The reversible work is the maximal useful work which can be obtained, W r e v = W m a x , and can only be fully utilized in an ideal reversible process. An irreversible process produces some work W a c t u a l , which is less than W r e v . The lost work is then W l o s t = W r e v − − W a c t u a l ; in other words, W l o s t is the work which was lost or not exploited during the process due to irreversibilities. In terms of lost work, the theorem generally states W ˙ ˙ l o s t = T 0 S g ˙ ˙ where W ˙ ˙ l o s t is the rate at which work is lost, and S g ˙ ˙ is the rate at which entropy is generated. Time derivatives are denoted by dots. The theorem, as stated above, holds only for the entire thermodynamic universe - the system along with its surroundings, together: W ˙ ˙ l o s t , t o t = T 0 S ˙ ˙ g , t o t where the index "tot" denotes the total quantities produced within or by the entire universe.