FAQ
Solar electric or photovoltaic (PV) technology converts sunlight directly into electricity.
You may be more familiar with PV cells as solar cells that power watches and calculators.
But PV can do much more. It can provide electricity for your home and provide the opportunity for you to take advantage of net metering.
The sun generates enough clean energy in one day to provide a year’s supply of energy for your home. Why not tap into this abundant resource of clean energy? PV preserves the earth’s finite fossil-fuel resources-coal oil, natural gas-and reduces air and noise pollution associated with those energy sources. Considering the state incentive program along with the state and federal tax credit program, have greatly reduced the cost of PV ownership. PV system reliability and durability are outstanding typical PV systems may last 40 years with minimal maintenance.
Photo-voltaic, also known as “PV,” produce electricity from the sun. The solar “cell” is the basic building block of PV technology. Solar cells are wired together to form a PV module. PV systems begin with the solar module. Modules gather solar energy in the form of sunlight and
convert it into direct current (DC) electricity. The more sunlight they receive, the more electricity they produce. Solar modules are the heart of the PV system. In essence, they are the power generators. To convert DC power to alternate current (AC) power, the type of electricity used in your home, a device called an inverter is used. Depending upon the size of the PV system, inverters may be mounted by the PV manufacturer right on the back of a PV module. Inverters may also be wall-mounted separately from the PV system, but within close proximity to the solar panels. This type of inverter’s location can be in the basement, attic or garage. If an excess of electricity is produced through the PV system, the power will flow back through your meter for distribution back to the grid system. This is known as “net metering”.
Your electric meter spins forward when electricity is flowing from your utility provider into your home. The meter spins backward when power is flowing from your home back to the utility provider. That is, if more electricity is generated through a PV system than your home requires, the excess energy flows back to the provider’s electric grid system, which turns your electric meter backward. At the end of the month, you are billed only for net consumption – the amount of electricity consumed less the amount of electricity distributed back to your utility provider’s grid.
A typical PV panel consists of solar cells connected electrically to form a module that can measure two to four feet wide and four to six feet long. Some solar modules look just like traditional roof shingles. Many interconnected PV modules are called an array.
Your PV system will continue to produce electricity during cloudy weather, although the total amount will be reduced. Power will automatically be provided through your utility provider at night or during inclement weather.
Proper placement of your Photo Voltaic (PV) system is critical to the success of the PV systems output. Having even one cell in a PV module in the shade, especially during the best sun hours of the day, can decrease the output for the entire module and possibly the entire system. Since the shaded cell is not producing any moving electrons, it acts like an open circuit to the entire string. This is similar to having one Christmas tree light burned out in a string of lights. No current can flow! When looking to place your PV system, please watch for any type of shading. Shading can include trees, chimneys, TV
antennas/satellite dishes, dormers of the house.
Step 1- Review your roof for good access to the sun. Does the area for PV system receive full sunlight with no shading?
Step 2- If you have 100% of sunlight on a south-facing roof, you have an ideal roof for PV, although east and west-facing roofs are very good also. If there is shading on your roof, consult with a person trained on PV.
A PV system needs unobstructed access to the sun’s rays for most, or all of the day, throughout the year. PV panels are relatively unaffected by changing weather. In fact, some PV cells actually work better at colder temperatures. PV modules are angled to catch the sun, not snow, so any snow that does collect melts quickly. To maximize the energy production of photovoltaic electricity, PV systems are installed on a southern exposed roof and mounted parallel with the roof at a 35 degree roof pitch with no shading. However, roofs that face east or west may also be acceptable. PV panels should have their surfaces exposed to the sun’s rays for most or all of the day, with minimal or no shadows from trees, chimneys and gables between 9 AM and 4 PM.
Home Energy Efficiency and PV: An energy efficient home means you can install less PV. Every dollar on energy efficiency in the home saves $20 on your PV electric panels. To become energy efficient in the home, use compact fluorescent light bulbs, which use 75% less energy than the equivalent incandescent. A 15 watt compact fluorescent will produce as much light as a 60 watt incandescent. Also look for energy saving appliances (clothes washers, refrigerators, room A/C, dishwashers. Refrigerators before 1990 are power hogs!
