Excluding hardware costs, what is the USD cost of electricity required to generate 1 BTC?

That's the price around which BTC will stabilize, I think. If the price is higher than that cost, miners will generate new coins and sell to push the price down. Below that cost and the miners will turn off their mining rigs. Since they are not mining at that point, they won't have new BTC to sell. Of course other sellers could continue to aggressively push the price down but at least the miners won't be the ones doing the selling.