How Solar Works Ac vs. Dc
Electrical current is transmitted in two main ways, Direct Current DC and Alternating Current AC.  Direct Current is when the current maintains a relatively constant flow without changing directions.  DC current is produced by solar panels and stored in batteries.
Alternating Current changes directions in a sinusoidal pattern several times per second.  In the United States, the frequency is 60hz, meaning that it goes from positive to negative 60 times per second.  In Europe and many other countries the frequency is 50hz.  Alternating Current is what the utility grid distributes and what inverters produce from DC sources.
When transmitting current over long distances Alternating Current is preferred because of the more dramatic voltage losses that take place when using Direct Current.  The reason for this is that with DC, each electron has to travel from the source to the destination.  AC on the other hand, is just electrons vibrating into each other.
The standard voltage of AC power coming out of an electrical outlet is 120V, but when it is transmitted over the grid it is at a much higher voltage, at least 240V depending the particular section of the grid.  The higher voltage is preferred because it reduces line loses.
In single phase AC there are two conductors and an optional ground.  These two conductors oscillate opposite of each other.  Since one full oscillation is 360 degrees, the two conductors would be 180 degrees out of phase.  In three phase power, there are three conductors and an optional ground.  Each conductor is 120 degrees out of phase from the other two conductors.  Three phase is commonly used for large electric motors.
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