Site considerations include location (roof or ground mounted), area, orientation and tilt, and shading.
A few guidelines to consider when determining if your roof or site is a good fit for PV:
The location is free of shading between 9 a.m. and 3 p.m.
Preferred orientation allows for south facing.
Tilt (pitch of the roof) is close to 20 degrees, for Hawaii.
No roof repairs are needed in the near term. If you need to reroof soon, you may want to postpone your installation or reroof in advance of installing PV or a solar water heater.
Roof structural members are sound.
Roof warranty will not be voided if penetrations are made.
The available roof area is sufficient for the size of system to be installed.
Most PV arrays and solar water heating systems are installed on new or existing roofs. However, they can be ground mounted on support structures as well. Generally, roof installation is less costly and reduces the risk of theft, damage and vandalism. In Hawaii, an ideal location for maximizing annual output is a structurally sound (rafters, purlins, and other roofing materials) non-shaded roof that faces due south (180 degrees) and has a tilt of approximately 20 degrees, with enough space to accommodate the size of the system desired. See Orientation and Tilt below for more information.
Your location and the micro-climate that exists there, will also determine the amount of sunshine that your system will receive. The more sunlight that a PV or solar water heating system receives, the more energy will be produced. If you live in a very sunny area, the number of PV modules or solar water heating panels that you need to produce a set amount of energy will be less than if you live in a location that is often cloudy. Generally speaking, a PV or solar water heating system will work in almost all locations in Hawaii, but the size of the systems will vary to produce the same amount of energy. Refer to the section on Sizing Your System for more information.
Roof Type Considerations
For roof-mounted PV systems, the condition and type of the roof that the PV array is to be mounted on should be considered. A main consideration is the condition of the existing roof and the underlying roof structural members. A PV system may be in place for 20 plus years. If the roof will need to be replaced or structural members repaired within that period of time, consider reroofing with long-lasting roofing materials and doing any repairs prior to the installation of the PV system. Ground mounting may also be a viable option. If reroofing, repairing, or ground mounting are impractical, the costs of removing and replacing the PV system should be considered in your cost analysis.
Another consideration is whether the installation of a roof mounted PV system will void or otherwise impact any existing roofing material or weather-tightness warranties. Consult the roofing manufacturer and/or roofing contractor for warranty details. The type of the roof may allow for the use of a variety of mounting methods. In consultation with your contractor, it is important to consider which mounting method will be best for each roofing type. For a flat roof, a ballasted mounting system may be appropriate. For standing seam metal roofs (vertical ridges) some mounting system are designed to clamp to the standing seam, thereby reducing or eliminating the need for roof penetrations. For Spanish and concrete tile roofs, one method used is to remove the tiles where the array will be located and replace the roofing with shingles or another roofing product rather than making penetrations in the tile. Other contractors prefer to use mounting systems specifically designed for tiled roof installations. Based on the roofing material and working with your contractor, consider choosing the mounting system and installation method which ensures the best weather-tightness and meets the structural and wind/array loading requirements.
Orientation and tilt go hand-in-hand. The objective should be to get the greatest amount of direct sunlight possible while taking into account site specific conditions as the output of a solar water heating system or PV array is directly proportional to the amount of direct sunlight received. Generally, for maximizing year around performance, optimal orientation and tilt in the Northern Hemisphere is due south with a tilt equal to the site’s latitude (approx. 20 degrees In Hawaii). However, if you live in an area that routinely clouds up in the early afternoon, an easterly orientation may be best. System performance will be severely reduced if panels or modules have a northerly orientation or are excessively tilted. The more the PV array or solar water heating panels point away from south or are tilted from the site’s latitude, the less electricity they will produce over time. Although it may be tempting to use all available roof surfaces to increase the size of your PV system, PV modules (or solar water heating panels) facing the wrong direction or excessively tilted are not a good investment. An approximation of the impacts of different orientations and tilt can be obtained by using a performance calculator and by inserting different combinations of orientation and tilt in the “array tilt” and “array azimuth” fields. An experienced and locally knowledgeable contractor should be able to help guide you in determining the best location, orientation, and tilt for your specific site, conditions, and objectives. If your PV modules do need to be at different orientations you may also want to consider the use of micro-inverters.
Shading is a very important consideration when it comes to the installation of your PV array and solar water heating system because it can have a critical impact on performance. Though unobstructed direct sunlight is not as critical for solar water heating systems, it is very important for PV. It only takes a small amount of shading to significantly reduce a PV array's output. Shading can be caused by trees and other vegetation, vent stacks, other buildings or structures, and poles and wires. As a rule of thumb an array should be shade free between the hours of 9:00 am and 3:00 pm, the solar window or optimum electricity generation. The annual path of the sun should also be considered in determining if shading will impact the system especially during the winter months when the sun is at a lower angle. A good gauge is to use the winter solstice, December 21st, as a worst case scenario. In Hawaii at this time the sun will be approximately 45 degrees above the horizon at noon and objects in line with the location of the array and the sun that stick above the sun’s arc will shade the array. Tools are available to measure shading and its potential impacts upon system performance that your contractor may have.
A photovoltaic (PV) system needs unobstructed access to the sun's rays for most or all of the day to be effective. Shading on the system can significantly reduce energy output. There are various tools to help you better understand how the sun affects your area by helping you identify the amount of sun hours compared to its path. One tool available for the island of Oahu is the Oahu Solar Map.
Here is a tip on how to use the Oahu Solar Map:
As you can see on Oahu Solar Map, Ewa is in a 500 solar zone. On the solar energy conversion table you will note that a 500 sun zone produces 5.8 peak sun hours on average.
Sizing Your System
Sizing your PV or solar water heating system is a very important decision as it will impact the cost of the system and the amount of electricity or hot water that will be produced. A number of things need to be considered - system cost, available roof area, amount of sunshine that your location receives on an annual basis, if your needs will be changing over time, interconnection requirements (for PV). Many of these considerations affect each other.
Particularly for PV under the Net Energy Metering program, you must decide what portion of your electricity consumption (load) you want to produce on your own. Consult with a professional PV licensed contractor to determine the system size and number of panels that are appropriate for your energy consumption needs. The contractor should be able to size a system based on your average electric usage or a portion of your monthly use and provide you with an estimated return on investment, so you can see how long it will take for the system to pay for itself.
Inverters are used in PV systems to convert DC power or direct current, the power produced by either batteries or solar modules, to AC power or alternating current, the power supplied by the utility used to run household lights, appliances, etc. There are three categories of inverters; grid-tied (the most common), grid-tied with battery backup, and standalone or off-grid applications.
Of the grid-tied inverters there are three main types, string-inverters, micro-inverters, and AC PV modules. Generally for string-inverters, such as in NEM systems, one or two are used and they are located on a wall near the array or utility meter. DC is run from the array to the inverter(s) where it is converted to AC. Micro-inverters are located under each of the PV modules and convert DC to AC at that location and then AC is run to a circuit panel. A micro-inverter generally cost more than a central inverter and may outperform a central inverter if the modules are subject to shading. AC PV modules are similar to micro-inverters but the inverter and the module form an integral unit.
Each type of inverter, string, micro, or AC PV module has its advantages and disadvantages and may have different code requirements. We recommend that you discuss which type of inverter would best suit your application with your contractor.