Also depending on how much peak you use, buying the electricity at peak rates may be a better spend than an additional battery.
The more batteries you add, the lower their returns. They can quite easily give negative ROI once you have ‘enough’ and ‘enough’ doesn’t mean zero peak rate use.
For 9 months of the year, the benefit the additional battery will generate is likely to be pretty low. Sure you can energy trade but cycling the battery at ~7p/kwh after losses (for now) will not generate enough benefit to repay its initial outlay when you are paying retail rates plus installation.
It doesn’t feel good to be buying peak rate electricity but it may well be the most cost effective option.