ASTM E2848 is a short-term capacity test standard used to verify that a PV system performs as expected under real-world conditions. Conducted over several clear-sky days, it collects high-frequency power and environmental data, filters out anomalies, and applies regression modeling to normalize system output. Paired with ASTM E2939 for reference conditions, it helps stakeholders assess system performance before project handover. Commonly used for acceptance testing, performance guarantees, and issue detection, E2848 ensures problems are caught early—protecting investments and supporting contractual compliance.
Understanding ASTM E2848 & Its Role in PV Capacity Testing
In the solar industry, proving that a photovoltaic (PV) system performs as expected is critical—especially at milestones like commissioning or financial close. ASTM E2848 supports this by providing a short-term capacity test method to verify power output under real-world conditions.
Often paired with ASTM E2939, which defines standard reference conditions like irradiance, temperature, and wind speed, the two standards ensure results are consistent and comparable. This combination helps stakeholders evaluate performance against design expectations using a reliable baseline.
What Is ASTM E2848?
Developed by ASTM International, ASTM E2848 provides a standardized method for measuring and normalizing the output of a PV system over a multi-day test window, typically five or more consecutive clear-sky days. During this period, high-frequency data is gathered from the system and its operating environment. The collected data is then used to calculate a normalized capacity value that represents how the system would perform under either standard test conditions (STC), typically 1000 W/m² irradiance and 25°C temperature (often referred to a Rated Capacity Test), or reporting conditions (RC) as derived from test data or site-specific typical mean year metrological data. The stakeholders must weigh the pros and cons of what to use as the common reference point.
Why It Matters
Before a project reaches completion, key stakeholders—including developers, EPC contractors, financiers, and owners—require verification that the system meets its performance targets. ASTM E2848 enables this by offering a data-driven approach to catch issues early, such as improper inverter settings, shading miscalculations, or module installation errors. Addressing these discrepancies before handover can prevent costly delays or disputes.
How the Test Works
The ASTM E2848 method includes three main steps:
1. Data Collection
Over the test period, data is collected at short intervals—typically 1-minute averages aggregated into 15-minute bins. Similarly, a representative dataset from the agreed-upon Energy Model is processed:
- AC power output at the Point of Interconnection
- Plane-of-array (POA) irradiance
- Module temperature
- Wind speed
Stakeholders must also consider secondary parameters impacting facility production and agree on appropriate correction methods to account for their influence on the primary metric used to assess liquidated damages.
2. Data Filtering
Before analysis, both measured and simulated test data are filtered to exclude non-representative values.
- Low-irradiance periods (e.g., early morning or evening)
- Shading events or inverter clipping
- Sensor errors or anomalous readings
3. Regression Modeling
Using a multiple-variable non-linear regression, the method isolates how environmental conditions affect system output, enabling normalization. The standard regression model looks like this:
P = E (a1 + a2 x E + a3 x Ta + a4 x v)
Where:
P—photovoltaic system power at the point of interconnection, W
E—plane-of-array irradiance, W/m2
Ta—ambient temperature, °C
v—wind speed, m/s
a1, a2, a3, a4 : Set of regression coefficients derived from either the test data or the simulated data.
This model is then evaluated at the agreed-upon reference point to calculate a normalized capacity value. If the actual output deviates significantly from the expected design output, it signals that performance issues need to be addressed before proceeding.
Key Applications
- Acceptance Testing – Verifies system performance prior to project sign-off
- Performance Guarantees – Supports contractual obligations for short-term energy output
- Issue Detection – Identifies issues such as soiling, shading, or design mismatches
Bottom Line
ASTM E2848, supported by ASTM E2939, offers a rigorous and consistent framework for short-term PV system capacity verification. While it is not designed for continuous monitoring (like IEC 61724, which covers long-term performance ratios), it serves as a critical checkpoint—ensuring systems meet expectations before money changes hands or ownership transfers.