Soon, pervasive computers will enormously outnumber humans. Devices requiring sufficient energy to operate maintenance-free for periods of years and beyond render today's technologies insufficient. With the gap between energy requirements of embedded systems and achievable levels of harvested power reducing, viable hybrid energy and power management subsystems have emerged that combine harvesting with finite, rechargeable energy buffers. Coupled with advances in wireless power transfer and energy storage, the authors suggest that an energy design space is emerging. There are, as yet, no tools or systematic methods for design space exploration or engineering in this context. It's important to develop such a methodology, and critical to link it with methodologies for system design and verification. The authors discuss key factors such an energy design methodology should incorporate, including size, weight, energy and power densities; mobility; efficiencies of harvesters and buffers; time between charges, (dis)charge speeds, and charge cycles; and availability and predictability of harvestable energy. This article is part of a special issue on energy harvesting.