Residential Solar Site Measurements

New Developments

As the PV industry has matured, the expectations for accurate measurements have been ratcheting up. Increasingly, for the solar installer, competitive pressure to reduce cost of sales and BOS costs must be balanced with the financing companies’ requirements to provide accurate up-front site measurements, design estimates and energy production guarantees. The industry’s tools and best practices are evolving and maturing to keep pace. Here I describe recent developments and trends, including tools and best practices for measuring roof dimensions and shade, estimating system performance, and evaluating the impact of solar leasing options and performance guarantees on site measurement approaches. In addition, I address when, where and how the available technologies are most appropriately applied to help balance the needs of the different stakeholders associated with a PV system.

Stakeholder Perspectives on Site Measurement Accuracy

Throughout a typical residential PV system sales and installation life cycle (see Figure 1), the various stakeholders have different motivations and perspectives with regard to the accuracy of site measurements and the corresponding system performance estimates and guarantees.

Homeowner. Homeowners invest in solar energy because they want to reduce their energy costs and do so without adding new hassles and headaches. Their primary metric is their monthly electricity bill, before and after the installation, combined with any new financing payments. They may also want to view the system’s instantaneous or historical performance with a simple web interface or smartphone app. The method the homeowners use to finance the system may also influence how they view the measurements and performance. If they own the system, they want optimum production and may be concerned about ongoing maintenance. If they have a solar lease with an energy production guarantee, they may want to compare energy production to the guarantee and may not be as concerned with optimizing production.

Installer. The company responsible for selling, installing and maintaining the system typically feels pressure to close a sale quickly, with moderate and predictable costs. The inside salesperson’s goal is to close a sale over the phone. The outside salesperson attempts to close on the first site visit. Either way, sales representatives require accurate roof parameters and shade measurements so they can perform accurate system sizing and energy production estimates for the sales quote. After the sale, often an auditor or designer performs a more detailed on-site evaluation and makes any required adjustments to the initial design and system performance predictions. After installation, the installer wants assurance that the system performs to expectations within the warranty and/or performance guarantee period. Ultimately, the installer wants satisfied customers and minimal long-term risk to ensure repeat business and financial success.

State and local governments. In recent years, state programs have driven many of the industry’s best practices for site measurements. Public accountability and political pressure to ensure that subsidized systems meet a minimum quality and performance standard have led to the development of required procedures for installers. For example, many of the leading states that support solar energy have solar access measurement requirements for their incentive programs. Some programs require that the proposed PV system meet a minimum solar access value, while others adjust the incentives in proportion to the available solar access.

State or utility programs that pay an up-front incentive based on system capacity often adjust the incentive to include shade values. In some programs, 10% shade means a 10% reduction in rebate value. The California Solar Incentive, for example, prorates the rebate based on the amount of shading. Frequently, the financial impact of shading is reflected in the actual energy production of the installed system to a greater degree than in the rebate payment.

Solar financing company. With the dramatic rise in thirdparty financing in the form of solar leases and power purchase agreements (PPAs), there has been a significant shift in the dynamics of residential site measurements. Financing companies and their investors want to optimize financial returns while controlling risk. Site measurements are supplied by the installer and are critical to determining the project’s financial success. Increased measurement accuracy improves predictability and reduces guard bands built into the investment model to account for system performance variability, thus allowing for better all-around terms for the investor, the installer and the customer. Better terms, such as lower interest rates, favorable performance guarantees and lower baseline energy rates, enable an attractive offering that helps win the deal by reducing the customer’s payments and providing an assurance of energy production.


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