Modern heat pumps can get a coefficient of performance of around 3, meaning that for every watt of electricity going in, you get 3 watts coming out inside your home (the other two watts are being removed from the air outside). For everything that isn't a heat pump, energy in = energy released in your home (coefficient of performance=1). So in the end, you get like a 33% rebate on cpu usage via heating cost savings(very rough number, since the actual coefficient of performance varies wildly on outside air temp)...non-negligible, but it doesn't make it free unfortunately.