Damage per Energy: DPE is basically the average damage that is done per Energy Point spent. Multiple hits are factored in. Fractions are rounded to the nearest tenth since some weapons do less then 1 DPE, and having one decimal point helps in determining which weapons are more energy efficient.
DPE is calculated by dividing the total DPT by the energy consumption of the weapon. The higher the DPE, the more energy-efficient the weapon is.