I assume you only use the block heater IF you plan to start the engine, otherwise its pointless right? Electric here has gone up 37% though I don't pay for it due to solar panels if I use it then I don't get to sell it anyways so its the same thing.
With my previous hybrid, I would use the block heater on a timer most mornings. It drew 400 watts. Two hours was always enough. 20 minutes was sort of a point of diminishing returns. Even at the outrageous price of 37 cents, I think you could heat up for 45 minutes or so and still come out ahead. I developed a very simple test to figure out the maximum amount of gas you could save before you install a block heater (or precisely how much you save after the fact). I bet you would like it. PM me if you are interested, and I will send it to you.
With the Energi I rarely do a cold start from home; so I never installed a block heater on it. If I often did cold starts from home, I surely would install a block heater and return to using it regularly.