To start your PV project you'll want to know a few key points about your energy use, installation location, cost, and the buying process. The least expensive energy you have is the power you don't use, so Energy Efficiency First is key.

1. Basic Operations
2. Components
3. To Battery Backup?
   • Installation Issues
   • Energy Use Analysis
   • Working With Your Installer
4. What You Need to Know
5. Maintenance
6. Conclusion

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Basic Operations

When sunlight hits the PV cells, direct current (DC) flows through the inverter, which converts it to alternating current (AC). The AC power then flows directly into the building (if there is demand), or into backup batteries if the system has them, or to the utility. When the power is flowing back to the utility grid, the electricity meter turns backward.

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The Components

PV cells are the core of the system. They are made up of at least two layers of semiconductor material (usually pure silicon infused with boron and phosphorous). One layer has a positive charge, the other a negative charge. When sunlight strikes the cell, photons from the light are absorbed by the semiconductor atoms, which then release electrons. The electrons, freed from the negative layer of semiconductor, flow to the positive layer—thereby producing an electrical current. Since the electric current flows in one direction (like a battery), the electricity generated is called direct current (DC).

Many individual cells are wired together in a sealed weatherproof unit called a module. There are three types of PV modules: single crystal, “multi” or “poly” crystalline, and amorphous silicon. Each of these PV types is estimated to last at least twenty-five years. Some estimate that forty years is a reasonable expectation. The longevity rating of a module refers to the number of years before the unit starts producing only 80 percent of its original power rating. For instance, some modules are warranted to produce at least 80 percent of their full-rated power after twenty-five years. Instead of stopping production completely, a PV module will gradually produce less and less power over decades.

Single-crystal modules are currently the most efficient type available, meaning that they produce the most power per square foot of module. The cells are fragile so they must be mounted in a rigid frame, and the modules usually have a polka dot or checkered pattern.
Multicrystalline modules are made of cells cut from multiple crystals that are grown together in an ingot. They are similar to single crystal cells in module structure but slightly less efficient since they require a bit more surface area to produce the same amount of electricity.

Amorphous silicon modules (e.g. thin film) are made from cells created by depositing a micro-thin layer of silicon directly onto a sheet of glass, plastic, or other substrate. Although they are less efficient and require up to 50 percent more space, they can be mounted on a flexible backing, making them easier to transport and ideal for building-integrated uses, such as roofing tiles or shingles.

You should expect an inverter to function for at least ten years. The actual longevity depends on the location of the inverter (the hotter the temperature, the shorter the lifespan) and whether or not regular maintenance is carried out. A humid environment will affect the lifespan as well. The ideal place to install an inverter is in a cool, dry, and preferably air-conditioned environment.

To Battery Back-Up Or Not?

There are grid-tied systems with and without batteries, and also stand-alone systems. Stand-alone systems are not connected to the utility grid and always have a battery backup component if power is needed when the sun isn’t available. Grid-tied systems are connected to the utility grid. This most standard of systems feeds the PV-generated electricity through the inverters and into a power panel for direct use, with any surplus power fed back into the utility grid for credit against future kWh use. The current net-metering law in California requires retail-priced financial credit for each kWh sent into the system by the PV equipment. Grid-tied systems without a battery backup have no stored stored power for nighttime use if the utility power goes offline.

With a battery backup system, the inverter first sends DC to charge a bank of batteries. When the batteries are full, the system begins sending AC to the power panel for use in the home or business. If the utility power shuts down, the inverter delivers back-up power from the batteries to the building. In most cases a grid-tied PV system without batteries is sufficient for residential and commercial installations.

A battery-supported system is more expensive, since it includes more equipment and requires more maintenance. Depending on quality (based on current available technology), the batteries will have to be recycled every five to ten years. A battery backup system is essential when medical, industrial, and security equipment or appliances such as aquariums, refrigerators, and cooling fans, must be powered during blackouts.

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What Do I Need to Know to Install PV?

It is helpful to understand from the outset that a typical residential PV system will likely cost at least $15,000 (installed), after all state and federal incentives and tax credits. While prices are steadily declining over time, expect to invest at least this baseline amount for a smal lto medium-sized residential PV system. Commercial PV investors will spend a good deal more for a system that makes a significant dent in their energy use, although the state and federal incentives cover a larger portion of the system cost. PV technology has proven to be a solid business investment in California. For more information on the financial aspects of PV see Clean Power Estimator or articles from OnGrid .

Step One: Installation Issues

Shade, Direction, Tilt, Space, Weight

First, do you have an installation location? In California, you are looking for a south-facing unshaded space (usually on your roof). You can install the array facing west or even east, but north-facing systems are not advisable due to low kWh production. For shading analysis, pay attention to trees that may grow tall enough to shade the system over the next thirty years. Also, be aware of Solar Access Laws (see article in this guidebook) that protect your access to sunlight. In most cases municipal and homeowner association restrictions are not legal . Check your local ordinances to see if your solar access is protected from a neighbors remodel.

If you have tall trees that are shading your ideal installation location, adjust your location. The trees are probably on the south or west side of your home and are providing significant energy savings by shading the building on hot days. Please don’t sacrifice a natural energy-saving feature for a high-tech energy gain. If a potential site for a PV system receives even partial shade, we recommend you do a solar site assessment. A solar professional or trained homeowner can use simple tools such as the “Solar Pathfinder” to determine how much shading will occur and when. From this, she or he can determine the expected effect of the shading and whether to adjust the system design or components accordingly.

Since the modules are at maximum production when sunlight hits at a perpendicular angle, the system array will usually be tilted at an angle close to the latitude of the installation. You may want to adust the tilt and angle of the array to maximize summer exposure in order to take advantage of time-of-use metering (see article on Financial Analysis from OnGrid.net for a full explanation of TOU.) However, many factors affect your decision on tilt angle, and the system installer will recommend the most effective installation for your particular situation.

