Electric Company trying to back charge me two years for their mistake ($20k)

Electric Company trying to back charge me two years for their mistake ($20k): I purchased a place about 2.5 years ago now. When I purchased the property I was told from the previous owner that the electric rate was on a special grandfathered rate that was very cheap. It was an old pants factory that he had owned for about 25 years, and in the last 5 years or so turned into a living space. I was even shown a bill that showed just how low his bill was.
With these great rates, I purchased the house. My bills that I got from the electric company backed up what the previous owner told me and my bills were very low. Because of this, I added a 5-ton air conditioning unit for upstairs, and when the winter months came also switched all of my heating over to electric heat coils in my forced air handlers. Both of those together was about a $6k investment.
My bills continued to read extremely low. Because of this, I continued to run my heat at about 76 degrees non stop in the winter, and my air conditioner non stop in the summer to around 70. So that means they were working pretty much all the time. Again... my bill stayed very low over those two years, so I am thinking everything is great.
I even had someone from the electric company stop by to look at my place when they had a question if my place was a residential or commercial building. After determining it was residential everything was fine.
My bills still continued to be great and at the same I had been receiving.
In April... I was visited by an electric company worker who said that they had changed out my meter under routine 10 year change.
They said at that time they noticed that they had not been reading my meter correctly. There should have been a 40x multiplier applied to my reading each month. This means that I was only being billed for 1/40th of my bill.
I was called later that week to be told that I was going to be back charged for the actual usage for the past 2 years, totaling just over $20k!!

I am the only one that is being back charged. The previous owner is not. The previous owner who was paying the same rate that I was for at least the past 10 years, and likely the last 25 years that he owned the building.

My question is, are they allowed to do this? It is now pressuring me to have to sell my home as I can not afford to pay this electric rate. Not to mention I had to pay again to have my heat transfered back to gas heat. Plus I now have a pending $20k bill.
I already have an informal complaint into the PA utilities commission, but it has been over 2 and a half months with no results, or them even contacting me to hear my case.

A realtor that is now helping me sell the house stated that I might have a claim to sue the electric company as well as the former owner. The electric company because I am the only one they are charging, and they are charging me for their mistake. She thought I might be able to sue to also force them to keep my rate at the discounted (1/40th) rate as that is what I purchased the house under. Also going after the previous owner for showing that the bill was as low as it was, and me purchasing the house under that knowledge, and it now not actually being the case.

I would be very interested to hear any advice or information any one could give me. I don't even know what type of lawyer would handle a situation like this. - Is this your question? Add additional information