Aheem, isn't it the other way round? From what i have tested, a drill always consumes 15E/s (unless they throttle down when your source of electicity is delivering insufficient power, but i'll ignore that case for now). Then, the basic ore rate seems to be Concentration/20 per second, so if the concentration is 8%, you'd get 8%/20 = 0.004 ore per second. You can calculate using the given rates of fuel cells, drills and the ISRU converter, that you will need a ore rate of slightly above 0.02 ore per second and drill to achieve equilibrium, so a higher rate will result in a fuel win. This means, for unmanned drill vehicles, you would need an ore rate above 40% (i have only scanned kerbin so far, on kerbin you will never find a spot close to 40%, no idea if there is a body which such a high rate, probably not or only very rare). Now, for actually effectively farming ore cell-powered, you wanna take an engineer with you. Engineers simply increase the ore rate of drills by being in the craft. Only the highest engineer is counted, so you just need one. In my test world i only have 5* Engineers(Sandbox), so i could only test out that five-star-engineers will multiply your rate by 25, so in the 8% example, you would get 0.1 instead of 0.004 ore/s, which means that 4/5 of your mined ore will be effective fuel, while 1/5 of the ore is enough to supply the Fuel Cell Arrays.