You have small solar system, so your options are:
- Get a small battery (2-3kWh). A large battery will never charge fully in winter with a small system!
- Get more solar (at least another 3kW) and a larger battery (6-10kWh).
- Don't worry about the battery, and get more solar.
Option 3 will almost certainly give you the best return on investment. Here's why.
You can get quotes for either solar, batteries or batteries + solar here.