Basically how grid-tie works, is that when your system is generating electric, the extra electric is sent to the utility thus running your electric meter backwards (you are either selling or getting an electric credit from your utility). With a Grid-Interactive System, when your system is not making electric you are either drawing electric off your batteries or if your batteries are low, from the utility thus running you meter forward (you are buying or withdrawing your credit from your utility). In some cases you may not presently see any money from your utility company, however each year that has been changing and it depends on your utility company. Depending on your system and usage, you will be able to either lower your utility bill to none existent or at the very least have lower electric bills and in some cases may receive a check at the end of the month. A good example of Grid-Tie at its best is where you have long sunny days in the summer (selling to the grid) and short snowy days in the winter (collecting your summer credits). So my advise is, if you have the utility near by and expect that someday it will be going past your property or if you are already on the grid and are looking to make an investment in the future to seriously consider a system that can be tied to the grid. Also, in an area like northern New Mexico where power surges and brownouts seem like a daily routine than a Grid-Interactive System makes not only a good back-up system but may very well save a lot of unforeseen expense on replacing sensitive electronic equipment such as computers.
All Grid-Tie systems stop selling to your utility should there be an interruption of utility service. This is required by law to protect any linemen that may have to work on the outage problem. A Grid-Interactive System on the other hand will provide you with uninteruped back-up power as long as your batteries are full enough.
Presently there are three types of Grid-Tie Systems.
- GRID-TIE WITH STRING INVERTERS: This is where your system sells when you are producing and you purchase when you are not producing. The reason why the inverters are referred to as String Inverters is because each inverter must be matched to a string or strings of solar Photovoltaic (PV) Modules or Solar Panels.
When you are setting up your system you need to match the inverter you choose to the size and make of module you plan on purchasing. In other words, you can't purchase some panels because you think you are getting a deal and hope that they may work with a grid-tie inverter.
You need to decide what you can afford to invest and make your purchase at one time. You can always add additional inverters and panels to increase your system at a later date.
- GRID-TIE WITH A DEDICATED INVERTERS: This is a relativity new technology in the solar market. This system will cost you less to get started. Each solar PV module (solar panel) is connected with its own dedicated inverter. As you may see, with this system you can purchase as little as one inverter and one module and you could be up and running.
To expand your system you just add modules and inverters as you desire. Another advantage is that if some of your modules are clouded over, the remaining will still be selling energy to your utility.
- GRID-INTERACTIVE INVERTERS: These systems also have their advantages and disadvantages. Their main advantage is that they can provide you with electric when there is a utility outage because they have a battery back-up. They function much as any Grid-Tie System does except you will have a battery backup when your utility goes down. Here in the Taos area, those on the grid experience frequent brown outs.
Unlike an Off-Grid System you do not have to have a large battery bank, usually enough storage to supply your critical loads for a day or two is all that is normally required.
Another advantage is this type of system can have additional solar modules added at a later date so you can increase your output over a period of time.
This system is not as efficient as the other two Grid-Tie Systems mentioned above