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Grid-Tie Solar Power Systems
Grid-tie solar power systems are the most common systems used in residential and commercial solar installations. These type of photovoltaic systems maximize your return on investment while allowing the use of clean solar energy. Because of their simplicity, there is not much difference from installation to installation of a grid-tied photovoltaic system, which translates to a lower installation cost. Most gird-tie photovoltaic systems have an annual return on investment of 10%-15%, and in some cases closer to 20%, depending on the rebates of your local utility. The diagram below illustrates the general layout of a grid-tie solar system, and how it will interface with your existing electrical system.
(A) Strings of Photovoltaic Panels
This are the photovoltaic modules that will be on your roof. They are wired in series "strings" for maximum efficiency and minimal labor costs. Each solar installation may have one or multiple strings depending on the circumstances. Each of these strings connect in parallel at the combiner box, which is explained below.
(B) DC Combiner Box
The combiner box serves as a location to electrically "combine" the multiple strings of solar panels. The box is typically located close to the panels to eliminate the need to have multiple long wire runs back to the inverter. At the output of the combiner box, there is one set of direct current (DC) wires running to the inverter.
(C) AC Inverter
The Alternating Current (AC) Inverter is used to convert the DC power created by the photovoltaic panels to the 120VAC typically used in an american household. Other countries may have a different output voltage to comply with their local standard. In reality, the output of your solar inverter is typically setup for a 240VAC split-phase operation, since this is what is actually coming down from the utility line transformer. For a more in-depth explanation of "split-phase" and why we use AC in our house instead of DC, see our electricity 101 page. This 240VAC output connects directly to your existing utility panel which is explained further below.
(D) Existing Utility Panel
Your existing utility panel contains your electrical usage meter, your electrical breakers, and power line connection coming into your house from the utility pole. You are effectively making a direct connection (through a breaker and your power meter) between your inverter and the utility grid.
(E) Your Existing Loads
There is no re-wiring required to your existing electrical system in your house! Your existing loads can't "see" the solar system. Your solar system works interactively with the grid, and is effectively supplementing the power you receive from the utility. When the sun is up and your system is producing more power than you are using, you are essentially using 100% clean solar power!
(F) The Grid and the Payoff
Since your inverter is directly connected to the power line coming from the utility, along with a NET Metering Agreement from the local utility company (which the solar contractor will facilitate), you are enabled to back feed power into the grid when your photovoltaic system is producing more power than you are using in your house or business. This is where the investment of a solar power system shines. As you are back feeding power into the grid, you earn credits with the utility company, which are then used to offset the amount of power you use from the grid.
(G) Earth Ground
Electrical codes require your entire system to be connected to an earth ground. The earth ground provides life safety and fire protection from both electrical faults and lightning strikes. If you would like to know more about the theory of grounding, see our electricity 101 tutorial.
Take note, that alone, grid-tie systems do not provide back-up power in the event of a grid failure. Solar inverters have a feature known as "anti-islanding," which cause them to switch off during a power failure so that they do not energize the grid when the lineman is working on it. This can be overcome by utilization of a hybrid solar system, which incorporates an independent backup source, such as batteries or a generator.