Considering "Classic" when inactive/idle will "sleep" itself, battery life will remain similar to OS 9/X but some OS 9 applications are prone to more battery drain than OS X native--that is my observation on my 2003/2004 PB G4s. If you use more OS 8-9 era software, better to keep a spare Mac just for that task alone.
Remember Classic doesn't allow direct access to certain hardware/chipsets so your mileage varies with older software, in my experience some older IDE/PATA to USB 1.1 bridgeboards can't have the HDD formatted to HFS+ with journaling but hybrid USB 1.1+FW400(Oxford chipsets) don't have this issue with Classic.