United has a partner relationship with Reliant Energy where you can earn up to 27,000 miles.
Here are the details:
When you sign up for their 24 month plan, you will get 15,000 bonus points after you sign up and 500 points per month for the 24 month which will equal a total of 27,000 miles. The plan is a 2 year contract and if you cancel before, you will incur a penalty. I believe you have to be a new Reliant customer as well.
I have this plan but with AA and it has been great so far. Many have argued that the price per kw is higher but I have found it to be comparable to a regular plan. If you are unsure then call the number and ask what the rate differences are to see if this will work for you. The transition is seamless with your current electricity provider. Everyone needs electricity, why not earn miles in the process!