In the long run Bitcoin mining always moves toward zero profitability. In the state of dynamic equilibrium, the cost of mining would be more or less equal to the value of the mined coins (block reward and fees). Hence there is an upper limit to how much electricity mining will consume. As the block reward halves, the power usage also halves over the long term (assuming a steady price). As the value of BTC increases, the power usage will also increase. This upper limit is independent of the number of users or the number of transactions per second, at least while fees are a small fraction of the block reward.