Performance Metrics

The solar industry is introducing new technologies that will reduce the cost of solar power and simplify the installation process. The early adopters of solar have been knowledgeable about the technology and conversant in the language of the energy industry and therefore understand how to evaluate products and system performance. However, as the products enter the mass market, the industry must address the fact that currently available metrics, both at the component and system level, are neither uniformly used by the industry nor readily understood by solar consumers.

The lack of consistency and consumer-friendly metrics is an issue when evaluating individual components and the system as a whole. First, at the component level (individual solar modules, inverters, racks, etc.), it is difficult to compare and evaluate the performance of products in the marketplace. At the total system level, it is difficult to anticipate the energy output of a system, gauge how it relates to current energy consumption, and compare installation quotes from different system integrators. The lack of standard metrics and language contributes to a reluctant marketplace that slows the growth of a promising market. Finally, although certification processes for solar equipment are well-understood by manufacturers, there are limited means to usher products through the certification cycle, introducing uncertainty and costs for new products.

Objectives

In order to mitigate market confusion and assist the introduction of new products, the industry needs to educate consumers, standardize performance metrics at the component level, and use a uniform language to present system performance. To further assist the introduction of new products, the industry should support additional testing laboratories to shorten the certification cycle. SolarTech has identified the following recommendations in these areas, which we will work to address and correct as an industry.

Consumer Education

The industry must make a concerted effort to educate the consumer of the various components of a solar system, what factors impact a system’s performance at the time of installation, how to project and measure output of a system over its lifetime, and how to identify degradation of system performance independent of short-term weather conditions. Although elements of such an education program exist today in various forms, the information is not consolidated, varies in quality, and utilizes inconsistent language and metrics.

In order to address these issues, SolarTech intends to prepare a Solar Manual that defines and describes, among other things, the following:

• A means for consumers to calculate annual energy usage based on past bills,
• Existing performance metrics used in the industry,
• Factors that impact system performance, such as geographic location, shading, array orientation, and system components & efficiencies,
• A standardized formula or metric for consumers to project system output in annual kilowatt hours based on equipment ratings and other factors.

This Solar Manual, prepared and backed by the industry through SolarTech will be a critical foundation for consumers to evaluate components and systems and ultimately make a more informed decision. As this manual is developed, existing resources, such as those created under the California Solar Initiative, will be referenced to ensure consistency.

Standardization of Reporting for Module Specifications

As the primary energy converters, solar modules are an important component of a solar system. Most modules undergo safety certification and reliability testing that are widely understood and, in many cases, required (e.g., UL, IEC). However, the power output of these products is reported by module manufacturers using varying metrics. For example, module manufacturers typically report the Peak Power (DC power output) of their modules under Standard Test Conditions (STC), while some report power under PVUSA or “Practical” Test Conditions (PTC). In California, PTC ratings are calculated and published by the CEC.

Compared to the STC metric, the PTC metric more accurately presents the expected power output of a module, which is more relevant to the end user. However, wide variamce exists in the performance tolerance between manufacturers’ STC and PTC ratings, further supporting a recommendation for module manufacturers to report both ratings and to standardize on testing criteria. We recommend industry-wide agreement on a single standard for testing and certifying modules.

Standardization of Language and Metrics for System Performance

A solar system’s performance is a function of many factors – the DC power rating of the modules, the performance of various components in a system (inverter(s), etc), panel/inverter wiring and string optimization, shade, dust and debris, ambient temperature, the geographic location, and how it was installed (tilt, azimuth, etc.). Such diverse issues add to the multitude of performance metrics and varying language faced by potential solar consumers (see below table), which makes it difficult to compare systems and evaluate the real performance potential of a system. In order to minimize confusion, the industry must adopt a uniform language when presenting system performance and a standard set of factors to calculate/anticipate system performance.

In terms of using a common language to describe system performance, the most relevant and meaningful measurement of system performance is the annual energy output of each system measured in kilowatt-hours rather than the instantaneous measures of generation under ideal or laboratory conditions outlined above. Annual kWh of generation is a metric end users are familiar with (their existing electric bills are measured in AC kilowatt-hours), is a metric that can be easily compared when evaluating different systems, and is a required input when measuring the economic value of a system. The utilization of a common, understandable metric – annual kilowatt-hours – will set accurate expectations for system performance and facilitate informed purchasing decisions by consumers.

The primary reason that a standard does not currently exist for quoting a system’s potential performance is due to a host of factors that drive a system’s ultimate performance. Known as “derate factors”, these convert the DC rated power of the modules to an AC power rating for the system as a whole. There are tools today that utilize reasonable estimates of these derate factors to estimate AC power output. California’s Expected Performance Based Buydown (EPBB) calculation, for example, utilizes regional weather factors, the effect of shading on the system and other estimates of component derate factors to calculate estimated system performance. Because it is used to calculate rebates, most installers in California are familiar with the EPBB calculator.

There are a number of tools that are used to model system performance. A standard tool should be identified that takes into consideration most of the factors impacting a system’s performance and describes system performance using a relevant and understandable figure (annual kilowatt-hours). One standard tool that can be uniformly adopted by solar system installers and integrators is California’s calculator for estimating system performance and EPBB output – estimated annual kilowatt hours and peak performance kWH (summer months); however, this calculator is not universely recognized today as a standard in all circumstances nor does it take into account all existing derate factors.

Establishment of Local Certification Lab

PV modules and inverters (the two main products in a solar energy system) undergo rigorous safety and performance testing before they can be made available to the market. U.S. market testing is currently performed at the Arizona State University (“ASU”) Photovoltaic Testing Facility with standards specified by Underwriters Laboratory (“UL”) and the International Electrotechnical Commission (“IEC”). These tests can take six months or longer. Part of the challenge in this delay is the limited capacity available at ASU to meet the increasing market growth of the solar industry.

To help address these issues, SolarTech supports efforts to establish a second certification facility in Silicon Valley to test both PV modules and inverters to existing industry standards. This local effort will yield several benefits: 1) products will receive certifications in a timely manner, and consequently new products will get to market more rapidly, and 2) the testing center will provide a direct feed-back loop to local industry to support more rapid product improvements.

Summary of Recommendations

SolarTech supports industry efforts at defining standard performance metrics, and believes such standards will improve transparency and trust, and facilitate industry growth. In addition to establishing performance standards for modules, and establishing a common metric to communicate system output, performance of other system elements should be addressed. Specifically, PV modules, string sizing, and inverter wiring are other areas for further standardization and improvement. Moving forward, SolarTech will measure success against the above initiatives, and will actively support development of useful metrics and standards across the PV industry.