Solar Power Buying Guide
A quick guide to strategic product selection
► Consider the overall energy efficiency of your home or office before purchasing a solar power system. These systems are only cost-effective if the power the system generates is used efficiently. Ensure that energy-efficient lighting, HVAC equipment and appliances are installed in the building before the solar system is purchased. Also ensure that insulation is effective (high R-value) and windows insulate well (low U-factor). Compare HVAC, insulation and lighting technologies by visiting our Compare Technologies pages. Visit our Buying Guide Directory to learn more about energy-efficiency in lighting, HVAC systems, appliances, insulation and windows.
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Solar cells
A solar cell, also called a photovoltaic (PV) cell, converts sunlight into electrical energy that must be used immediately or stored in batteries to be used later. Each cell is made of silicon, similiar to a computer chip. The silicon is treated so that it generates a flow of electricity when light shines on it.
Cell, module, panel, array
The solar cell is one small (approximately 4"x4") area on the surface of a solar module. The module is made up of many solar cells set into a frame for support. A solar panel is several solar modules installed together on a rack. Put the racks together and you have a solar array.
Photovoltaics
Photovoltaic (PV) means the direct conversion of light into electricity. Photovoltaic solar cells, which directly convert sunlight into electricity, are made of semiconductor materials. This can include very simple cells that power calculators and watches, and complex systems that can provide power to houses.
Solar power in different weather conditions
Solar modules do work in cloudy/overcast weather although they produce less electricity than on a sunny day. Under a light overcast, the modules might produce about half as much as under full sun, ranging down to as little as 5 - 10% under a dark overcast day. If the modules become covered with snow, they stop producing power; if the snow melts or is brushed off, they resume operation immediately (see "How much sun will I get?" below for more information).
Technologies to choose from
- Monocrystal (high output) - the most efficient
- Polycrystal (high output) - almost as efficient as monocrystal
- Thin film/amorphous (heat- and shade-tolerant) - less efficient
See the U.S. Department of Energy's website for more details.
Efficiency
PV efficiency is measured using the "conversion efficiency" of a PV cell, expressed as a percentage. Cell conversion efficiency is the proportion of sunlight energy that the cell converts to electrical energy. Conversion efficiency is also measured for the photovoltaic module (called module conversion efficiency).
► Eco-rate rates solar modules according to modular conversion efficiency. Click here to see the product ratings.
Improving conversion efficiency is vital to making PV energy competitive with more traditional sources of energy. If one solar panel can provide as much energy as two (less-efficient) panels, PV system costs will be greatly reduced.
Today's PV systems produce electricity at a fraction of the cost of early PV systems. The earliest PV devices converted about 1%-2% of sunlight energy into electric energy. Today's PV devices convert 10%-20% of light energy into electric energy.
Solar thermal water heating
Solar thermal hot water heating technology is different than PV technology. Solar hot water heaters use the sun to heat either water or a heat-transfer fluid in collectors while photovoltaic solar cells directly convert sunlight into electricity. As much as 70% of the sun's energy can be extracted using solar thermal technology, making this a cost-effective choice. Storage is typically the issue for most consumers, however. Please visit our Water Heater Buying Guide to learn more.
Residential PV systems
How it works
Solar modules, mounted on your roof, convert sunlight directly into direct currect (DC) power (if the roof application is not feasible, a ground-mount, trellis or pergola application may also be an option). An inverter converts this DC power into alternating current (AC) power that can be used in your home (for electric power purposes such as to run your appliances and lights).
The system can be interconnected with your utility, referred to as "on-grid, grid-connected or grid-tied" ("off-grid" refers to systems that are not connected to the utility electrical grid). With an on-grid system, if your solar system produces more electricity than your home is using, your utility may allow net metering (see "Benefits" below). Your utility would provide power as usual when your electricity demand exceeds that produced by your solar system.
Systems are also available with a battery backup. Part of the power produced by your solar system during the day is used to charge the batteries, which provide power for your critical loads in the event of a power outage.
Choosing the right size
The array size you need depends on your average electrical usage, climate (see below), roof angle, shading tendencies and many other factors (your installer will be able to give you details during a site visit to your property). In general, solar PV systems sized between 1 to 5 kilowatts are usually sufficient to meet the electricity needs of most homes. You will buy multiple modules that range in size (power output, measured in watts) that when put together, will make up your solar PV system.
