Criteria to use the battery?

What is the criteria for using the battery?

My system is set to the defaults (I think) and I often see the battery has enough power to supply the house, yet the system uses grid energy.

Suggest you login to the portal and make sure it is set to Eco.
Good idea to set something other than Eco and hit save and then set Eco again to make sure it is correctly set

MartinG as @SteveCook says, but also worth noting that the data you see in the app in Away mode is only snapshot every 5 minutes, and it takes a finite amount of time for your inverter to respond to house load changes, so its possible that you were just seeing a momentary lag in the inverter starting discharge to meet house load.

It shows no battery discharge which could be due to Eco mode not on, but also shows no solar which is odd assuming this was done during the day?