A grid-tied system with battery back-up generates electricity, stores it, supplies it to household loads, and forwards any excess to your utility. Like a simple grid-tied system, it disconnects automatically from utility lines when power is down, and it draws power from the utility when your system doesn't produce enough to meet demand. The big difference is that that a battery back-up system continues to generate useable electricity when utility power is down.
A battery back-up system is 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 additional components. It also includes extra electrical work to establish a separate breaker panel for items you want on your back upload—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.