Is a battery worth it with solar?
Often, yes, if you're out during the day. Without a battery, much of your solar power goes back to the grid at a low export rate. A battery lets you use it in the evening instead, when electricity costs you more.
How a battery helps
Your panels make most power in the middle of the day, when many homes use little. A battery stores that power so you can run the kettle, oven and TV on it in the evening.
When it makes most sense
If you're out at work during the day, if you use a lot of power in the evening, or if you're on a time-of-use tariff with cheap night rates, a battery usually earns its keep faster.
When it might not
If you're home all day and already use most of your solar as it's made, a battery adds less. The same goes for very small solar systems.
Sizing it right
Bigger isn't always better. Match the battery to how much you use in an evening. Your installer can work this out from your bills.
What to ask
Ask how long the warranty runs, how many cycles it's rated for, whether your current inverter works with it, and where it'll be fitted. Fitting a battery at the same time as panels usually costs less than adding one later.