Grid Tie System (Utility interconnected system) Grid connected PV systems without backup capability are the most common type of PV system. This is because they require the least amount of maintenance and are the most cost effective to install. Grid connected PV systems are connected to your home's main service panel in parallel with the electrical lines from the utility. The PV system can be sized to handle 50%, 100%, or any other amount of your overall energy usage. Your main circuit breaker panel will always take as much electricity as it can from your solar electric system, and since the panel is still connected to the utility grid, any remaining energy that is needed is pulled from the utility. On sunny days when your system might be producing more energy than your home is using, the generation from the PV system will actually be sent out into the utility grid, causing your meter to spin backwards. These are hours when the utility is buying energy from you!
Grid Tie Systems are also virtually maintenance free. One drawback to Grid Tie systems, however, is that during a power outage, your PV system will automatically shut down. The main reason for this is that if the grid goes down, the inverter in your PV system will have no outside resource to export excess power, nor will it have anywhere to pull the remaining power from if your system cannot power all of the appliances in your home. In order for your PV system to continue operating during a blackout, you will need to have a battery back-up system (For more information, see the section on Battery Back-Up Systems)
Grid tie with battery backup
Battery backup allows your solar electric system to provide power to your home even when the utility power is out. Grid tie battery backup systems usually provide power only to critical circuits since it is not cost effective, or necessary, to provide battery backup for your entire house. It will be necessary to determine what items are to be run from the batteries in order to size them correctly to meet your requirements. A sub-panel must be added to connect these critical loads to your inverter. Battery backup systems will cost more than grid tie systems. And, the batteries will require periodic checks.
Off grid system
Off grid systems are the most complicated type of PV system because they must be designed to meet all of your electrical needs without any room for error. They must be able to function during adverse weather conditions and provide adequate power to your home. All off grid systems contain batteries to store electricity, and it is important that the batteries be sized correctly to ensure enough energy will be available for the In designing an off grid system, it very important to determine all of your loads so that you can size the system correctly.
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