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Retrofitting a Photovoltaic System
Residential photovoltaic (PV) solar electrical systems are becoming more and more affordable lately, not only because of federal and local rebate programs but also in light of ever-rising prices for oil, natural gas and electricity.
So how does a homeowner interested in installing a PV system retrofit one onto an existing home? What kind of equipment is required? Is it difficult to install? And what about professional versus do-it-yourself installation, and other installation and equipment factors? We'll explore all the issues involved in retrofitting a PV system onto a typical home.
What is PV?
PV panels, also called solar panels or modules, contain hundreds of small silicon cells that collect the sun's energy and change it into electricity that can be used in the home. This differs from solar space heating and water heating, which are based on solar collectors that change radiation into heat energy using the physics of heat absorption and conduction.
PV cells are made mostly of silicon, the second most abundant material in the earth's crust after hydrogen. One side of the material is electrically positive, the other negative. When light strikes the electrically positive side of the material, the negative electrons are activated in a way that produces an electric current, and voila - energy.
Groups of PV cells are electrically configured into modules and arrays, which can be used to charge batteries, operate motors and power any number of electrical loads. With the appropriate power conversion equipment (called an inverter), PV systems can produce alternating current (AC) compatible with conventional household appliances, and operate in parallel with and connected to the electric utility grid.
More than 75 percent of these PV systems are retrofitted onto existing homes and only more recently have been used in new construction due to an increasing demand for solar. Typically, the process begins with a homeowner motivated by rising energy costs and environmental concerns calling up a solar contractor to find out how it all works.
A good solar contractor understands that solar is expensive and should first encourage homeowners to deal with the "low-hanging fruit" - the easy ways to make their homes more efficient. In other words, if you're interested in installing a PV system, consider switching to Energy Star appliances and compact fluorescent lighting. Make sure the walls and attics in your home have adequate insulation, upgrade to double-pane low-e windows, and seal and weather-strip all cracks and leaks to the outside. These energy-efficiency steps will help to minimize your energy needs and enable you to purchase a smaller, less costly PV system.
Size the System Properly
Once your house is as "tight" and energy efficient as possible, you should examine your energy bills over the past 12 months - the amount of time necessary to understand your household's energy consumption while factoring in seasonal changes and eliminating unusual spikes. When you figure out your energy needs and what portion of those needs you want to come from PV, you can size the system appropriately. (Generally, residential systems range in size anywhere from 1 kilowatt to as high as 10 kilowatts, though typical systems are in the 2- to 4-kilowatt range.)
It's important to keep in mind that you don't want to produce more energy than you consume due to a concept called "net metering." In net metering, a home with a grid-connected PV system is allowed to supply excess electricity (more than the household uses) to the utility grid, which runs the home's electric meter backward. When the home needs more power than its PV system can produce (at night or on overcast days, for example), it uses utility electricity, which runs the meter forward again.
When you configure your system this way, you don't have to invest in batteries to store the PV power, which can greatly increase the cost of buying and maintaining a PV system. You simply bank your electricity with the utility and withdraw it as needed.
Although this system works well, there is little financial benefit to producing more energy than you use. For example, in Colorado (where I live) some utility companies credit homeowners the retail price of their excess energy up to the point where their energy production equals their energy consumption. But once a home produces more energy than it consumes annually, the energy companies pay a much lower wholesale price for the excess energy, as if the homeowner was an energy producer. So unless you live somewhere like Germany, where the utility pays retail prices for excess energy production, the financial rewards are negligible when compared to the cost of the equipment. This is why you should size the system appropriately.
The Site Survey
Once you've tightened up your home and sized a system for your needs, it's time for a site survey. You need excellent solar access, so ideally the system should be installed on a roof that faces south. The system can also face southeast or southwest, although there will be an efficiency loss of about 20 percent. While some homeowners choose to install solar panels on the ground, 90 percent install systems on the roof, where there is better access to the sun and less chance for vandalism.
The system should have no shade over it for as much of the day as possible. This is critical, since PV systems are wired together like Christmas tree lights. Just as a whole string of Christmas lights stops working when one bulb goes out, so too does the PV system shut down when even a hand shades a single cell. (Fortunately, bird droppings will not shut down the system, since the droppings are not big enough to cover an entire PV cell. So there's no need to get out your slingshot.)
Interestingly, PV systems are significantly more shade-sensitive than solar thermal (hot water) systems. Since solar thermal systems are not dependant on interconnected and well-functioning cells, only their shaded portion reduces their efficiency; they won't shut down entirely when shaded like PV systems.
With PV, you need to be especially cautious of chimneys, roof vents, trees and other obstructions that could prevent the system from working properly. Choose an open area of the roof that has as much access to full sun as possible.
The Nitty Gritty Installation
Generally, it takes one to three weeks to install a PV system on a home. During this time, there is little or no change in homeowner lifestyle, as minimal work is done inside of the house.
In addition to the solar modules on the roof, the PV system installation requires three shoe-box-size pieces of equipment on the outside wall next to the electric meter: two safety disconnects that enable you to isolate the system and shut it down when necessary, and an inverter that converts the solar electricity to alternating current for residential use.
Although it's not especially difficult to install a PV system, professional installation is recommended for a number of reasons. First, PV manufacturers typically provide a 25-year warranty that could be voided by a less-than-professional installation. Second, PV rebates are significant, and often you can get those rebates only when you have a warranty from a professional installer. Third, if you have a problem, it helps to have someone experienced come over and fix it. Fourth, PV involves high-voltage electricity that you don't want to mess with for safety reasons unless you are qualified.
In addition, professionals generally know how to work within the local code system. And finally, the industry has taken many years to shake off its "hippie" image. Professional installers are thus encouraged so the systems work properly and the industry is not tainted by unfortunate mishaps, such as roof leaks, incorrect wiring, incorrect equipment connections and incorrect tie-ins to the roof structure.
A Good Investment
Installing a PV system requires significant up-front capital, but like any good financial investment, you are hedging against energy costs that are all but guaranteed to rise, capturing the gains of a fixed energy cost for an extended period of time.
So what does an average system cost? Because a PV system is specific to the house on which it is installed, costs can vary widely. But to get a general idea, we'll look at the best-selling PV unit sold by Namast© Solar Electric, a Boulder, Colo.-based retailer and installer of PV systems ( http://www.namastesolar.com ). The company's 3.5-kilowatt system sells for $26,000 to $30,000, but after rebates and incentives are factored in the cost drops to $8,250 to $12,000.
Homeowners who install a PV system receive a federal income tax rebate for up to 30 percent of the cost of the system, capped at $2,000; the tax rebate will be offered through the end of 2007, though a new bill may extend and increase the amount of the rebate.
In addition, many states and localities offer rebates or incentives to homeowners who install PV systems. The rebates and incentives vary widely, in part because the price of electricity varies. For example, at the time this article was written, residents of Colorado could receive a rebate of $4.50 per watt on a PV system but Californians could receive a rebate of only about $2.80 per watt, in part because electricity is so expensive in California, so there is less need for incentives.
For up-to-date information on rebates in your state and locality, check the Database of State Incentives for Renewable Energy (DSIRE) at http://www.dsireusa.org . You just might be surprised to find that, once rebates are factored in, a PV solar electric system is within your budget.
Kim Master is executive director of the Boulder Green Building Guild and a senior associate at What's Working Inc., a Boulder, Colo.-based greenbuilding consultancy. She is also co-author of Green Remodeling (New Society Publishers).