It's not as useful, and harder to wire. To measure amperage, you either need a gauge in series with the main power feed, or an inductive pickup. It would tell you how much load was on the system, and it would tell you whether the battery was charging or discharging. You can infer these things by the voltage reading, so having both gives you very little additional information.

IIRC, the hole diameter is 1", or maybe 26mm.