Another suggestion is that you look for other ways to save energy, so you can use your oven and stovetop to cook.
Energy prices here in Texas are much lower than what they are in the UK, but I still watch my usage. All of my lights are LED, I wait until I have multiple errands to run before I get in the car and drive, and I adjust my thermostats as needed -- such as being at home vs being away from home.
Using an oven in the winter, the cost is partially mitigated by the fact that the oven helps heat the house, making the furnace work less.
As for the running cost of sous vide, I have never done the math, but I would think it would be less than running an oven for the same amount of time. Plus, in summer months, it doesn't change the temperature in the house, which matters a lot when you live in Texas where it can be over 100F/38C for weeks/months at a time.
CD