This is not a straight-forward answer as some companies might tell you. The honest truth is it depends on the amount of energy you are currently using along with the condition and location of your roof.
As a general rule of thumb an electric bill that is $75 or higher on average is where one will see immediate savings. If you aren't using a lot of power, then it doesn't matter if the power utility company rates are expensive, you won't see immediate savings. Only after the utility company raises their prices and you are already locked in at fixed rate for your solar energy will you begin to appreciate the savings.
For those that have a high energy usage, savings are normally immediately appreciated. Having a solar system can cut your energy costs in many cases up to half off your current bill. In a few cases, one might even save more than half off depending on where your roof is facing