In some instances a PV system will include a mechanical tracking system. The tracker can reduce the size of a PV system by getting more power out of each module by moving it to follow the sun. Single axis trackers rotate around one axis, usually a northsouth axis that allows the panels to follow the sun from east to west. The gain in output from tracking varies with the site and the local weather. Increases of 10 to 40 percent annually are possible depending on local conditions and latitude. The solar array must not be significantly shaded early and late in the day when the tracker achieves its greatest gain. With grid-connected systems, the gain in summer late in the afternoon is especially advantageous if the system is on time-of-use metering.

A high-efficiency system (single or poly crystalline) requires approximately 100 square feet for every 1,000 watts of modules. Less efficient thin-film modules require up to 50 percent more space, but are somewhat more effective in semi-shaded or lowerlight locations. Since a standard residential system size is 2–3kW, the space requirement for a standard residential PV system is 200 to 300 square feet. The highest efficiency modules weigh 4–6 pounds per square foot with mounting hardware, while thin film modules may be much lighter, especially if they are part of the roof structure itself.

Step Two: Energy-Use Analysis

The second step is to determine your average monthly kWh use over the last year, and how much of that power was charged at Tier-two and higher rates. In order to encourage conservation, California ratepayers are generally charged more for increasing amounts (tiers) of power they consume. To the extent that PV power is replacing the more expensive tiered-rate power; the return on your investment is higher. Although customers receive credit for kilowatts they send into the grid, it cannot offset more than 100 percent of your annual energy use. Generally people with grid-tied systems choose to install enough PV to produce between 50 and 80 percent of their projected demand because the savings from time-of-use metering helps eliminate an additional fraction of their electric bill and it’s usually possible to reduce at least 10 percent of your energy use by implementing basic energy-efficiency measures.

The general rule for estimating PV system size is that 1,000 watts of system capacity (1kW) produces between 135 and 150 kWh per month, on average. So, if your monthly use averages 700 kWh (slightly less than the average Northern California home), a 2kW PV system would produce close to 50 percent of your electricity per month, on average.

Sizing Your System

As you size your system, you should consider “economies of scale” that can decrease your cost per kWh as you increase the size of the system. For example, inverters are sized in terms of their maximum capacity to process the DC. If you are installing 3kW of modules initially, you may want to consider a 5kW inverter in case you want to add modules to your array later. PV systems are modular in nature. The cap to production is the inverter, meaning that a 2.5kW inverter can’t process more than 2,500 watts of DC. If you are unsure whether you need or can afford the larger system but may want to add on in the future, you should upsize your inverter in the initial installation and add panels to the array later. Labor costs for installing a small system may be nearly as much as for a large system. It’s worth remembering that your PV provider is likely to offer you a better price to install a 5kW system all at once, rather than installing the first 2.5kW this year and coming back to install the second half a few years later.

Step Three: Working with Your Installer

The third step, once you know how much kWh you use and you’ve identified an installation location and calculated the likely cost range, is to call a PV installer for a quote. There are several online analysis tools against which you can gauge the estimated of costs and benefits provided by your installer. (See “Tools” in the Resources section of this guidebook.) The Clean Power Estimator is a CECsponsored PV and wind analysis tool that enables users to input a wide variety of system specifications, installation data, annual utility bill, and the projected cost of the system per kW. Given this information, which can be easily modified for various scenarios, the Estimator provides a monthly cash flow analysis, production estimates, and projected simple payback period. The tool is supplied with real-time state incentive rates and utility-specific kWh costs. Estimator outputs can be reviewed with your contractor to clarify any discrepancies with contractor projections.

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Maintenance

Read the owner’s manual for all your PV equipment. This is the ultimate authority on maintaining your particular modules and inverter.

Cleaning the array

• Some experts contend that dirt-encrusted modules don’t lose much production capacity and advise against cleaning the system more than once a year. PV system owners, however, will tell you that they notice a minor jump in their kWh production after cleaning the modules. And if the array is visible from the street they enjoy the aesthetics of a clean system.
• The equipment manual will provide a suggested cleaning regimen and instructions on how to clean the modules. Some enterprising PV owners install a PVC sprinkler tube along their roof ridgeline so rinsing off the array is as easy as turning on the garden spigot.

Inverter maintenance

• Inverters are much like a desktop computer except they are processing hundreds of volts of power for five to ten hours a day. The unit starts automatically when there is enough sunlight to run minimum voltage through the inverter.
• Keep your inverter in as cool and dry a location as is available, and protect it from direct sunlight. Some PV owners use a small PV module (20 watts) to power a standard computer-cooling fan. When the sun hits the small module (installed on an outside south- or west-facing surface), the fan automatically starts to cool the inverter. This is not essential, but it helps in hot regions like the Central Valley.
• Try to minimize dust and cobwebs on the inverter unit as these inhibit cooling of the electric components. Production data capture Most inverters come with a data monitor, either on the inverter itself, or wired to an external unit. Most PV owners want to ensure the system is performing up to specifications. In addition, future financial incentives will be based on system performance.

Arranging for a data-monitoring system that transfers information to your home computer will enable you to track the information on your own schedule and over various climatic conditions. If you don’t arrange for this at the outset, it can be costly to purchase a data-capture system after your system is installed.

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Conclusion

PV technology is a great way to reduce production of global warming gases, shield your home or business from increasing electricity rates, and enjoy the fact you are using clean power from the sun. The financial implications of a PV investment are thoroughly investigated in the next article on Financial Analysis. For more information, and to meet others who have invested in PV technology, consider joining NorCal Solar. If you have questions about this article or any information in this guidebook, please contact us at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it .