Grid-Tied PV Systems
Grid-tied PV systems feed electricity to your household electrical panel and send any excess power to the utility company through existing utility lines. Grid-tied systems have the lowest initial cost because they don’t use back-up batteries or generators. They’re also easy to add modules to later. Grid-tied systems minimize power loss because the electricity your system generates travels through fewer components and encounters less resistance and energy loss than other systems. The biggest drawback to a simple grid-tied system is that when you lose utility power, your inverter shuts down your solar array, and you are without power even if the sun is shining.
You must sign a net metering agreement to be able to send electricity back to the utility company. Net metering allows utility customers to offset their consumption with electricity generated by their renewable energy systems. Your electric meter essentially runs backwards when your system generates more electricity than you can use. The utility credits your account at full retail rate and charges you only for the power you use in excess of what you’ve produced. Credits roll over month to month and are zeroed out at the end of the year. Learn more about net metering here.
Grid-Tied with Battery or Generator Back Up
A grid-tied system with battery back up generates electricity, stores it in batteries, supplies it to household loads, and forwards the excess to your utility. Like a simple grid-tied system, it disconnects automatically from utility lines when power is down and draws power from the utility grid when your system is not producing enough to meet demand. The big difference in this system is that it continues to generate useable electricity when utility power is down.
A battery back up system is slightly less efficient and costs more than a simple grid-tied system. Reduced efficiency is a result of electricity traveling through a greater number of components. Increased cost comes from buying, installing, and maintaining batteries and the additional charge controller and inverter. It also includes extra electrical work to establish a separate breaker panel for items you want on your back up
load—essential items like refrigerators, freezers, lights, and fans that you want to run during power outages.
During a power loss, the electricity stored in the batteries is instantly available. How much power you store depends on the type, number and capacity of batteries and if sunshine is available to recharge them during the day.
An alternative back up option is a stand-by generator tied to all
or some of the circuits of your house by an automatic transfer switch. When power goes out, the transfer switch automatically starts the generator and restores power within a few seconds. Generators run on propane, natural gas, diesel, or gasoline. A good generator with controls and installation is often more expensive than back-up batteries and charge controller, but it can be used with a simple grid-tied system for uninterruptible power.
An off-grid PV system supplies electricity to homes that are not connected to a utility company. When the sun is not shining, a battery bank provides power to the electric panel. It’s common to size battery banks to provide power for 3-5 days. An off-grid system typically includes an additional source of power such as a wind turbine or generator. If there are prolonged days of cloudiness, a back up generator automatically starts to re-charge the batteries and provide power. Net metering is not available to off-grid homes.
Off-grid living can be financially justified if the cost of bringing power lines to the property is so high it would offset much or all of the cost of the PV system and generator.