I would question the point of owning a BE: Ideally you run it with a rPi. So consumption at around 10W  (including PSU) is not so much of an issue. About .25kw/h per day of non-stop operation. However it might be well over 150 days before the BE paid for itself. Another 50 days or so for the pi. During that time you had to pay for 50kw/h. Another 10-20 days. All provided that there won't be a flood of asic devices pushing the difficulty even further (the current trend doesn't look exactly linear). Take your BTC and trade it, the daily spread gives you more than a BE could ever generate.