ACE is computed starting when a storm becomes a tropical storm and ends when it is no longer a tropical storm (weakening/dissipation, extratropical transition, and so on). Longer-lived and stronger storms have higher ACE values, since it is a sum over all times of the wind speed squared.

It is a measure of what activity has already occurred and has little to no predictive value in terms of future activity. It is not a quantity that is 'expended' -- and, in fact, is not really an 'energy' at all. It's just a way of quantifying tropical activity by weighting longer-lived and more intense storms greater than shorter-lived and/or weaker storms.

Tropical systems contribute in a small way to the global energy budget, but we do not as of yet know why there are only about 90 worldwide per year. It quite likely has absolutely nothing to do with ACE, however. Controls on hurricane activity may arise out of other, truer energy considerations -- such as upper ocean heat content -- but again, this has nothing to do with ACE.