The amount of space needed by a PV system is based on the output of the PV system and the type of PV system installed. Most residential systems require as little as 50 square feet (for a small “starter” system) up to as much as 1,000 square feet for larger systems. A typical two-kilowatt system could occupy approximately 200 square feet. If your location limits the physical size of your system, you may want to install a system that uses more efficient PV modules. Greater efficiency means that the module uses less surface area to convert sunlight into a given amount of electric power.
| PV Module Efficiency* | PV Capacity Rating, watts (in bold) | |||||||
| 100 | 250 | 500 | 1,000 | 2,000 | 4,000 | 10,000 | 100,000 | |
| 4 | 30 | 75 | 150 | 300 | 600 | 1,200 | 3,000 | 30,000 |
| 8 | 15 | 38 | 75 | 150 | 300 | 600 | 1,500 | 15,000 |
| 12 | 10 | 25 | 50 | 100 | 200 | 400 | 1,000 | 10,000 |
| 16 | 8 | 20 | 40 | 80 | 160 | 320 | 800 | 8,000 |
* Although the efficiency (percentage of sunlight converted to electrical energy) varies with different PV Modules available today, higher-efficiency modules typically cost more. So, a less efficient system is not necessarily less cost-effective. – The Vermont Renewable Energy Resource Center.
As a starting point, you might consider how much of your present electricity needs you would like to meet with your PV system. For example, suppose that you would like to meet 50 percent of your electricity needs with your PV system. You could work with your PV provider to examine past electric bills and determine the size of the PV system needed to achieve that goal.
You can contact your local utility provider and request the total electricity usage, measured in kilowatt-hours, for your household over the last 12 months (or consult your electric bills if you save them.). Ask your PV provider how much your new PV system will produce on an annual basis (also measured in kilowatt-hours) and compare that number to your annual electric demand to get an idea of how much you will save.
Energy From the PV System
Determine the size of your PV system in kilowatts (kW). This value represents the “kW of PV” input for the equations below. Based on geographic location, the “Energy Production
Factor” is about 1,900. To estimate the annual energy produced by your system, use the following equation: Energy from the PV system = (kW of PV ) x (Energy Production Factor) = kWh/year Divide this number by twelve if you want to determine your estimated monthly energy reduction.
Energy Bill Savings
To estimate the annual energy bill savings from your system, use the following equation: Energy bill savings = (Annual kWh/year ) x (Residential Rate) = $/year saved (The residential rate in the above equation should be in dollars per kWh; for example, the rate of 17 cents for kWh is input as $0.17/kWh.)
For example, a 2 kW PV system (assuming a residential rate of .17 cents per kWh)
would provide the following: Energy from the PV system = 2kW x 1,900 = 3,800 kWh/year.
Energy Bill Savings = 3,800 kWh/year x $.17/kWh = $646.00.
Note: The uncertainty of the contoured values is generally ±10%. In mountainous and other areas of complex terrain, the uncertainty may be higher.
A PV system can be installed on any well-structured roof. Typically asphalt shingles are easiest to work with, while slate is the most difficult. If your roof is older and needs to be replaced in the very near future, you may want to replace it at the time the PV system is installed to avoid the future cost of removing and reinstalling your PV system. PV systems are also available that integrate PV cells into the roofing materials themselves. This allows the roof itself to act as the PV collecting device and serves as an excellent option when replacing a roof or designing and/or building anew home. One benefit of an integrated PV system is its ability to offset the cost of roofing materials.
The total cost of a PV system includes the PV system and installation. Your PV system’s cost will depend on a number of factors, including system size and the energy efficiency of your home, whether the home is under construction and whether the PV is integrated into the roof or mounted on top of an existing roof. The cost also varies depending on the PV system rating, size, manufacturer, retailer and installer. Small-scaled PV systems with built-in inverters that produce about 600 watts of power may cost about $10 per watt ($6,000). These small systems will offset only a small fraction of your electricity bill. A 2-kilowatt system that will offset the needs of a very energy-efficiency home may cost $8 to $10 per watt ($16,000-$20,000). At the high end, a 10-kilowatt system that will completely offset the energy needs of many conventional homes may cost $7 to $8 per watt ($70,000-$80,000). These prices of course, are just rough estimates, and your costs will depend on your system’s configuration, your equipment options and other factors.
Remember, the state incentives can significantly reduce the cost of purchasing a PV system.
In addition, the state and federal tax credit can reduce your overall costs much further.