One of the wonderful things about solar power is how well it works with the utility grid. When your solar energy system produces more power than your home currently needs, you can pump that power out to the grid and your utility will give you a credit for the value of that electricity. When your home demands more energy than the solar energy system is generating, you can draw power from the grid to make up the difference.
Unfortunately, when the utility power is out, your solar energy system still needs another energy source to act as a backup — a place to send electricity when the system is producing more power than you need and a place to pull extra power from when a cloud passes overhead. Your utility doesn't want you to do that during a power outage because it endangers line workers trying to restore power. So solar installations are required to disconnect from the grid during a power outage.
One common backup strategy is to add batteries to your solar installation. Unfortunately, the price of batteries hasn't fallen as fast as the price of solar panels. That means a battery backup system can easily add 30–40% to your overall cost of installation.
Another idea is to combine solar with a backup generator. This makes a lot of sense for buildings — like our schools and municipal buildings — that already have a backup generator installed. Properly designed backup generators disconnect from the grid during a power outage — operating like an island and supplying all their own power. A well-designed solar energy system can easily integrate with your backup generator, letting the solar panels carry the load when the sun is strong and the backup generator picking up the slack during evening hours. The NY Times has an excellent article about a school that survived Hurricane Sandy by doing just that.