Is it a sound investment ?
We can't predict the future. Batteries are still relatively expensive, and much will depend on your actual situation. Do you generate a lot of unused power during the day, and do you charge your car every night? Then you should absolutely get a battery. Do you only have a few PV panels and consume very little during the night? Then don't bother, you won't benefit financially -which doesn't mean a battery can't still be nice-to-have in some cases.