We describe an energy-based analogue of the time-based roofline model. We create this model from the perspective of algorithm designers and performance tuners, with the intent not of making exact predictions, but rather, developing highlevel analytic insights into the possible relationships among the time, energy, and power costs of an algorithm. The model expresses algorithms in terms of operations, concurrency, and memory traffic; and characterizes the machine based on a small number of simple cost parameters, namely, the time and energy costs per operation or per word of communication. We confirm the basic form of the model experimentally. From this model, we suggest under what conditions we ought to expect an algorithmic time-energy trade-off, and show how algorithm properties may help inform power management.