In off-grid situations, batteries make perfect sense. You want to store whatever excess electricity your solar panels generate for later use. Without batteries (or a connection to the utility grid), you would end up wasting a lot of electricity (and money)!
If you`re grid-tied, the situation is completely different. It doesn’t matter if you produce more electricity than what you`re capable of consuming. Excess electricity is simply put onto the utility grid (through net metering or feed-in tariff schemes). As a result, your power meter spins backwards, and you receive bill credits accordingly.
In one sense, the utility grid is really like a virtual battery. Tap it for energy during times when your solar panels aren`t producing (e.g. cloudy days and nights), and recharge the battery when you`re producing too much.
Grid-tied solar panels can be combined with solar batteries for extra protection against utility grid blackouts. However, we still recommend backup generators over batteries due to obvious cost differences.
Solar batteries are expensive. A properly sized battery bank will typically cost you $5,000-$10,000. Also note that solar batteries should be replaced every 10 years.
The bottom line is this: Homeowners with a grid-connection should steer away from solar panel batteries. They simply do not need them. Only consider solar panel batteries if you`re in an off-grid situation.