Hi Spruce, actually when you reduce the voltage to any motor the amperage will also get reduced, and so therefore your battery lasts longer.
The varistream doesnt just reduce the voltage, it moniters the pump motor and will keep the flow rate constant no matter what height you work at. It also detects when the flow has been stopped, ie at your pole control valve, and will stop the pump, you can actually change a setting inside that controls the pressure that it will stop the pump at. So you dont need to physically switch off the varistream each time you stop the flow, I leave it on all the time i'm in an area, just switch it off when i'm gonna drive off.
Tim