I should have been clearer in my "miles per kw" or "kw per mile" reference. If you can make batteries half the size, then you can double up on the range, which means you will have more choices to pick and choose your recharging times and locations. More people will be able to recharge overnight, rather than, say, at work during the day.
In most areas, electrical demand is lower at night and typically costs less. Sometimes there is excess electricity. In the case of hydropower, if there is no demand for the electricity, it goes to waste (there is little to no storage of electricity on the grid), or they can reduce power generation at the hydropower turbines if they have that option*.
* In many cases, they have to let some water through the hydropower dams due to:
1) minimizing flood risk by not storing too much water in the reservoir(s) upstream, and
2) in-stream flow regulations - you can't just turn off the water, as it is needed downstream by fish & wildlife, municipalities, irrigation districts, recreation, etc. (In Texas, for example, fresh water needs to reach the estuaries on the coast to reduce hypersaline areas in the bays that can impact many forms of fish, wildlife and vegetation).
Dam operators have the option of flowing water through a spillway rather than the power-generating turbines in the powerplant, but why not generate electricity if you have the ability to do so for relatively free? That would be like owning construction equipment and not using it.... it is costing you more money to leave it sit.
