are 1% and 3% respectively, and soiling
is minimal.
System Adjustment Factors:
1. Temperature [1 - (46°C-25°C) × (0.0045/°C)] = .905
2. Irradiance: (IRR ÷ 1000W/m2) = 800 ÷ 1000 = .80
3. Inverter Efficiency: 0.97
4. Mismatch and DC and AC wire losses: 4% total = 0.96
5. Soiling: 0% = 1.0
Inverter Expected Output Power = 5500 × (0.905) × (0.80) × (0.97) × (0.96) × (1.0) = 3708 W
Therefore, in this example, inverter expected output power is 67% of the PV array DC rating:
3689 ÷ 5500 = .674 = 67.4%
*Note, under ideal irradiance conditions of 1000 W/m2, the expected inverter output is 4635 W, or
~84% of the PV array DC rating: 5500 × (0.905) × (1.0) × (0.97) × (0.96) × (1.0) = 4635 W
Example, expected inverter output power (microinverter):
How much power should an Enphase M250 microinverter produce when paired with a SolarWorld 275
monocrystalline PV module when the array temperature is 46°C (115°F) and the irradiance is 800 W/m2?
*Note that an array temperature of 46°C is expected under environmental conditions of 800 W/m2
irradiance and 20°C (68°F) ambient air temperature. The inverter efficiency is 97%; module mismatch
and the DC/AC wiring losses are 1% and 3% respectively, and soiling is minimal.
System Adjustment Factors:
1. Temperature [1 - (46°C-25°C) × (0.0045/°C)] = .905
2. Irradiance: (IRR ÷ 1000W/m2) = 800 ÷ 1000 = .80
3. Inverter Efficiency: 0.97
4. Mismatch and DC and AC wire losses: 4% total = 0.96
5. Soiling: 0% = 1.0
Inverter Expected Output Power = 275 × (0.905) × (0.80) × (0.97) × (0.96) × (1.0) = 185.4 W
Therefore, in this example, inverter expected output power is 67% of the PV array DC rating:
185.4 ÷ 275 = .674 = 67.4%
*Note, under perfect irradiance conditions of 1000 W/m2, the expected inverter output is 231 W, or
~84% of the PV array DC rating: 275 × (0.905) × (1.0) × (0.97) × (0.96) × (1.0) = 232 W
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Works Cited
William Brooks, J. D. (2012, March). NABCEP-PV-Installer-Resource-Guide-August-2012-v.5.3.pdf.
Retrieved September 03, 2014, from NABCEP: http://www.nabcep.org/wpcontent/uploads/2012/08/NABCEP-PV-Installer-Resource-Guide-August-2012-v.5.3.pdf
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