ASTM E2848-11

Because there are a number of choices in this test method that depend on different applications and system configurations, it is the responsibility of the user of this test method to specify the details and protocol of an individual system power measurement prior to the beginning of a measurement.

Unlike device-level measurements that report performance at a fixed device temperature of 25°C, such as Test Methods E1036, this test method uses regression to a reference ambient air temperature.

System power values calculated using this test method are therefore much more indicative of the power a system actually produces compared with reporting performance at a relatively cold device temperature such as 25°C.

Using ambient temperature reduces the complexity of the data acquisition and analysis by avoiding the issues associated with defining and measuring the device temperature of an entire photovoltaic system.

The user of this test method must select the time period over which system data are collected, and the averaging interval for the data collection within the constraints of 8.3.

It is assumed that the system performance does not degrade or change during the data collection time period. This assumption influences the selection of the data collection period because system performance can have seasonal variations.

The irradiance shall be measured in the plane of the modules under test. If multiple planes exist (particularly in the case of rolling terrain), then the plane or planes in which irradiance measurement will occur must be reported with the test results. In the case where this test method is to be used for acceptance testing of a photovoltaic system or reporting of photovoltaic system performance for contractual purposes, the plane or planes in which irradiance measurement will occur must be agreed upon by the parties to the test prior to the start of the test.

Note 1—In general, the irradiance measurement should occur in the plane in which the majority of modules are oriented. Placing the measurement device in a plane with a larger tilt than the majority will cause apparent under-performance in the winter and over-performance in the summer.

The linear regression results will be most reliable when the measured irradiance, ambient temperature, and wind speed data during the data collection period are distributed around the reporting conditions. When this is not the case, the reported power will be an extrapolation to the reporting conditions.

Accumulation of dirt (soiling) on the photovoltaic modules can have a significant impact on the system rating. The user of this test may want to eliminate or quantify the level of soiling on the modules prior to conducting the test.

Repeated regression calculations on the same system to the same RC and using the same type of irradiance measurement device over successive data collection periods can be used to monitor performance changes as a function of time.

1.1 This test method provides measurement and analysis procedures for determining the capacity of a specific photovoltaic system built in a particular place and in operation under natural sunlight.

1.2 This test method is used for the following purposes:

1.2.1 acceptance testing of newly installed photovoltaic systems,

1.2.2 reporting of dc or ac system performance, and

1.2.3 monitoring of photovoltaic system performance.

1.3 This test method should not be used for:

1.3.1 testing of individual photovoltaic modules for comparison to nameplate power ratings,

1.3.2 testing of individual photovoltaic modules or systems for comparison to other photovoltaic modules or systems,

1.3.3 testing of photovoltaic systems for the purpose of comparing the performance of photovoltaic systems located in different places.

1.4 In this test method, photovoltaic system power is reported with respect to a set of reporting conditions (RC) including: solar irradiance in the plane of the modules, ambient temperature, and wind speed (see Section 6). Measurements under a variety of reporting conditions are allowed to facilitate testing and comparison of results.

1.5 This test method assumes that the solar cell temperature is directly influenced by ambient temperature and wind speed; if not the regression results may be less meaningful.

1.6 This test method is not applicable to concentrator photovoltaic systems; as an alternative, Test Method E2527 should be considered for such systems.

1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.