Designing a System

Except for small, fully integrated PV systems, you should carefully design your system so that the components are properly sized and matched to arrive at a cost-effective and safe electric generating system. As a general rule, efficiency first; then look at renewable energy generation.

  • Define Site Conditions: Will the collectors be located where they are free of all shading from at least 9 a.m. through 3 p.m.? What is the most aesthetic mounting method? Will local ordinances or utility regulations allow installation of a PV system at your location? How energy-efficient is the building or appliance to be served?

  • Estimate Loads: Estimating your electric load is fairly straightforward, but it requires thoroughness. List every light, appliance, tool, and gadget that uses electricity and will be using the power from your PV system. Then list the watts of power required to operate each item, and the time of day and length of time you normally expect to use that item. Often this step is the place to identify opportunities to improve your energy efficiency.

    An 18-watt compact fluorescent bulb provides the same level of light as a 100-watt incandescent light bulb. If your major appliances are more than 10 years old, they often can be replaced with new appliances that use significantly less energy. Traditionally, we don’t use all our electric appliances at the same time. Avoiding the simultaneous use of major electric appliances can reduce your peak load, thus reducing the total system size required.

  • Determine the Type of System: Will your system be a "stand-alone," "utility-intertie," or a "hybrid"? This decision will determine whether you need batteries and what size array you will need.  Read more about types of PV systems.

  • Determine the Size of Battery Banks: The type of system you choose is critical to deciding on the size of your battery bank. With a grid-intertie system, which is connected to the electrical grid, some people choose to do without a battery bank. With a stand-alone system, you need sufficient battery storage to see you through extended low-light situations. When deciding on the size of battery packs for hybrid systems, you need to balance cost issues with load demands.

  • Determine the Size of Array and Components: As you have figured out by now, deciding on the size of a solar array and its components involves tradeoffs between load, storage, array location and tilt, and system type. Twenty-five or more photovoltaic simulation programs exist for sizing solar arrays and matching them to Balance of System (BOS) components. The best way to ensure proper system design is to work with a reputable solar dealer whose references you have checked. Like any other major purchase, shopping around is a good idea. While Missouri does not have a large number of photovoltaic dealers, you can find out how to contact those that are available through various web sites (American Solar Energy SocietyInterstate Renewable Energy CouncilSolar Energy Industries Association) and periodical publications.

A very complete discussion of the system design process is provided in:

Stand-Alone Photovoltaic Systems, A Handbook of Recommended Design Practices, Sandia National Laboratories, SAND87-7023, National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161, 308 pg. plus Appendices, 1995.

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