The depth of discharge is a percentage showing how much charge has been used from a battery compared to a full charge. So if a battery has a total capacity of 100kWh and 40kWh are used then the depth of discharge is 40%. DOD is the opposite of state of charge, in this example the state of charge would be 60%.
Batteries also have a rated DOD in their specifications to show the how much of the battery’s total capacity can ideally be used. For example a battery with 100kWh total capacity and a rated DOD of 80% means in reality there is only 80kWh that can be used before affecting the battery’s health.
Some battery providers give the depth of discharge figure as the battery capacity, protecting the top and bottom state of charge for the battery in the battery management system to keep the battery healthy and extend it’s life e.g. state the battery has a 60kWh capacity when in reality it 100kWh to protect the top and bottom 20% (20kWh). This makes the 60kWh DOD 100kWh reality battery look expensive but the battery life will be far longer than a 60kWh real capacity battery which uses the top and bottom 20%, so could work out cheaper overall.
Check before purchasing a battery to prevent unnecessary costs later.