Not all the power in the battery is usable. The battery will switch off at 10% to protect it from being over-discharged. This is a safety feature to ensure that the battery isn’t damaged by an over discharge situation. That said it is also a good idea to set the inverter to disconnect from the load when the battery reaches  15% State of Charge. This will allow the battery to keep the inverter on until solar or the utility supply return to charge the battery.