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Questions to Ask a Contractor
For a list of questions you might ask solar contractors, review our Guide to Going Solar. Click here >
Additional Technical Details
Selection of the type of PV module specified for any installation should be part of a complete design review to ensure that the customer's requirements are met, including total kW output, cost, and aesthetics.
Monocrystalline Silicon PV Panels are composed of silicon PV cells that are made from a single large crystal. Monocrystalline PV modules are the most efficient PV modules due to the purity of the silicon used. These modules are typically rated at a higher capacity (watts) for a given size. They also tend to perform better during low-light conditions. However, the higher efficiency comes at a higher cost for the modules. Module efficiency for the end user is likely most important where the installation is planned for a location with limited space. The sizing of a micro-inverter in comparison to the module rating needs to be properly matched to achieve optimal performance.
Polycrystalline Silicon PV Panels are composed of PV cells made up of multiple small silicon crystals. They are typically less efficient, have a lower performance at higher temperatures, and are sometimes cheaper than monocrystalline panels. The efficiency of polycrystalline panels continues to improve.
Thin Film PV technologies are relatively new, with thin PV material layered on a thin, flexible and lightweight film-like material. Thin film PV is less efficient than mono- or polycrystalline panels but may be attractive to those who wish to integrate the material on a building roof or sides rather than installing an array of roof-mounted panels. In some cases, installation of thin film may be easier and less costly than traditional rack-mounted PV modules. Thin film technologies today typically include amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium selenide (CIS or CIGS).
Understanding Peak Sun Hours
Solar radiation or "irradiance" is the amount of solar energy that hits a given surface and is generally expressed as "watts per square meter." Insolation is the amount of solar energy received by a given surface area, expressed as "watt-hours per square meter." Average daily insolation is sometimes referred to as "peak sun hours." Peak sun hours are used to give customers and installers an idea of how good the solar resource is at a particular location; in other words, how "sunny" a location is. Peak sun hours can range from about 3 hours to 6 hours depending on the altitude and microclimate of a location. Online, this is usually shown as a searchable map. Basically, the peak sun hours for a given location are the amount of time it would take if the sun were shining at its maximum to equal the same actual solar insolation received at that location over an entire day. It may be easier to understand this graphically:
Determining a PV Systems Output in kWh
A ballpark estimate to determine how many kilowatt-hours a PV system may produce uses peak sun hours and the nameplate rating of the modules in kW. As the manufacturer nameplate rating of a PV module is calculated under controlled test conditions, referred to as Standard Test Conditions or STC, it generally is different than what a PV module will actually generate on average in a typical installation. (The nameplate rating under STC can be found on the label attached to the underside of a module or in the module manufacturer's specification sheet.) To account for this difference between testing and real world conditions and other system losses, a derate factor is also applied. NREL in its PVWatts model uses a derate factor of 0.77.
Here's an example for illustrative purposes only:
- 16 modules with an STC of 215 watts per module
- 4.2 peak sun hours assumed for the location
- 0.77 derate factor
- 16 x 215 = 3,440 watts
- 3,440 / 1000 = 3.44 kW (kilowatts) STC output of total array
- 3.44 x 4.2 hrs. = 14.45 kWh
- 14.45 x 0.77 = 11.13 kWh = approximate daily average annual output of PV array
- 11.13 x 30 = 334 kWh = approximate monthly average annual output of PV array
The actual kWh output of a PV system will depend on a number of factors including proper installation, type of modules, orientation, tilt, location (insolation), weather conditions, shading, soiling, inverter efficiency, and age of modules.
When comparing contractor bids and the system output they are claiming, it is important to know what assumptions, especially peak sun hours, they are using. NREL's PVWatts modeling is simple to use and is recommended as an easy way of estimating the kWh output of a PV system.
