A home photovoltaic (PV) system uses the available sun shining on a residence (usually the roof area) to convert sunlight into electricity through the use of specialized photovoltaic cells. These systems are becoming more popular in areas of the US where the clear and sunny sky conditions are generally consistent. Most photovoltaic systems are constructed as an array of multiple panels, and are fixed to the roof, or are moveable to track the angles of the sun throughout a day or year. Some are positioned on the ground near the home.
A home photovoltaic system is recommended for homes when:
- There are more clear, sunny days than partially cloudy or overcast days where you live.
- The main roof area on which the panels will be installed is facing due south or close to south, and is free of shadows cast from neighbor buildings, trees, or other obstructions.
- You have enough space in the home for the components (batteries, inverters, charge controllers, etc.).
- You can determine how much electricity you need or want to produce, and it works for you economically.
Photovoltaic energy systems are most cost-effective in areas with plentiful sun resources (dependable clear sky conditions each day).
Reduce energy consumption
Before choosing a photovoltaic system for your home, you should consider reducing your overall energy consumption by making your home more energy efficient. Reducing your energy consumption will significantly lower your utility bills, and will reduce the size of the photovoltaic system you need.
On homes that are already energy efficient and utilize some types of natural heating, cooling, and daylighting, home photovoltaic systems can lower your electricity bill by 50-90%, and they are nonpolluting. See Passive Solar Design for Homes.
Photovoltaic (PV) systems are made with semiconductors similar to the materials used for computer chips. These semiconductors convert sunlight into direct current (DC) power.
The materials used to make the semiconductors are created from silicon. The most basic part of many PV systems is the cell or wafer. A cell is typically a small silicon square (or rounded off square). Each cell generates about 1/2 of one volt. Cells are combined to create a PV Module or panel. A panel is a standalone combination of wafers on a single sheet of material that is enclosed between sheets of glass or plastic to protect the cells from damage and water. Modules are installed side-by-side together on a roof, called a PV array.
Parts of a Home Photovoltaic System
Many manufacturers of home photovoltaic systems provide consumers with a complete package of all of the necessary components and assemblies.
The parts of a typical photovoltaic system for a home include: (see diagram below)
- The photovoltaic array: PV cells in panels are typically one of three types of material: mono-crystalline, multi-crystalline, or amorphous (thin-film). The thin-film panels are often referred to as building integrated photovoltaics (BIPV), because they usually integrate into shingles, siding, or other exterior building surfaces without the use of mounting racks. An array is designed to generate a specific amount of energy. There can be two or many modules in an array. Modules (panels) arranged (wired) in series (positive to negative) increases the voltage while arrays wired in parallel (positive to positive, negative to negative) increases the amperage. Most photovoltaic arrays are wired in both parallel and series.
- The Mounting Rack: The PV mounting rack is a series of small metal structural pieces in tube or I-beam shape that attach the PV panels to the roof surface. On PV systems that track the sun angles each day and/or season, these mounting systems also contain moveable pivots and sliding hardware to allow for smooth movement of the panels and array. The mounting rack is typically attached through the roof into the roof structural system (rafters, trusses) to prevent the rack from damage from high winds, hail, or other impacts. Because the mounting system penetrates the roofing material in many places, special flashings or pipe boots are installed at the penetrations to prevent water leaks.
- The Inverter: The inverter changes the direct current (DC) power generated by the photovoltaic array into 120 volt alternating current (AC) power, which is the type of electrical power found in a residence.
- Disconnect: The disconnect breaker or switch is similar to the disconnect switch that is on a typical house. The purpose of the disconnect switch is to shut off the power to the house for safety or maintenance.
- Batteries (Battery Array): Whether the system is connected to the utility company grid, or if it’s a stand-alone system, a series of batteries (also called an array) can store excess power for use in the home when the sun is not shining, especially at night. The battery array should be enclosed in a separate room that is ventilated to the outside as the batteries usually contain chemicals (such as acids) that may be hazardous. Batteries for the array are usually deep cycle batteries that can be recharged over and over and are specially designed for wind turbine systems. Battery arrays wired in series (positive to negative) increases the voltage, while arrays wired in parallel (positive to positive, negative to negative) increases the amperage. Most battery arrays are in multiples of six and are wired in both parallel and series.
- System Monitor: This device allows a homeowner to view the amount of power that the photovoltaic array is generating versus the amount of power that is being used from the battery array or utility company.
- Charge Controller: The charge controller is connected to the battery array and the inverter. It controls the voltage and amperage of the electricity from the batteries to produce a constant power source free of power spikes or undercurrent. The controller also prevents the batteries from overcharging.
- Grounding system: The grounding system is a series of wiring and metal rods that penetrate the earth around the panels and mounting system to protect the system from lightning strikes and other electrical hazards. A grounding system is required on photovoltaic systems by building codes and utility companies.
- Wiring: The wiring system connects all of the components together. The wiring gauges (sizes) and types should be verified with the manufacturers of the photovoltaic panels, controller, and inverter.
- Utility Meter: Depending on the requirements of your utility company, the meter is either the same meter that is connected to your utility company supplied power, or it is a separate meter connected to the utility company meter. For a single meter, the meter spins forwards or backwards, depending on whether the home is using the photovoltaic power or utility company power. On two meter systems, the extra meter spins forward to show how much power is generated by the photovoltaic system. In either case, the utility meter is monitored by the utility company and is used to determine your utility bill (or refund!).
Costs of a Photovoltaic System
Depending on the size and configuration of photovoltaic array (especially the number of panels), the angle of the roof, and type of mounting system, local building codes, utility company requirements, and the availability of the other components, a complete photovoltaic system for a home typically ranges from $5-$8 per watt of electricity generated.