To approximate the array size you need, multiply your average daily electrical demand in kilowatt-hours by 0.25. The result is the approximate size of solar array, in kilowatts, needed to meet your electrical demand. Viewing your electricity bills is helpful in making this calculation. Alternatively, use the American Solar Energy Society's solar power calculator.
How much energy can one expect a 1-kilowatt PV system to produce?
A 1-kilowatt PV system will generally produce approximately 1,600 kilowatts of energy on average per year; a 2-kilowatt system 3,200 kilowatts, a 3-kilowatt system 4,800 kilowatts, and so on, depending on the location and amount of sunlight the PV array receives.
A module's "Rated Power" or estimated energy produced per year, is measured at 25 degrees Celcius (77 degrees Fahrenheit) per square meter.
► Eco-rate features solar modules that range in Rated Power output from 123 - 230 watts. Click here to see the product ratings.
How much sun will I get?
To determine the amount of energy you can expect to generate from your PV system, estimate the average solar radiance of your property. Solar radiance is the amount of solar energy received on a given surface area in a given time, commonly expressed in kilowatt-hours per square meter per day (varies according to weather and latitude of the given location).
The Florida Solar Energy Center conducted a study to examine how a 2-kilowatt photovoltaic system would perform if installed on an energy-efficient home in climates across the U.S. The study accounted for all factors that impact a PV system's performance, including the temperature effect on the photovoltaic cells, the amount of sun peak hours in various regions, and the efficiency of inverter to convert solar derived energy from DC to AC. The study revealed that solar photovoltaic systems work just about anywhere in the U.S., even in colder/less sunny regions such as the Northeast and Northern Pacific West. In New York or New Jersey, for example, a 1-kilowatt system should produce about 1,270 kilowatt hours of electricity per year. In Seattle, a 1-kilowatt system should produce about 1,200 kilowatt hours per year. Electricity generation is higher in warmer climates such as the Southwest. See the American Solar Energy Society's website for a map of estimated kilowatt-hour generation in climates across the U.S.
Installation
You will need a building permit to install a residential solar system. You will also need to check-in with your utility to discuss their solar power rules and procedures.
Click here to find a verified solar installer in your area.
Benefits
- Clean, renewable energy: solar power is the Earth's most available energy source, easily capable of providing many times the total current energy demand. It does not emit any carbon emissions or toxins into the environment as a result of its production.
- Net metering: all solar electricity generated that is not used at time of generation will be fed back to the utility grid, so that your solar-generated power can be used by other people. During these times, your utility meter will actually run backwards so that you are credited for this power. Your utility bill at the end of the month will be the difference of what you generated and consumed. For those summer months that you generate more than you consume, these “credits” will be forwarded to you the next month.
- 30% Federal Tax Credit and other rebates: receive a 30% tax credit on the installed cost of your solar electric system, up to $2,000, thanks to the recently renewed Federal Tax Credit for Energy Efficiency (photovoltaic systems must provide electricity for the residence, and must meet applicable fire and electrical code requirement). Use Eco-rate's Rebate Directory to find solar rebates specific to your area.
- Increased appraised value of your property: based on a recent study by ICF consulting (funded by HUD and the EPA and published in The Appraisal Journal, October 1998), energy saving measures such as solar can add up to $20 of home value for every $1 of yearly energy cost saving.
Products
ENF Study
The following rankings were generated in 2007 by research conducted by ENF, a photovoltaic information company, among directors of photovoltaic installation companies in 45 different countries.
The top ranking companies receiving awards are:
Top Ranked Solar Panel Manufacturers
- No.1 in World - SunPower (USA)
- No.2 in World - Schott Solar (Germany)
- No.3 in World - SolarWorld (Germany)
"Best Solar Panel Product Quality"
"Best Solar Panel Value for Money"
Top Ranked Solar Inverter Manufacturers
- No.1 in World - Kaco (Germany)
- No.2 in World - OutBack (USA)
- No.3 in World - Mastervolt (USA)
"Best Inverter Product Quality"
"Best Inverter Value for Money"
Top Ranked Solar Mounting System Manufacturers
- No.1 in World - Direct Power and Water (USA)
- No.2 in World - UniRac (USA)
Best Solar Trackers
- No.1 in World - Zomeworks (USA)
Best Solar PV Systems
- No.1 in World - Sharp (Japan)
ENF's research found that 49% of companies thought the cell brand in a panel was important, and the most commonly named cell brands that installation companies thought were high quality were Sharp, Q-Cells and BP Solar.
How are PV modules rated on Eco-rate?
Please click here to discover our PV module rating system.
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