Understanding the basics of solar energy technology, equipment, and terminology.
Solar Energy For Beginners – An introduction to the common equipment and terminology used in solar technology. Topics of discussion include solar PV systems, solar terms, system components, net metering, and financial considerations with regard to solar development.
1. Solar PV Systems
Solar electric systems convert the energy in sunlight into electrical current, which can power electric loads, be fed back to the electric grid, or be stored in batteries. All solar electric systems consist of the same basic components but vary widely in terms of size and complexity.
This tool focuses on utility grid-tied residential solar PV systems under 25 kW in size. Solar electric systems should not be confused with solar thermal systems, which are a separate technology that captures the sun’s thermal energy to heat water and air.
When sunlight strikes a solar electric array, electrons in the array are agitated into motion, creating direct current (DC). The
electrical current flows along conductors from the array to an inverter. The inverter transforms the DC into alternating current
(AC), which powers most common electrical appliances.
Solar Energy For Beginners – The AC flows through conductors to the site’s electric service panel, and then to individual branch circuits and loads. If the solar PV system is grid-tied (connected to the electric grid) and produces more electricity than is used at the site, the excess current is pushed back into the utility grid. This basic description of a solar electric system applies to most installations.
2. Solar Terms
The following terms are frequently used when discussing solar energy and associated technologies.
- Alternating current: AC describes one type of electric charge flow. The AC stream of charges periodically reverses itself, whereas direct current (DC) describes a stream of electrons that moves in one direction only. AC is the standard electric current for power grids worldwide. Solar electric cells capture particles of light and convert them into DC electricity. An inverter translates DC into AC for consumers to use in their homes and businesses.
- Ampere: Abbreviated as amp, this unit is used to measure electric current.
- Balance-of-system: BOS costs refer to the costs of all aspects of a solar PV installation, except the cost of the modules and inverters. BOS costs include all wiring and miscellaneous materials, along with soft costs, such as time and administrative costs associated with selling and signing a contract, system design and permitting, installation labor costs, inspections, travel to and from the installation site, and other costs of doing business. These costs account for as much as 50 percent of the total solar PV system installation. New York State has focused on reducing BOS costs to reach its goal of installing 6 gigawatts of solar by 2025.
- Direct Current: DC describes the direct, constant flow of electricity. Unlike AC, DC does not periodically reverse direction. A solar PV system comes equipped with an inverter that converts DC into AC, the standard electric current for power grids in the United States.
- Energy payback: Gauges how long it will take to recover the energy originally required to manufacture a solar PV system. Because most solar PV systems last 20 – 25 years, there is a pronounced net environmental benefit over the system’s life span. The U.S. Department of Energy estimates an energy payback of 1-4 years for rooftop solar PV systems. The original energy used is often referred to as embedded energy.
- Feed-in tariff: FITs are long-term generation contracts that have favorable terms designed to encourage the production of renewable energy by individuals and businesses. FITs are typically offered for long periods of time, such as 10, 15, or
20 years. - Inverter: A key component of any solar PV system that converts direct current (DC) electricity into alternating current (AC) electricity, which is the standard current in the United States.
- Kilowatt: kW is a unit of measure that equals 1,000 Watts and is the main mechanism for measuring the size or capacity of a solar PV system. The Watt is named after Scottish inventor and mechanical engineer James Watt (1736 – 1819).
Kilowatt-hour: 1 kWh is equivalent to the electricity generated or consumed at a rate of 1,000 Watts over the period of one hour. - Net metering: A common feature of grid-connected solar PV systems whereby excess electricity produced by a solar array is fed back into the utility grid. System owners can earn credits on future energy bills for the excess electricity their systems generate. The credits can then be used later when homeowners need power from the local utility, such as at night or on cloudy days.
- Power purchase agreement: PPAs are becoming a popular way for homeowners to take advantage of solar power without the financial responsibility associated with installation costs. Under the agreement, a third party installs the solar PV system and the homeowner agrees to buy the electricity (kWh) it generates, typically at a rate lower than what the utility offers.
- Photovoltaic: PV technology converts solar energy into direct current electricity. The technology uses semiconducting
materials that exhibit the photovoltaic effect, a naturally occurring phenomenon in which photons of light emitted from the sun knock electrons off their valence shell into a higher state of energy, creating electricity. A solar PV system uses solar panels, which are composed of a number of solar (PV) cells, to convert sunlight directly into electricity. - Photovoltaic cells: PV cells are thin layers of semiconducting material that are usually made of silicon. When the silicon is exposed to light, an electrical charge is generated. Solar (PV) cells form the basis of a solar PV panel, which together make up a solar PV system.
