The size of compact pulsed power generators capable of producing pulses with ∼100 ns duration at Gigawatt power levels is primarily determined by the specific energy density of the utilized energy storage medium. Capacitors capable of delivering large pulsed currents at several 10 kV voltage levels have been most frequently used as the energy storage medium for portable pulse generators. To increase the specific energy density of the pulsed power generator, the capacitors are often voltage overstressed at the cost of capacitor life. However, rapid charging (milliseconds) of the capacitor immediately followed by discharging alleviates somewhat of the lifetime problem. For repetitive operation of the pulsed power generator, the charging/discharging energy loss is the more important parameter. The energy, Wc, needed to charge a capacitor to a set voltage is measured along with the energy released, WR, by the capacitor under conditions corresponding to a compact Marx generator operating with ∼ 10 Hz rep-rate into a ∼ 20 Ohm load. For the tested capacitors with Mica as dielectric, the capacitor efficiency, η, i.e. the ratio between WR and W c, is roughly equal to 97% and largely independent of the charging time. Also tested ceramic capacitors revealed an efficiency of ∼90% for fast charging and an efficiency of ∼ 94% for slower charging (from ∼2 to 35 μs time constant).