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The federal government has installed more than 2,900 solar photovoltaic (PV) systems. The electricity generated from these on-site systems has increased 12-fold over the last 10 years. PV systems have 20- to 30-year lifespans. As they age, their performance can be optimized through conducting proper operations and maintenance (O&M).

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Solar PV Performance Initiative

The Federal Energy Management Program (FEMP) PV Performance Initiative aims to understand the performance of the federal PV fleet. FEMP is performing studies to compare existing systems’ performance to expected metrics (given age, location, etc.). Anonymized and aggregated study results will be posted here along with future opportunities for federal agencies to participate in the PV Performance Initiative.

Understanding Solar Photovoltaic Performance

Performance ratings of PV module are measured under standard test conditions (1,000 W/m2 of sunlight; 25°C cell temperature). In practice, however, the intensity of sunlight is usually less than 1000 W/m2, and the temperature is typically hotter than 25°C.

Additionally, once PV systems are deployed, several factors can impact their expected production (or electricity generated). They include:

  • Solar Resource: Although the solar resource is variable, most of the variability is predicable based on time of day, time of year, and the angle that sunlight hits the PV module surface. In fact, the solar resource would be perfectly predictable based on clear-sky models if not for clouds, which are not as predicable. So, we typically rely upon a site’s historic climate data rather than the purely theoretical clear-sky model when assessing the expected performance of a PV system.
  • Age: PV module efficiency unavoidably degrades at about 0.5% per year. Failure rates are also higher in later years as the equipment ages.

Events such as severe weather can also impact PV system performance in unpredictable ways. Comparing the system’s actual output with the expected output can quantify and identify underperformance.

Measuring Your System’s Solar Photovoltaic Performance

Two key indicators of PV performance are performance ratio and availability.

Performance ratio refers to the fraction of the expected power output when the plant is available. The performance ratio can be evaluated over any time period (instantaneously, monthly, annually). It is calculated as the ratio of actual production (measured by a production meter on the PV system) to model production, which is based on a computer model of the same measured solar resource and temperature data over the selected time period (taking into account age of the system).

Performance ratio = actual production/model production (%)

Availability refers to the percentage of time that the system is operational and capable of delivering power if the solar resource and the grid are both working. Availability is calculated from time-series data according to:

Availability = (1-downtime)/total time (%)

These two key performance indicators affect utility cost savings and life cycle cost in different ways.

Operations and Maintenance for Photovoltaic Systems

Proper O&M helps ensure that a PV system is performing as expected over its 20- to 30-year lifespan. Comprehensive O&M can help increase performance ratio and availability.

Actuarial data indicate that comprehensive PV system O&M could improve the average performance ratio of systems from 92% to more than 95%, which is a substantial increase in revenue and environmental benefits.

O&M benefits include:

  • Increased efficiency and energy delivery
  • Decreased downtime
  • Extended system lifespan
  • Improved safety and reduced risk.

See Best Practices for Operation and Maintenance of Photovoltaic and Energy Storage Systems to learn more about the benefits of O&M and how to properly maintain your PV systems.

Challenges to conducting proper O&M include the high costs associated with maintaining small or remote systems, lack of budget, and lack of in-house expertise. Although O&M for PV systems is usually low compared to O&M for other types of equipment, it is important to measure performance and have a plan for repairing equipment that fails.

A comprehensive PV O&M plan should include:

  • Administration, contracting, record keeping, budgeting, and warranty enforcement
  • Interaction with electric utility operations
  • Preventive or routine planned maintenance and cleaning
  • Corrective maintenance or unplanned repairs
  • Materials recycling and disposal.

Typical Causes of Photovoltaic System Failure

– Monitoring contracts expire, staff rotate to new positions, passwords are lost, and files get misplaced. Therefore, it is important to keep monitoring systems up and running to keep an eye on PV system performance. 

– PV modules have no moving parts and require very little maintenance. However, some issues requiring repair include hot spots in the module, cracks in the cells, water intrusion into the junction box or module laminate, or delamination of the module.