- Remote net metering: A variation on net metering whereby a solar PV system’s production is credited to an electricity
consumer(s) located at a different physical site. - Solar photovoltaic (PV) systems: A technology that converts sunlight directly into electricity. A PV system is made up of solar cell modules (panels), which are made up of solar cells.
- Solar thermal systems: A technology that uses sunlight to heat water or air. In contrast to a solar PV system, a solar thermal system uses mirrors to concentrate sunlight to produce heat.
3. System Components
3.1 Modules
A solar PV module or “solar panel” is an electrical generation device that produces DC current when exposed to sunlight. Most modules consist of 60-72 small, conjoined solar cells, an aluminum frame, and a tempered glass front piece. Modules are roughly three feet by five feet and are mounted in either a portrait (a vertical rectangle) or landscape orientation (a horizontal rectangle). In monocrystalline modules, individual cells are made from single pieces of silicon. Polycrystalline modules feature cells made from multiple pieces of silicon.
Installers wire together multiple modules to combine their voltage. Multiple strings of modules can be combined to add their
current (amperage).
Solar Energy For Beginners – The size of solar PV systems is typically given in rated DC capacity at standard test conditions (STC). For example, a system with 10 modules rated at 300 Watts each is a 3,000-Watt (3 kilowatt) system. Most solar PV modules come with a manufacturer’s production warranty of 25 years and are expected to have a useful life of approximately 30 years.
3.2 Inverter
All utility grid-tied solar PV systems have at least one inverter, which converts DC to AC. Most residential solar PV systems have one or two string inverters, which are connected to one or more strings of modules. Inverters are generally mounted vertically on basement, garage, or exterior walls, and can be located indoors or outdoors.
Microinverters are a special type of inverter that are mounted on the underside of individual solar PV modules. Unlike string inverters, each microinverter services only 1-2 modules, which permits greater flexibility in system design. Most solar PV professionals describe system size in terms of module capacity (kilowatts DC), whereas most electric utilities refer to system size by inverter capacity (kilowatts AC).
| String Inverter | Microinverter |
|---|---|
3.3 Balance of System Components
“Balance of system” (BOS) generally refers to all equipment in a solar PV installation except the modules and the inverter. (Occasionally, inverters are included in the term.) BOS components include racking, conductors, conduit, disconnects, fuses, mounting hardware, combiner boxes, and occasionally batteries.
3.4 Racking
Solar Energy For Beginners – Most solar PV arrays are mounted to roofs using specially designed aluminum racking systems. Typically, L-shaped brackets are connected to the roofing members of a house with lag bolts. Long aluminum rails are bolted onto the L-feet, and individual modules are attached to the rails with clamps. All roof penetrations must be flashed to prevent leaks and roof damage1, and the system designer must ensure that the roof is structurally strong enough for the additional load of a PV system. Any necessary replacement or repair work on a roof must be done prior to the installation of the solar PV system.
System designers may choose to use a ballast mounting system on flat roofs. Instead of using lag bolts to anchor the racking to the building’s structural members, heavy concrete blocks weigh down the array. Ballasted systems are less likely to create leaks in the roof membrane, but add substantial weight and may be too heavy for some roofs.
Solar electric arrays are also commonly ground-mounted. Arrays can be mounted on racks directly on the ground or atop a metal pole. As with roof mounts, metallic racking must be bonded (made electrically continuous to provide a path for fault currents). When designing a ground-mounted system, the designer must account for soil conditions. Voltage drop is a concern for ground-mounted systems, which often have long conductor runs.
Ground-mounted solar PV arrays sometimes include tracking equipment, which rotates the array throughout the day to follow the sun’s trajectory. Tracking may occur along one or two axes. The additional energy produced by these systems must be weighed against their additional cost, complexity, and maintenance.
3.5 Conductors
Solar Energy For Beginners – Conductors (wire) coming from the modules are typically factory-assembled “PV Wire” with a factory-formed termination (see
NEC 690.31). These factory leads are labeled “PV Wire” or “Type USE-2” and are rated to withstand all weather conditions. They are then spliced with standard building wire, using appropriate connectors and enclosures.
The standard building wire is installed in a raceway (conduit) to its next point of connection. Under certain conditions, conductors may be directly burial or part of a cable assembly. NEC 690.32 and NEC 310 provide guidance on allowable conductor types and methods.
The maximum allowable voltage for residential solar PV systems is 600 volts DC, but nonresidential systems may run up to
1,000 volts DC (NEC690.7(C)). Conductors must be protected from accidental contact. When exposed, they must be installed in a raceway (such as conduit) or otherwise rendered inaccessible. For example, the exposed conductors on the back side of a ground-mounted array must be guarded or located at least eight feet above ground.
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