There is no question in the mind of any battery professional that the only way to establish the capacity of a battery is to do a load test.
Yet, for many battery owners, the question is: why do I need to know the battery capacity when that was already calculated when the battery was sized? In a sense, that is true. However, all batteries will lose capacity as they age. And in order to meet the required load and runtime during its life, the battery should typically be oversized as part of the selection process, and therefore, should always exceed the capacity requirement until it reaches end of life.