You do not need batteries. Batteries are not included with solar systems by default because they are still quite expensive when compared to what they offer.
Most solar systems in Las Vegas and surrounding areas are connected to the power grid, or grid tied. The power company’s grid acts like a virtual battery “storing” excess power production as credits on your power bill because of net metering. Those credits then pay for the power you need at night.
Unlike adding a physical battery which can cost anywhere from $13,000 to $50,000, you will not pay any additional cost for this “virtual battery.” You should understand that when the power goes out, a standard grid-tied solar system will shut off too. This is required by electrical code to assure the safety of electrical workers while they restore power.
Unexpected power coming from your house would be a hazard for these workers if the grid is down. As soon as the grid is back on, the system will activate on its own and start making power again. Here in Las Vegas, we have a very stable power grid and rarely see outages of more than a few minutes. For these reasons, adding a battery does not make sense for most homeowners.