Energy intensity—an expression normalizing pump energy use by water volume, also called specific energy—is becoming a more commonly used key performance indicator as water utilities seek to analyze and optimize their energy use. Its theoretical basis, however, has not been well documented. Beginning with Newton's second law, we derive the pump power equation and the pump energy intensity equation, provide specific values for engineering reference, and discuss applications. In a perfectly efficient system pumping water, the minimum energy intensity is 0.00272 kWh/m³ per meter of head or 3.14 kWh/Mgal per foot of head. Considering typical pump efficiencies, these references may be scaled and used for analyzing pump performance or estimating future energy uses. As a key performance indicator, energy intensity is a convenient input-output quotient where both parts can be observed directly, enabling tracking over time and comparison among individual facilities. As a planning tool, energy intensity can estimate expected energy use without having to know details of the pumping system. Such are the high-level applications water systems may find as a prelude to deeper energy analyses in pursuit of sustainable infrastructure.