The battery drain increases by a third if you use a high current mic at 48 Volts. Fortunately, the most common professional mics are relatively low current (such as Sennheisers). Approximate battery life will go from 4.5 hours at no phantom, to 3.7 hours with a low current mic, and to 3 hours with a high current mic. Battery life can be improved for some low voltage, high current Schoeps by running them in the 18 Volt position. A number of popular performing microphones are just as happy at 11 Volts as they are at 48 Volts. There is absolutely no advantage to running them at 48 Volts; it is just wasted battery power. Switch the UH transmitter to 18 Volts with these mics and reduction in battery life will be reduced by more than half.
So the overall answer here is, check the specs of the microphone to see what minimum voltage it really requires and then set the UH to 18 Volts if possible. If you do that you should lose even less than a half hour of battery life. If the specs are not at hand, try running the mic at 18 Volts and see if you are happy with the results. In fact, have someone else switch the voltage and see if you can tell the difference, no matter what the specs say. You may be able to save some battery money.