JeremyP If your battery has been filled with Solar energy during the day and you haven't used it all through the peak period then you might not want to top it again with cheap grid energy which does cost you, even during the off-peak period.
The setting of, say 80%, means that if you currently have 80% in the battery then you have enough to get through to the next solar charge period so don't need to charge from the grid at all. But if the next solar charge period is predicted to be poor, say, below 5kWh then charge from the grid anyway as the predicted Solar will not fill the battery the next day leaving you with the possibility of running out of charge right in the next evening peak period.
During the summer you may find that your solar panels produce enough energy for you not to need to fill from the grid at all hence this option. If you didn't have this option then you would fill from the grid overnight then the following morning quickly get back up to 100% charge from the solar panels and export the rest all through the day. You might not want to do this - especially as you said you didn't get paid for exports.
So whether this option is useful to you depends on the size of your battery, whether it can carry you through the night and whether your solar array can cover the house load and charge up the batteries every day.
Edit: Don't forget that during the summer the solar is available right through the Flux peak period of 4-7pm and is probably still producing enough for your house load unless you have an East facing array.
