Wouldn't the solution to charging be the same as any other consumable utility? Charge x dollars per month for the base keeping the wires hooked up fee, and then x cents per kilowatt hour?
If you look carefully at your bill, you'll find that this is basically how it is structured in the US. Even commercial customers pay a demand charge (highest kW used in any 15 min period over the month, on the order of 10 USD/kW peak) for the site's peak power use plus a separate fuel charge (total kW-hr over the month) for their total energy use.
The issue is that the actual fuel charge for energy varies widely over the course of the day, and customers aren't exposed to that fluctuation. This leaves utilities to absorb the cost fluctuations, which turns them into finance companies as much as electric service providers.
