NABCEP PVTS key terms

The instantaneous power density of sunlight falling on a surface, typically expressed in watts per square meter. It drives module output at any given moment and is measured with a pyranometer or reference cell.

The accumulated solar energy delivered per unit area over a period of time, such as a day or year, usually expressed in kilowatt-hours per square meter. It is the energy quantity behind production estimates.

The number of hours per day at 1,000 W/m² irradiance that would deliver the same total energy as the actual sun over that day. A convenient way to compare sites and size arrays.

The laboratory reference conditions of 1,000 W/m² irradiance, 25°C cell temperature, and a specified air mass spectrum used to rate module nameplate power. Real-world output rarely matches these values.

The direct-current power a module is warranted to produce under Standard Test Conditions, printed on its label and used as the basis for system sizing and incentive calculations.

The percentage change in module power for each degree Celsius of cell temperature change, usually negative. It explains why hot rooftop arrays produce less than their rated output.

The actual operating temperature of the photovoltaic cells, which runs well above ambient air temperature because of absorbed sunlight and restricted airflow behind the module. It directly affects voltage and power output.

Nominal Operating Cell Temperature: the cell temperature reached under 800 W/m² irradiance, 20°C ambient air, and specified wind speed. It lets designers estimate real operating temperatures from ambient conditions.

A multiplier accounting for losses that reduce a system's output below the module nameplate value, including temperature, wiring, soiling, inverter, and mismatch losses. Multiplying all derates gives overall system efficiency.

The ratio of actual delivered energy to the theoretical energy the array would produce at its rated power under the same irradiance. It normalizes output and is a standard quality benchmark for completed systems.

A forecast of annual or monthly energy output for a proposed system, built from site irradiance, array size, and loss factors. It underpins savings projections, payback analysis, and customer expectations.

The process of identifying obstructions whose shadows fall on an array and quantifying the energy lost. It may use a site survey tool, solar path chart, or modeling software and often drives module layout decisions.

The degree to which a location receives unobstructed sunlight over the year, considering trees, buildings, and terrain. It is a primary screening criterion for array placement.

The compass direction a module surface faces, usually measured in degrees clockwise from true north. In the northern hemisphere, a due-south azimuth generally maximizes annual energy collection.

The angle between the module surface and horizontal. It affects how much irradiance the array captures and how well rain cleans the glass, and is often set near the site latitude.

A general term for the solar radiation received at a location, often used interchangeably with irradiation in sales conversations. It describes the solar resource available for a project.

A diode inside a module that provides an alternate current path around shaded or faulty cell groups, limiting hot spots and power loss. Its presence affects how partial shading degrades array output.

The ratio of alternating-current power delivered to the grid or loads to the direct-current power drawn from the array. It varies with load and is a key input to production estimates.

A record of a customer's electricity consumption over time, showing hourly, daily, and seasonal patterns. Comparing it with solar production reveals how much generation is consumed on site versus exported.

A billing arrangement that credits customers for electricity exported to the grid, often at the retail rate, offsetting power drawn at other times. Rules vary widely by utility and jurisdiction.

An electricity tariff in which the price per kilowatt-hour changes by time of day and season. It can make solar energy more valuable if production aligns with expensive periods.

A utility charge based on the highest interval of power drawn during a billing period, common for commercial accounts. Solar can reduce it only if it reliably cuts peak demand.

The share of solar generation used directly at the site rather than exported. Higher self-consumption usually improves project economics when export credits are low.

The technical and administrative process of connecting a generating system to the utility grid, including application, review, and approval. It determines timeline and possible equipment upgrades.

A financial benefit such as a tax credit, rebate, or grant that reduces the net cost of a solar purchase. Eligibility rules and timing affect the quoted customer price.

The time required for cumulative savings and incentives to equal the initial system cost. Simple to explain to customers but ignores the time value of money and post-payback savings.

The value today of all future project cash flows, discounted at a chosen rate, minus the initial investment. A positive result indicates the project creates value over its life.

The rate used to convert future cash flows into present value, reflecting the cost of capital or an investor's required return. Higher rates reduce the apparent value of long-term solar savings.

The average cost per kilowatt-hour of energy produced over a system's life, accounting for capital cost, incentives, operations, and output. It allows comparison with utility rates or other generation options.

A contract in which a third party owns the solar system and the customer buys its output at an agreed rate, often with little or no upfront cost. Terms cover price escalation, duration, and end-of-term options.

A financing arrangement in which a third party owns the system and the customer pays a fixed monthly amount for its use. The customer typically receives the energy but not the tax benefits.

Selecting the number of modules wired in series so that voltage stays within the inverter's window across expected temperature extremes. It protects equipment and maximizes harvest.

The voltage measured across a module or string with no current flowing, which rises in cold weather. It sets the maximum voltage the inverter and conductors must safely withstand.

The operating voltage and current combination at which a module delivers its greatest power under given conditions. Inverters continuously track this point as irradiance and temperature change.

An on-site inspection gathering roof condition, dimensions, shading, structural, and electrical information needed to design and quote a system. It confirms or corrects assumptions made from remote imagery.

The written offer presented to a customer describing system design, expected production, price, incentives, savings, warranties, and terms. It is the core sales deliverable and must match the final contract.