Many people are surprised to find out that solar power is NOT new technology! Solar electricity uses photovoltaic cells to harness sunlight for electrical energy; the photovoltaic effect was discovered in 1879.
Photovoltaic = light + electricity Photovoltaic is often abbreviated “PV.” PV is used synonymously with the term solar electricity.
When sunlight hits a photovoltaic cell, the photon knocks loose an electron, and the flow of direct current (DC) electricity begins. Several photovoltaic cells are wired together by a manufacturer to do a photovoltaic module.
A solar installer then buys these PV modules, which mount the modules together both physically and electrically to form a solar array. By nature, the photovoltaic array generates DC electricity when it is in the sun. If AC (alternating current) electricity is desired, then the DC power is wired through an inverter, which converts DC to AC.
Some small items, such as an attic fan, can be direct-powered with solar cells. The fan is wired directly to the solar cells. When the sun hits the solar cells, the fan turns on. When there is no sunlight, there is no power for the fan. There is no switch to turn the fan on or off, and no energy storage to power the fan when the sun is not available.
Direct-powered with battery
Many items can be sized to work from a battery that is charged from solar cells. The solar cells provide power when the sun hits them, and that power is wired through a battery charger, and then to batteries. The battery charger prevents the batteries from getting over-charged and also prevents the batteries from being over-discharged.
When the load (holiday lights, radio/CD player, refrigerator…) is turned on, the power comes from the batteries. The batteries are re-charged from the solar cells. If there is not adequate power from the sun to keep the batteries charged enough to run the load, the battery charger prevents the load from running to protect the batteries.
A whole-house system that is configured this way is called off-grid, meaning the solar array charges a large battery bank, and the house loads (lights, appliances, electronics) are all run off of the battery bank. Many small items can be run directly off of the DC power from the solar cells and stored in the DC batteries. For a whole-house system, AC power is most often needed, so an inverter is added between the batteries and the loads.
Most homes in the United States are already connected to the utility grid. When a solar electric system is grid-connected, the utility grid connection most often is not changed in any way. The solar array generates DC power that is wired to an inverter that is UL 1741 listed for a safe grid connection.
The AC power coming out of the inverter is then wired into a 240-volt circuit breaker in the main electric panel. Just like lights and appliances draw power from a circuit breaker, solar power feeds a circuit breaker. When the sun hits the solar array, electricity is generated.
The electricity goes through the inverter and then feeds power to the solar breaker. The electricity travels up the buss bar in the main electric panel; if anything in the house is drawing electricity, the solar power first goes straight to the house, feeding those breakers.
If the solar power is not used by the house, it continues traveling up the buss bar in the main electric panel and goes out the main breaker. The power goes through the utility meter the other way and joins the electricity in the main utility lines. Logistically, the excess solar power from my house might help power my neighbor’s house.
Excess power will be utilized!
In this way, any extra power my home is generating and not using is ‘stored’ by the utility as a credit on my account, and any excess power I need but do not produce. Power I use when the solar system cannot generate power (at night) is still available from the utility grid. The grid serves as storage and back-up, without my needing to have a battery bank.
A utility meter does allow power to travel in both directions, to the house, and from the house, and many states have a net-metering law that governs a distributed energy source like solar power on a home. When we first test a newly-installed solar electric system, it is cool to see an older dial-style meter stop, and then start spinning backward!
However, most meters are not configured to accurately read power going in both directions, so once we have tested the system, we turn it off, wait for the local officials to complete safety inspections. Then we meet with the local utility for official commissioning and permission to turn the system on. At the time, a bi-directional meter is often installed.
The bi-directional meter will separately log the power coming to the house and the power generated by the house, and these readings are then compiled for the monthly bill, with power used charged as normal, and power generated credited to the homeowner. If a home were to generate the same amount of electricity as it used over a month, we would call that home ‘net zero.’
— Redsun Solar (@RedsunSolar) March 18, 2017
It is important to note that while a grid-connected system performs simply and automatically if the utility grid goes down, a grid-connected solar home would also be without power. At first, this doesn’t make sense, because the solar array is generating power any time the sun is available.
But the grid-connected solar system could also be feeding power out through the main and onto the utility lines – and during a power outage, this would be unsafe for the utility line-workers. For this reason, a grid-tied inverter must meet UL 1741, which means it shuts down automatically in the event of a grid failure.
The usual waveform of an AC power circuit is a sine wave. A grid-tied inverter is not designed to generate its sine wave; it reads the sine wave from the grid and matches it. So without the grid, this type of inverter is unable to function.
Because grid failure is rare, 99% of solar homes are simply grid-connected. It is certainly possible to have a grid-connected system that has a battery back-up for use in the event of a grid outage. There is a cost involved in installing batteries, and even if they are rarely (or never) used, batteries have a limited life expectancy. Also, there is an environmental cost to batteries. Whether or not your home needs batteries depends on your circumstances.