Tilt and PV Output
There are a number of variables that should be considered when designing a PV system, including the benefits and drawbacks of a fixed tilt compared to a single-axis or two-axis (dual-axis) tracking array. To get the most out of a PV system, the array of solar modules needs to point in the direction that captures the most sun. The most efficient collection of solar radiation happens when the sun's rays strike the module's surface perpendicularly (at 90 degrees).
The most common mounting method – a fixed tilt array – does not adjust to track the movement of the sun. A single-axis tracking array generally tracks the sun's daily path across the sky from East to West. A two-axis tracking array generally tracks the sun's daily path from East to West as well as the sun's seasonal path (Declination) from South in the winter through North in the summer (Northern Hemisphere). Though a single-axis tracking array for a system installed in Hawaii may produce about 30% more energy annually and a two-axis tracking array about 38%, the vast majority of residential and commercial PV systems are installed at a fixed tilt, usually the tilt (roof pitch) of the roof face they are mounted on. This is because a fixed tilt is the simplest and generally least costly mounting method. Additional advantages of a fixed tilt are that there are no moving parts, periodic adjustment is not necessary, no additional equipment or equipment maintenance is needed, and fixed mounting structures are relatively lightweight and able to be roof mounted.
Another tilt method occasionally used allows for the array to be tilted twice a year, generally in the Fall and Spring by the use of different length mounting legs. This method partially accounts for seasonal solar radiation differences and may increase annual energy output around 4% in Hawaii. Though requiring less moving parts than either single or two-axis tracking, this method is generally more expensive than fixed mounting, requires additional parts, maintenance, and labor to seasonally adjust the array tilt.
For maximum overall annual output a general rule-of-thumb used by the solar industry for a fixed tilt installation is to tilt a south facing array to the location's latitude. For Hawaii this is approximately 20 degrees. However, if the PV system's output is desired to be optimized in the wintertime, a higher tilt may be used and if the output is desired to be maximized in the summertime, a lower tilt may be used. For an easy to use tool to model the effects of different array tilts and orientations on energy output, visit PV Watts.
Building Integrated and Building-Attached Mounting Systems
There are two broad classifications for how PV arrays are mounted on buildings including residential and commercial buildings, building integrated PV (BIPV) and building-attached (BAPV). In BIPV systems the solar modules are architecturally integrated into and may form part of the structural design of the building. Examples of these are solar roofing shingles and tiles, exterior PV walls, parking shade structures, and skylights. BIPV mounting systems are most often used for new construction applications and are not as common as building-attached mounting systems. Building mounted PV installations have the advantage of utilizing the building structure as part of the PV mounting system thereby reducing costs, placing the PV array in a location less assessable to vandalism, and optimizing available space.
Building-attached mounting systems are generally used in existing buildings and in retrofit applications and generally do not form a part of the building's structural members. In building-attached mounting systems a support structure to attach the PV modules to is generally fastened to the roof's existing structural members by a series of hanger bolts or mounting brackets often called "feet".
Building-attached mounting systems fall into two broad categories – stand-off and rack mounting systems. They both generally use extruded aluminum rails designed to withstand the wind and structural loads which the PV array may be subjected to. In most applications the rails are secured by stainless steel fasteners to the roof's structural members. The modules are then mounted to these rails using aluminum clips designed to fit the particular module being installed. Also, there are a number of mounting systems that use ballasted feet to reduce or eliminate penetrations into the roof's structural members. These are generally used on flat or very low-pitched roofs.
Regardless of the mounting method used to attach a PV array to a building, sealing of the roof penetrations is critical. Leaking penetrations can result in water damage to the building's interior spaces as well as to the roof's structural members.
In stand-off mounting (flush-mounting) the PV modules are elevated above and parallel to the roof's surface. This is the most common method of mounting. The advantages of stand-off mounting are that it is generally the least costly, quickest to install, minimizes wind loading compared to rack mounting, and if properly installed provides for air circulation and minimal debris buildup potential.
