A grid tie inverter is a special type of power inverter that will condition power from renewable sources like solar and wind so that you can flow the electricity out into the power grid and actually sell electricity back to the power company.
The 1978 Federal Public Utilities Regulatory Policies Act (PURPA) states that private citizens have the right to sell renewable energy back to their local utility company. This law came about because of the 1973 arab oil embargo and it is what guarantees you the right to sell excess energy you produce from solar or wind back to the electric company.
A standard power inverter takes direct current (DC) power from a source like solar panels or batteries and converts it to alternating current (AC) power, like the electricity that comes out of the wall sockets in your home. It also converts the 12 volt or 24 volt electricity coming from the solar panels, wind generator or batteries to 120 volts, the standard for electricity coming from the power grid. Many solar or wind power systems include inverters to allow you to run standard appliances with renewable energy. However, you can’t just plug any inverter into the public electric grid.
The AC electricity produced by a power inverter is basically the same as the power on the grid – both use alternating current, 120 volts, 60 cycles per second. The problem is electricity in the two systems may not be cycling in phase with each other. If the electricity from your inverter is cycling up when the grid is cycling down, things aren’t going to work. A grid tie inverter monitors the power from the grid and makes sure the power coming from your inverter stays in sync with the phase of the electricity from the grid.
Keeping the phase in sync isn’t the only thing that a grid tie inverter does, however. You can think of electricity as flowing downhill. It flows from higher voltage to lower voltage. This is, for example, how you can charge a battery. You connect a power source that is outputting more voltage than the battery is putting out and electricity flows from the higher voltage source into the lower voltage battery. (This is also why you need more than 12 volts to charge a 12 volt battery.)
So, in order to feed electricity from your renewable source back to the power grid, your electricity must be at a slightly higher voltage than the power grid. A grid tie inverter continually monitors the power from the grid and outputs slightly higher voltage than what is coming from the grid. The effect of this is any electricity that you aren’t using flows out to the grid, your power meter runs backwards and you sell this excess electricity back to the power company.
Because of the extra work involved in monitoring and conditioning the output power to match the power from the grid, a grid tie inverter is more expensive than a more simple power inverter that can’t tie into the grid. However, if you think you will have excess power and want to sell that excess electricity back to the power company you will need a grid tie inverter to do so and it may well be worth it to spend the extra money. A grid tie inverter will work fine for providing AC power to appliances in your house and then when you’re ready to sell excess electricity back to the power company, you’ll be all set!