– The majority of downtime and maintenance is associated with inverters. Small micro-inverters and string inverters are generally replaced, whereas larger central inverters are usually repaired by replacing components.

– Capacitors and printed circuit boards are common sources of inverter problems.

– Faults in wiring caused by physical damage include short circuits, open circuits, and ground faults. Rather than just replacing fuses or resetting ground-fault protection devices, it is important to discover and repair the root causes of the problems.

– Capacitors and printed circuit boards are common sources of inverter problems.

– Faults in wiring caused by physical damage include short circuits, open circuits, and ground faults. Rather than just replacing fuses or resetting ground-fault protection devices, it is important to discover and repair the root causes of the problems.

Tips for Developing an Operations and Maintenance Plan

In creating a comprehensive PV O&M plan, include the following elements:

  • Contact information for responsible parties, including site owners and off-takers of power, utilities, local jurisdictions, local landowners, and emergency personnel
  • System documentation including: 
    • As-built drawings, specifications, site plans, photo records, special safety considerations, electrical single-line diagrams, schematics, drawings, and installed components' "cut sheets" and warranties (including warranties from the system installer)
    • Performance estimates
    • Insolation/shade studies (including a description of nominal conditions to make it easier to see malfunctions or deviations)
    • Operation manuals associated with any of the equipment (including emergency shutdown and normal operating procedures)
    • Contracts for preventive maintenance, service, and other operations documents, including contacts for each
    • Specified response times and availability
  • Schedules for manufacturer-recommended preventive maintenance measures to preserve warranties and optimize system energy delivery. Include details such as the cost and current supplier of each preventive maintenance measure and special instructions regarding:
    • Hours that work is to be performed
    • Means of access to site
    • Locations where vehicles may be parked and equipment staged
  • Descriptions of operational indicators, meters, and error messages; physical monitoring setup and procedures for archiving and reporting performance data; and procedures by which data are regularly examined for system diagnostics and analytics
  • Inventory of on-site or easily accessible spare parts and a process for determining when other spare parts need to be ordered based on component failure history
  • Clearly defined availability and performance metrics and events outside of management control
  • Focused training program for all O&M staff on processes relevant to each worker and the equipment they may be working on
  • Chronological O&M log containing:
    • Work order and task tracking, including initial commission report
    • Inspection reports
    • Ongoing O&M history
  • Procedure for responding to alerts from monitoring diagnostics, error messages, or complaints from the building owner; a troubleshooting guide for common problems
  • List of all equipment including its make, model, serial numbers, and a supplier of replacement parts. Include a diagram of the system with locations of all equipment (to spot trends in manufacturing defects by placement in system) and serial number
  • Determine criteria to decide whether to repair or replace a component, criteria to decide whether to "cannibalize" a string of modules to source replacement modules or to order new parts, and criteria to decide if an energy storage system with declining energy storage capacity should be replaced
  • Procedures for reacceptance testing following a repair
  • Budget for an O&M program including costs for monitoring and diagnostics, preventive maintenance, corrective maintenance, and means to minimize exposure (i.e., a line of credit) to replace the inverter or perform more expensive corrective maintenance if needed
  • Plans to address degradation in performance of and perform periodic replacement of the energy storage system. The plan should be revised based on test results and actual battery system performance.
    • On-site PV systems that are implemented through a performance contract, such as an energy savings performance contract or a power purchase agreement, often require that an O&M contract be included. Note that hundreds of federal systems have been purchased with appropriated funding and may not currently have an O&M contract in place. 

Key Resources

3rd Edition Guide encourages adoption of best practices to reduce the cost of O&M and improve the performance of fielded systems.
Fact sheet provides an overview of design specifications that include best practices for system survivability identified from 2017 hurricanes.
Presentation covers estimating PV O&M costs, polynomial expansion, and implementation of net present value and reserve account in cost models.
Covers on-site solar photovoltaic systems with an emphasis on third-party designed, financed, owned, and operated systems.