A more thorough explanation:

wantage (wahnt-ij), n. A deficiency of something; specifically, a vessel's deficiency of not being full, due to leakage.

• There was a wantage of water in the pitcher because it had a crack.
• The wantage of oil in the engine caused it to overheat.

These examples illustrate the definition of wantage by showing a lack or deficiency of something due to leakage or other causes. In the first example, the pitcher had a crack which caused a wantage of water, meaning it was not full. In the second example, the engine had a wantage of oil, which caused it to overheat and not function properly.