This again? For his application, that is absolutely incorrect. He was clearly trying to take into consideration the cost of charging his Volts vs the cost of running non-EVs, and for that it all gets billed at the latter tiers (probably some 3 and some 4 in his scenario). Assuming his previous load always put him in Tier4 as he said, the answer to his "how much did my bill go up" its going to be VoltKWHUsed * Tier4CostPerKWH. Simple as that. VoltKWHUsed * AvgKWHCost is totally the incorrect answer for what he was solving for.

And this again? By your logic, if he now goes out and buys a replacement refrigerator that uses 100 kWh/month, that refrigerator is now costing him the highest possible rate because it was the last thing added to the power bill. Now, because the old refrigerator is out, and the new one is in, the Volts move down to a Tier 2 rate instead of a Tier 3 rate? Arbitrarily determining which Tier an electronic device falls under based solely on the point in time in which it was added to the system is illogical.

If he wants to know how much his power bill will go up after buying a Volt, that answer is simple. Old bill subtracted from new bill. The rates of the various Tiers do not and should not be considered.

If he wants to know how much it is costing him to charge the Volt, he can either track specific rates, times, and usages of the Volt, or he can simply take the average rate he is paying for electricity and multiply that by the number of kWh the Volt uses.

In my opinion, the simplest solution, though possibly hard for many non-EV owners to fathom, would be to total all energy expenditures. Pre Volts, his power bill was X + $700 (gasoline). Post Volts, his power bill is (X + $200) - $700.

The trick is figuring out what you are solving for, and he, specifically, wants to know how much it costs to charge the Volt. So in his case, Don is right: He should be using his average rate. Sure, that makes all of his other electronics more expensive to operate, but you can't, arbitrarily, assign all of those increased expenses to one electronic device.