Photovoltaic System Basics
What is a photovoltaic (PV) system?
Photovoltaic (PV) systems (or solar electric systems) use PV cells to convert sunlight into direct current (DC) electricity. PV cells are made from silicon and were originally developed to power spacecraft and space stations. The cells come wired together in panels that typically measure about 4 feet by 1 foot by 1.5 inches deep. A group of panels mounted on a frame is called a PV array.
To provide electricity reliably and safely, PV systems include several pieces of equipment in addition to the PV array. This balance of system components typically includes racks and other mounting equipment for the solar panels, an inverter, wiring, and if desired a form of electricity storage (typically batteries).
Most PV systems convert the DC electricity that is produced by the solar panels into the common household form of electricityalternating current (AC)by using an inverter. AC electricity can then be used to power your appliances directly, or feed back to an electric utility's grid. The DC electricity can also be stored directly in batteries for later use. In this case, to prevent overcharging or discharging of the batteries, a charge controller is used. Instead of an AC inverter, a DC converter can be used to convert AC electricity coming from the utility grid into DC electricity, which can then be stored in batteries for use during power outages. This type of configuration is often called a PV uninterruptible power supply (UPS).
Does a PV system replace an electric utility?
PV systems are typically used as either stand-alone systems or grid-connected systems. The role of photovoltaics in these two types of systems is very different, and the design decisions and performance requirements are very different as well.
Stand-alone PV systems generate all of the on-site electricity needs of a home. Therefore, they are not connected to any electric utility. Stand-alone systems can provide AC or DC electricity, and typically include batteries to store electricity for use when the sun is not shining. Stand-alone systems are often cost-effective when installed in remote areas where access by electric utilities is difficult and expensive.
Grid-connected PV systems are typically sized to meet 50% or more of a home's electrical load. These systems are not always sized to meet all of the electricity loads of a house because of the higher up-front costs associated with purchasing a larger system. A larger system will typically cost less per kilowatt-hour generated due to the economies of scale associated with the installation costs. Thus, a grid-connected system will generate all or part of the electricity required in a home, while the remaining electricity loads are met by the utility.
PV systems can be easily integrated with a utility's electrical grid providing clean, renewable electricity for homeowners, while still ensuring continuous power supply from your regular utility.
What are the benefits of having a PV system?
- Reduce your utility bill
It's generally too expensive to create a PV system to power everything in a house. However, if used wisely, PV systems can significantly reduce your utility bill. Any energy produced for "free" by the sun and your PV system is energy that you don't have to purchase from your utility. This translates into direct savings on your monthly utility bill.
In addition to the direct savings, the PV arrays also act as a sunshade for your roof, reflecting heat from the sun that would otherwise be absorbed by your house. A shaded roof area can reduce the air temperature of your house, reducing the energy required by your air-conditioner to keep a comfortable temperature in your home.
- Environmental responsibility
Another important consideration in installing PV systems is environmental responsibility. By using a renewable power source, you're helping reduce the impact of energy use on the environment for generations to come.
- Improved power reliability
If you use a grid-connected system with battery banks, then you can be assured of a continuous power supply in case of a power outage.
How much electricity can a PV system produce?
In Santa Clara, the average residential customer uses 6,500 kilowatt-hours (kWh) of electricity per year. A 2-kilowatt (kW) PV system installed in Santa Clara, on average, can produce about 3,700 kWh of electricity annually. This is about one-half the load of a typical residential customer.
Given a 2-kW rooftop PV system located in the Santa Clara region, where the average number of full hours of sunshine per day is 5.1 hours, the amount of electricity that this system could potentially produce is:
= 2 kW x average annual sunshine hours
= 2 kW x (5.1 hrs/day x 365 days/year)
= 3,723 kWh annually
Note: the rated output of PV modules is assessed using the full hours of sunshine per day where one (1) full sun hour is defined as being exposed to a solar intensity equal to 1000 watts per square meter. Full sun hours does not indicate the number of hours that the sun is shining in a day, but rather is an indication of the intensity of the sun.
Also important to note is that not all of the sunshine that strikes a PV cell will be converted into electricity. For the above example, let's assume a derating factor of 0.80. This factor accounts for inverter efficiency and wire losses, but it does not include battery losses. Thus, the amount of electricity that a 2-kW system located in the Santa Clara region could potentially produce is:
= 3,723 kWh x 0.80
= 2,978 kWh
If a system includes battery banks, then an additional derating factor must be considered in the above calculation.
This amount of electricity represents an annual cost savings of:
= 2,978 kWh x 10¢/kWh
= $298 per year, or almost $25 per month.
What is the lifespan of a PV system?
Solar Electric systems last a long time depending on the amount of active use, or the amount of sunshine converted into electricity by the PV cells. Most systems are only in use for 6 to 8 hours per day. Manufacturers will usually provide 20+ year warranty, but PV modules will likely last longer than that. The balance of system components, however, will have a shorter life.
What is involved in maintaining and operating a Solar Electric system?
Without any moving parts, maintaining a PV system is relatively simple. For grid-connected systems, minimal maintenance is required, including keeping the panels free of any debris such as fallen leaves, bird droppings, etc.
For stand-alone systems, the batteries usually require the most maintenance. Contact your PV system retailer for more information on maintaining battery banks.
PV system manufacturers will usually provide a 20+ year warranty on the PV modules, and a 10-year warranty on the balance of system components.