C58 is an expert (look for his builds). I've had extremely good luck (five years of off road use) with a BBS02 and 52V 10 ah Luna battery. Unlike many, the RPM-based assist works as well for me as the many more sophisticated versions I ridden, and the Haibike -Yamaha I own. The BBS02 adds about 10 pounds and seems like consumes 10 - 20 wh depending on the level of assist (i rarely use more than level "3" of "9"). I ordered a three and a half pound battery (52V, 4 ah) that should be adequate for one of my "normal" rides, 15 miles with 1000' - 1500' of ascent. We'll see when it arrives.