While I agree that the applicable statute of limitations is 6 years, it should only limit you to recovering the electric charges assessed more than 6 years back. If these charges are indeed wrongfully made (that is, the lease says nothing about it (and you might want to double check this)) then you should be able to not have to pay them in your new lease and potentially recover the amount you paid for the past 6 years.
So if this common area electric assessment is not listed in the lease, you may have a good claim based on ORS 90.315(2).. And check out the following subsection (3) which provides for double damages if the failure to disclose was made "knowingly." And I would ask for interest, and costs and attorney fees (if any) as well. But first have somebody familiar with leases take a look at yours to make sure the common area charge is not buried in there somewhere.