If you didn't put a battery in at install time, there's a recent IRS determination suggesting [1] you can still receive the 30% tax credit if the battery is installed at a later date. I would highly recommend pulling the trigger right before the tax credit begins its phase out at the end of 2019, assuming costs come down further between now and then.

The caveat is that your system must charge the battery from your panels, and cannot charge from the utility (trivial inverter setting). With that said, based on net metering laws in most locales, it behooves you to consume as much of your own production as possible. Exceptions abound.

https://www.reddit.com/r/energy/comments/82fchy/irs_letter_o...

We have net metering here, so the grid is basically my battery. It would be more worthwhile if I could charge from the grid but I’m way overproducing as is.