I was thinking of buying a DC-DC converter to charge USB devices from my battery bank rather than using my 110volt inverter output. Before I bought it, I wanted to measure my inverter efficiency and compare the wattage cost. My inverter gave me an average efficiency of 88% (as advertised). Most of the DC to DC converters are about 80% according to their specs, so it doesn't make sense to use them. FYI, my inverter is an AIMS 6Kw low frequency inverter/charger.