The Federal Energy Management Program (FEMP) helps federal agencies make informed decisions about the instrumentation, data acquisition, processing, and reporting platforms available to monitor the performance of photovoltaic (PV) systems and ensure that the systems deliver their expected benefits over a long performance period (greater than 25 years).

The federal government has installed over 3,000 PV systems at federal sites as of 2020. Operations and maintenance (O&M) of these assets is critical to ensuring their safety, performance, longevity, resilience, and expected cost savings. With PV system monitoring, agencies are able to identify and address challenges related to performance in real time.

Understanding Solar Photovoltaic System Performance: An Assessment of 75 Federal Photovoltaic Systems

This report offers recommendations for improving performance of federal PV systems through operations and maintenance.

Providers of PV Monitoring Systems

PV system monitoring platforms may be offered by:

  • Project developers and fleet operators
  • Manufacturers of microinverters, direct current (DC)-optimizers, string inverters, and central inverters
  • Engineering, procurement, and construction (EPC) companies
  • Independent third-party software platforms.

Methods to Deploy a PV Monitoring System

Platforms differ in hardware and software requirements and can include cloud applications (i.e., software as a service). The alternative deployment methods include:

  • Cloud implementation
  • Application on own device (e.g., tablet, computer)
  • Installation on own computer network
  • Use of provider's hardware.

Direct Measurements Required for a PV Monitoring Platform

Power conditioning equipment such as inverters often include metering and instrumentation that can easily report parameters for a monitoring system. Alternatively, additional instrumentation can be installed to measure quantities of interest, such as delivered power.

Electrical measurements include:

  • Real power and reactive power—Real power is actually consumed and measured in units of kilowatts, and reactive power flows back and forth (mainly due to the coils in motors) in units of kilovolt-ampere reactive (kVAR)
     
  • Voltage and current at the level of the whole PV system, each inverter, each series string of PV modules, or each PV module.

Weather data and satellite data are increasingly used in monitoring platforms instead of on-site measurements of environmental conditions. Environmental conditions include:

  • Incident sunlight
  • Ambient temperature
  • PV module temperature. 

Features of PV Monitoring Platforms

Process diagram shows satellite data processed to incident solar radiation, which is sent to a near-real-time production model and compared to measured data from a production meter. A dashboard communicates with utility operations via telemetry and curates data.

A PV monitoring platform integrates satellite data with solar resource data into a production estimate from a computer model (e.g., SAM), which is compared to measured data from a PV system production meter. A dashboard calculates key performance indicators, communicates with utility operations, curates data, and reports results.

Illustration by Andy Walker and Fred Zietz, NREL.

PV monitoring platforms may include some or all of the following features:

Calculations and analysis—Data interpretation based on comparison with neighboring systems or by comparison with a computer model based on PV system description and environmental conditions (e.g., System Advisor Model [SAM]). 

Reports of key performance indicators—Monitoring platforms can provide reports of availability and performance ratio. Platforms can host reports on websites or push reports to stakeholders by email.

Financial calculations—With the details of a utility rate schedule (tariff), platforms may calculate the economic revenue or utility cost savings associated with the measured energy delivery based on time-of-day and seasonal variation in utility rates. Financial calculations might also include the value of lost production due to downtime or reduced output.

Errors and alarms—Monitoring platforms can inform operators of abnormal conditions and faults that require immediate attention. Platforms can issue alarms by email or text messages.

Issue and track maintenance tickets—Monitoring platforms can allow users to issue maintenance tickets to address identified problems and to track tickets to the solution of the problem.

Interface with utility or larger system—Monitoring platforms can report required information to the utility company and may also receive signals from the utility for control of the PV system setpoints, such as curtailment of power delivery or to source reactive power when needed. 

Curate updated project documents—Performance monitoring is closely related to the configuration of the plant and any changes that have been made over time. Thus, it is beneficial to store updated as-built drawings, O&M contracts, and other project documentation within the monitoring platform.

Expected Cost of PV System Monitoring Platforms

Manufacturers of microinverters and solar optimizers that serve the residential market have built the instrumentation into their power devices to make monitoring available at a small scale. However, for most installations some additional costs may be incurred for power measurement instruments, internet connectivity, or other features not provided by monitoring capability internal to the inverter. In recent projects supported through FEMP's technical assistance program, power metering current transducers, voltage taps, and power meter electronics cost $1,800 for a single-phase 240 volt alternating current (VAC) meter in California and $3,800 for a three-phase 480 VAC meter in Washington, D.C. A budget of approximately $5,000 should be adequate for a complete set of instrumentation for monitoring.

In addition to the initial cost to install any new instrumentation and internet connection, there will be ongoing periodic (per month, per year, per contract length) subscription costs paid to platform operators to collect, curate data (archive and index, make available) and provide the features listed above. Some platforms may be free to the user (no cost, or paid in whole-system cost), but most entail a fee, and in the case of large utility-scale plants there is always a substantial fee. Costs may range from $0 to $100/year for residential and commercial systems up to about 100 kW; for significantly larger systems, the annual cost can increase to approximately $50,000/year for detailed monitoring and sophisticated data analysis of a utility-scale PV plant (e.g., 100 MW). For the size and type of PV systems typical of federal facilities, a budget of $1,000/year would be a reasonable expectation of a software platform subscription cost. Details of cost and features can be found in an internet search on keywords such as “PV monitoring software comparison” or “best solar monitoring systems.”

Best Practices in PV System Performance Monitoring Platforms

  • Consider instrument calibration and servicing requirements in the selection and operation of the monitoring platform
     
  • Proactively share information with stakeholders
     
  • Consider data presentation; good reporting is essential
     
  • Allow maintenance of hardware and software by a variety of service providers (non-proprietary)
     
  • Ensure "operational continuity" (ability to backup and restore data)
     
  • Allow third-party access to data as required for custom work
     
  • Provide the best available cybersecurity; consider back-haul of data other than the site federal network (e.g., cellular data) and supply-chain security 
     
  • Consider "bundling" small systems together for more efficient management and consolidation of data and platform contracts
     
  • Make sure the entire monitoring system is on an uninterruptible power supply.

Next Steps and Resources for Agencies Interested in Improving PV System Performance

Federal agencies are encouraged to consider subscription to a monitoring service that can report on energy delivery and key performance metrics, send notifications to staff, and diagnose some problems automatically. Check operational indicators frequently, examine periodic reports (monthly), and act on information to restore any measured underperformance. FEMP offers resources related to PV system performance and O&M, including publications, sample procurement specifications, and trainings.

Learn more about Optimizing Solar Photovoltaic Performance for Longevity.

Key Resources

Recommended FEMP Courses