Solar Energy Systems Glossary: Key Terms and Definitions

Photovoltaic technology, grid interconnection rules, and incentive structures each carry a precise vocabulary that shapes how installations are designed, permitted, financed, and inspected. This glossary covers the core terms used across residential, commercial, and industrial solar energy systems in the United States. Mastery of these definitions enables accurate communication with installers, utilities, permitting authorities, and financial institutions. The entries below are organized by functional category and cross-referenced with the regulatory frameworks that govern their application.

Definition and scope

A solar energy systems glossary serves as the controlled vocabulary for a multi-disciplinary field that spans electrical engineering, building codes, utility regulation, tax law, and environmental policy. Terms carry specific legal or technical meanings that differ from colloquial usage — "capacity," for example, means peak DC wattage under Standard Test Conditions (STC) per IEC 61215, not average output under real-world conditions.

The scope of this glossary covers:

• Photovoltaic (PV) hardware — panels, inverters, racking, and storage
• System architecture — grid-tied, off-grid, and hybrid configurations
• Interconnection and utility terms — net metering, feed-in tariffs, and islanding
• Permitting and code references — NEC, IBC, and AHJ processes
• Financial and incentive terms — ITC, MACRS, and PPA structures

The National Electrical Code (NEC), Article 690, published by the National Fire Protection Association (NFPA), defines the baseline electrical safety requirements for PV systems in the United States. Local Authorities Having Jurisdiction (AHJs) may adopt NEC with amendments, creating variation across jurisdictions. The International Building Code (IBC), maintained by the International Code Council (ICC), governs structural loading and roof penetration standards.

For a broader orientation to how these terms fit into system design, the Solar Energy Systems Topic Context page provides additional background.

How it works

Technical terms in solar energy systems map directly to measurable physical or financial quantities. Understanding the mechanism behind each term prevents misapplication.

Key hardware terms defined:

1. Watt-peak (Wp) — The rated output of a PV module measured at STC: 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum, per IEC 61215.
2. Inverter — A device that converts DC electricity from PV modules or batteries into grid-compatible AC electricity. Three primary types exist: string inverters, microinverters, and power optimizers paired with string inverters. Full classification is covered in the Solar Inverter Types resource.
3. Maximum Power Point Tracking (MPPT) — An algorithm within inverters or charge controllers that continuously adjusts the electrical operating point to extract maximum power from the array under varying irradiance conditions.
4. State of Charge (SOC) — The remaining capacity of a battery storage system, expressed as a percentage of its total rated capacity (kWh).
5. Depth of Discharge (DoD) — The percentage of battery capacity that has been discharged relative to the total. A battery rated at 10 kWh with a 90% DoD specification can safely discharge 9 kWh per cycle.

Key interconnection and utility terms:

1. Net Metering — A billing mechanism in which excess electricity exported to the grid offsets a customer's consumption charges, typically on a kWh-for-kWh basis. Net metering policies vary by state and utility under rules set by state Public Utility Commissions (PUCs).
2. Anti-Islanding — A safety function required by IEEE 1547-2018 that forces a grid-tied inverter to shut down during a grid outage, preventing energization of utility lines where workers may be present.
3. Interconnection Agreement — A contract between a system owner and the utility, required before a grid-tied system can operate. The Solar Interconnection Process page covers the application stages in detail.
4. Capacity Factor — The ratio of actual annual energy output to the theoretical maximum output if the system operated at nameplate capacity for all 8,760 hours in a year. Utility-scale PV systems in the US Southwest typically achieve capacity factors between 25% and 29% (U.S. Energy Information Administration, Electric Power Annual).

Common scenarios

Residential permitting context: A homeowner installing a grid-tied rooftop system encounters terms such as "service entrance," "backfeed breaker," and "interconnection application" during the permit and approval process. The AHJ will reference NEC Article 690 for electrical requirements and the local adopted building code for structural sign-off.

Commercial system design: A commercial array sized in kilowatts-DC (kWdc) versus kilowatts-AC (kWac) reflects the DC-to-AC clipping ratio, also called the inverter loading ratio (ILR). Commercial projects frequently encounter Modified Accelerated Cost Recovery System (MACRS) depreciation schedules, a federal tax provision allowing 5-year accelerated depreciation on solar assets under IRS Publication 946.

Battery storage integration: The terms "AC-coupled" and "DC-coupled" describe how a battery connects to the PV array. DC-coupled systems share the inverter with the array; AC-coupled systems use a separate bidirectional inverter for the battery. This distinction affects solar battery storage systems sizing and efficiency calculations.

Off-grid design: Systems without utility connection require autonomy calculations based on average daily load (kWh/day), peak sun hours for the site, and battery bank capacity. The term "days of autonomy" defines how many consecutive cloudy days the system can supply load without array input.

Decision boundaries

String inverter vs. microinverter: String inverters are cost-efficient for unshaded, uniform-orientation arrays. Microinverters are specified when 3 or more modules experience shading from different sources, or when modules face 2 or more orientations — because partial shading on a string inverter reduces output from all modules in that string proportionally.

Grid-tied vs. off-grid vs. hybrid: The classification boundary is determined by utility interconnection, not by the presence of battery storage. A grid-tied system with batteries remains grid-tied unless it includes a certified automatic transfer switch that isolates the system during outages per NEC Article 702. Hybrid solar systems occupy the middle classification: they maintain grid interconnection and include battery backup with islanding capability.

DC system voltage classes: NEC Article 690 establishes two voltage divisions for PV source circuits. Systems operating above 1,000V DC require equipment listed for that voltage range and are classified as "high-voltage PV systems," subject to additional wiring method and labeling requirements beyond the standard residential 600V threshold.

Federal tax credit (ITC) eligibility: The Investment Tax Credit, administered under Internal Revenue Code Section 48, applies to the installed cost of solar energy property. The Inflation Reduction Act of 2022 (Public Law 117-169) reset the base ITC rate to 30% for systems placed in service after January 1, 2022. Battery storage systems became eligible as standalone property under the same law. Detailed eligibility conditions are addressed in the Solar Federal Tax Credit (ITC) reference page.

References

• IEC 61215 — Terrestrial Photovoltaic Modules: Design Qualification and Type Approval (IEC)
• NFPA 70 / National Electrical Code, Article 690 — Solar Photovoltaic Systems (NFPA)
• IEEE 1547-2018 — Standard for Interconnection and Interoperability of Distributed Energy Resources (IEEE Standards)
• International Building Code (IBC) — International Code Council (ICC)
• U.S. Energy Information Administration — Electric Power Annual
• Internal Revenue Code Section 48 — Investment Tax Credit (IRS)
• Inflation Reduction Act of 2022, Public Law 117-169 (U.S. Congress)
• IRS Publication 946 — How to Depreciate Property (IRS)