A common figure of merit utilized for HPM sources is the instantaneous power efficiency at the time of peak output power. This conversion efficiency from the electron beam power to radiated microwave power disregards the overall energy efficiency of the system, which is vitally important in systems intended for practical application. Optimizing the overall system energy efficiency allows a reduction in volume, weight, and prime power requirements that makes for a much more practical and fieldable system. Texas Tech University has developed a vircator-based laboratory HPM system that has a high end-to-end energy efficiency relative to other HPM systems. The system utilizes a sealed-tube vircator source that requires no external vacuum pumping subsystem. For rep-rate use, the tube contains an integrated low-power sputter ion pump. The lack of need for an externally applied magnetic field saves complexity, weight, and potentially power. The vircator tube is simple and robust. The source is driven by a Marx generator utilizing pulse forming networks (PFNs) instead of lumped capacitors. Utilizing PFNs allows the driver to apply a more ideal pulse shape to the source than a traditional Marx, enhancing the performance of the source and reducing the amount of energy wasted in the rise and fall of the pulse. The system has demonstrated greater than 1% energy efficiency from the energy stored in the Marx to the radiated HPM, with potential for improvement. The peak total radiated output power of the system is between 50 and 100 MW.