Rack mounting or "racking" in general may use a similar mounting support structure as stand-off mounting but utilizes legs or other supports to raise one side of the modules higher than the other. Rack mounting is mostly used to reposition the face of the modules where the tilt or orientation needs to be adjusted to receive more direct sunlight.
Ground and Pole Mounting
Besides building mounted, PV arrays may also be ground mounted or pole mounted. Ground mounting and pole mounting are primarily used where there is inadequate roof space, roof shading, the condition of the roof or underlying support structure is not suitable or sound, roof penetrations may void the roofing warranty or otherwise want to be avoided, the array cannot be orientated or tilted as needed, aesthetic reasons, and adequate land is not available. Large-scale PV arrays, single-axis and two-axis tracking systems are predominantly ground mounted. Pole mounting is often used for small PV arrays using two-axis tracking.
Understanding UL 1741
When selecting which inverters to purchase for your project, please be aware that your utility will only approve inverters that are UL1741 certified and the project must be designed in compliance with IEEE1547. These certifications ensure compliance with industry standards to ensure the generation system will not adversely affect the power quality and safety within your home or business and for the other utility customers connected to the same utility circuit. These standards are updated as the industry knowledge base grows. The Underwriters Laboratories (UL) has established testing standards for grid-connected inverters based on IEEE1547, which is the Standard for Interconnecting Distributed Resources published by the Institute of Electrical and Electronics Engineers (IEEE). UL is an independent safety consulting and certification company that will test photovoltaic inverters for safety and compliance with IEEE1547. An inverter passing the UL1741 certification will ensure the product has been tested to highest industrial standard, enabling the product to be certified for grid interactive operation. All inverter based grid connected generation systems including those with battery backup must certified to UL1741 and system design must be in compliance with IEEE1547.
Grid-tied Battery Storage Systems
Photovoltaic systems interconnected to the utility typically do not incorporate battery backup systems, primarily because of the additional expense. Businesses and homeowners can choose to purchase a grid interactive photovoltaic system utilizing battery backup to supply electricity to critical loads in the event that electric service from your utility is not available.
In addition to higher initial cost, a system with batteries will require specialized equipment and additional maintenance, and will have additional siting considerations. The batteries must be located in a secure ventilated space and incorporate protective non-conductive shielding to prevent inadvertent contact with the batteries. Batteries have the potential to inject significant electrical current when an electrical short occurs, allowing current to flow, which could lead to fire and possible injury. Therefore the siting of batteries is very important and must be located in an uncluttered location that is not readily accessible by people and pets. If you are interested in a battery backup photovoltaic system, please be aware that contractors may provide quotes for batteries that provide differing amounts of storage capacity and life expectancy. Depending on battery longevity, replacement of the batteries within the life of the PV system should be factored into any cost analysis. It is recommended that you request quotes from contractors that either have experience with off-grid photovoltaic installations or can demonstrate their past experience installing grid-connected battery backup systems.
Concentrated Solar Power (CSP)
Concentrated Solar Thermal (CST) uses sunlight focused by a parabolic ("curved") trough or dish and a central receiver to produce heat or electricity. Parabolic troughs focus the sun's energy through long rectangular, curved mirrors. The mirrors move to follow the sun during the day, focusing sunlight on a pipe running down the center of the trough. An easily heated fluid flows through the pipe. The heated fluid is used to produce steam that can be used to turn a turbine to produce electricity. Parabolic trough systems have been reliably operating in the U.S. for over a quarter of a century.
PV Attic Fans
Attic Fans Roof-mounted PV-powered attic fans can reduce attic temperatures by 20 to 30 degrees. They have been successfully installed on open-beam homes to reduce indoor temperature as well and could be a cost-effective alternative or supplement to air conditioning or other venting. Roofers may provide this feature as an optional add-on as part of re-roofing estimates. Carefully check the warranty for roof leak coverage if you are considering such an installation.
Roof-Mounted PV Fan
PV Fan Vent In Open-Beam Home