Solar Electricity
Photovoltaic cells produce DC, or direct-current electricity, when they are exposed to light. In a solar-electric, or PV, system, many cells are wired together and encapsulated behind a piece of tempered glass that is bound by an aluminum frame for strength. This approximately 3ft. by 5ft. unit is called a PV module.
PV Modules
PV modules are hermetically sealed and designed to withstand 100 MPH winds and 1” hail stones. However, they may shatter from a blow such as a dropped hammer.
PV modules have a twenty-five year life expectancy, and in many cases they may continue to perform much longer at a slightly reduced output. They have no moving parts and require little or no maintenance over their entire life. In urban areas they may require cleaning every year or two if the air quality is poor.
PV Array
Multiple PV modules are wired together in series on a roof to form a PV array. This results in a high voltage DC array of 200-550VDC. The array wiring enters metal conduit near the array and proceeds to an array disconnect switch immediately adjacent to the utility-interactive inverter.
Utility-Interactive Inverter
An inverter transforms the DC array electricity to 240VAC, alternating current. This is the proper type of electricity to power a home or business, and or, send back to the utility company. The inverter sends its 240VAC electricity to a production meter, that counts the kilowatt-hours of electricity produced, and then to a 240VAC disconnect that can disconnect the system from the home or business side of the inverter. The electricity then travels to the main service panel. It connects to a backfeeding circuit breaker in the panel that is installed at the bottom of the existing breakers, specifically labeled as the PV system backfeed breaker.
A grid-tie, or utility-interactive, inverter will only produce power when it senses power from the utility company. If there is a power outage, the inverter will not function. This means that the PV system will power no loads in the home or business as a backup when the utility power is down. This condition is mandatory in the inverter design to ensure that no power can be supplied backward to the grid and harm a utility worker.
How Your PV System Functions
The inverter will not function unless there is power from the array. At night when there is no light, the inverter will effectively be turned off. When dawn comes the array generates electricity and the inverter turns on. The electricity from the inverter supplies electricity to the main service panel. This will power the loads in the home or business. If the loads are greater than the PV electricity, the remainder will be powered by the utility company and recorded on the meter on the exterior of the building. If the loads are less, the extra PV electricity exits the home or business out to the power line to your neighbors. As it passes out of the building, the excess power effectively turns the meter backwards and reduces your monthly bill.
In general, the power company will power loads at night while the PV system is not producing. During the day, the customer may use less than the PV produces and send the excess back to the power company. This also works on a yearly cycle. More PV power is produced in the summer, and more utility power is purchased in the winter.
Snow Accumulation
The majority of the production of your PV system occurs in the non-winter months. Do not fret if your PV array is covered with snow for a day. Secondly, your array produces very little on a cloudy day. Do not worry if the array is covered on a cloudy day. In general, an accumulation of snow on the array will slump down due to gravity, exposing an upper portion of the array to the sun. This portion will warm, and warm the array under the snow, causing the snow to slide off. A steeper array will clear more quickly. A flatter array will produce very little in the winter anyway.
Ice on the array will melt and slide when the sun passes through the clear ice, much like a window, and heats the modules beneath. Sometimes snow covers the ice and reflects the sun. In this case, wait for warmer weather, or if safe, brush the layer of snow away so that the sun can pass through the ice to heat the modules. Additionally, when some of the array is clear, that portion with produce electricity that forces its way through the covered modules, generating some internal resistance and heat.