Comment by mindslight

If you leave the EV charging out of your consumption, does a time of day plan still save you money on the remaining usage? Or does it cost you? If it saves you money, then it would make sense to be on a ToD plan regardless of EV charging. Which means it makes sense to consider your additional EV draw as costing the marginal off-peak rate. Essentially the EV load has the valuable property of being dispatchable.

You can do the same thought experiment with say a dehumidifier in your basement. It can easily be off during peak usage and still accomplish its job, so its cost of electricity is also the marginal off-peak rate.