Consumers use simplified measurements as a basis for residential energy decisions. We analyze their measurements of monthly consumption, changes through time, comparison of appliances, and length of payback period. Because of systematic errors in quantification, consumers choose ineffective energy conservation actions, and underestimate the benefits of previous actions. These errors do not result solely from lack of information since they are made even by consumers who understand technical energy measurement. To explain the persistence of a seemingly disadvantageous system, we show that consumer methods are cognitively efficient. They are easy to learn and use and are compatible with general-purpose budgeting tasks. However, they lead to higher energy use than would be economically optimal for either the individual or the nation.