> An EREV version could get by with 50-100kWh, which is also a lot less weight to carry.

It would most likely be closer to the top of that than the bottom. The standard range Lightning has a 98 kWh battery. So you can see what I mean, the battery would only be slightly smaller. Slightly less expensive, and slightly lighter -- but remember that you also have to add back in the weight of the engine and generator.