Photovoltaic (PV) solar installations typically fall within one of two categories. They are “grid-tied” and “stand-alone”. Here are my best explanations of these two systems.
Grid-Tied: A grid connected system is one that works in unison with the local utility power company. When the grid is operational, only then are the solar panels operational. There are a number of reasons why this is so. First, if the solar system produces more energy than is being consumed at any point, this power is then fed into the grid. The power produced must match the utility power in phase order to feed into the system. The only way this is done is by reading the utility power and then adjusting the inverter accordingly. The meter on the house will then spin backwards. This type of meter is called a net meter. If in the case the system produces more energy than is consumed in entirety, the local utility typically pays the customer for the energy produced. Rates will differ based on the utility’s policies. Another reason that the solar system will not work is for safety of electrical workers. If the power is out and the solar system is pumping power into the system, an electrical worker could be electrocuted working on the home, the business or even the local power line. The solar system works in conjunction with the utility by monitoring the phase balancing of the power supplied and matching it and then by providing power to your home or business.
Stand-Alone: A stand-alone system is one where the panels are not connected to the grid in any way and provide power to a home, business or any application that needs electrical power. An example of this is a road way sign with a solar panel on top of it. In a home or business, they are usually paired with a battery system or some form of energy storage. This application is for the remote locations where bringing utility power is either not feasible, not wanted or too expensive. The batteries in this type of system must be maintained on a regular basis. Stand-alone systems are also used in micro-grid applications. Micro-grid solar systems can be thought of as a hybrid between the two systems being described. The utility power can be a local generator or wind turbine and the solar system matches and supplements power accordingly. Depending on the micro-grid sophistication, a control system must be implemented to dictate what power source is considered the base utility power. The other renewable, or other energy sources, must then match what the base utility power is supplying.