ASTM E2848 Capacity Test for Beginners
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Capacity and performance ratio tests are essential tools used to showcase the efficiency of photovoltaic (PV) plants to potential buyers or lenders, helping de-risk their investment. These tests provide a clear and standardized way to evaluate whether a PV system performs as expected compared to its design model.
One widely recognized standard is ASTM E2848-13, "Standard Test Method for Reporting Photovoltaic Non-Concentrator System Performance." This test compares the actual performance of a PV system against its modeled performance, accounting for some level of uncertainty. By following a well-defined and controlled method established by a standards committee, this test ensures consistency in evaluation and sets a baseline for quality expectations.
**Why does this matter for project owners and financiers?**
Failure to meet the test criteria or delays in completing it can have financial consequences, such as triggering liquidated damages (LDs). Since these tests are typically conducted at the end of the project lifecycle, any issues found during testing can affect the final payment or financing terms.
**How is the test carried out?**
While the concept seems simple, the process involves detailed calculations that require careful attention to avoid errors. Here's a breakdown of the steps:
**Step 1 – Gather inputs**
To ensure a smooth process, make sure you have the following ready before starting:
- A valid PVSYST model.
- Experience with regression modeling and a working test sheet.
- A fully operational system with functioning equipment and calibrated weather stations.
- At least 5–7 days of system operation to collect data, which may vary depending on weather conditions.
**Step 2 – Export data**
Prepare the data for analysis by gathering the following:
- From PVSYST: Annual raw data including Global Horizontal Irradiance (GHI), ambient temperature, wind speed, correction factors for plane of array (POA) irradiance, and time/date stamps.
- From your data acquisition system: Raw data including date/time, production meter readings, temperature, wind speed, and POA irradiance.
- Review mechanical and electrical drawings to identify any potential issues like shading or inverter clipping that could affect performance.
**Step 3 – Filter the data for quality**
This is one of the most complex parts of the test. The data must be cleaned to remove low-quality or erroneous entries. Minimum requirements include:
- At least 50 fifteen-minute data points (or 750 minutes of data).
- Exclude data below 400 W/m² and ensure at least 98% of the inverter’s nameplate capacity is met.
- POA irradiance should fall within a +/-20% range.
- While there are no strict limits on temperature or wind data, they can be excluded if they don’t align with the system’s performance.
**Step 4 – Run the regression**
Once the data is clean, run the regression analysis to determine the power capacity and standard error of the measurements. If the ratio of measured power to modeled power is above 95%, and the regression error is under 5%, the test is considered successful.
If results are not satisfactory, common issues include:
- It may take longer than expected to gather enough high-quality data, especially depending on the season and system design.
- The PVSYST summary report isn't sufficient; you need raw, detailed data for accurate analysis.
- Improper setup during the first attempt is common, even with the ASTM guidelines.
- Poorly calibrated sensors can significantly impact the accuracy of the test results.
**Who conducts these tests?**
At Pure Power, our Owners Engineering team specializes in conducting ASTM E2848 and other performance tests. As an independent third party, we ensure that every step is followed precisely, providing a fair and reliable assessment of your PV system’s performance. Whether you're an owner or financier, our expertise helps you make informed decisions with confidence.