Peco Energy Co. crossed the wrong customer when it dismissed Mari Jensen's complaint two years ago.

"I'm told I am like a dog with a bone," she said.

Jensen lives in Concord Township, Delaware County, with her husband, Arthur. They spent about $30,000 in 2010 to install a rooftop solar system on their Sleighride Road ranch house.

For the last 19 months, the Jensens have fought a surreal David-and-Goliath battle with Peco over the formula the utility uses to pay them and other owners of solar systems for the power they produce and sell to the grid.

"It was absurd," said Mari Jensen, a retired school bus driver. "The more they kept fighting it, the angrier I got."

On Thursday, a unanimous Pennsylvania Public Utility Commission agreed with the Jensens and ordered Peco to recalculate its payments over the last five years to solar generators, who are known as net-metering customers.

Because of the Jensens' action, 540 solar customers will receive additional compensation of $5,360, including interest of $1,444, Peco spokesman Ben Armstrong said.

And the Jensens, whose dogged perseverance drove the case?

Their share amounts to $1.42. Plus interest. That won't even cover half the postage on one of the many certified letters they sent to the PUC.

"It wasn't about the money," Mari Jensen said in her kitchen Friday, as she paged through a pile of paperwork from the case. "It was the principle."

Armstrong, the Peco spokesman, said the utility intended to comply with the PUC's ruling. "We were unaware that we were not billing/compensating customers appropriately before this issue was raised," he said.

Though the dollar amounts are small, the issue is not as trivial as it may seem. Utilities such as Peco are facing a complex new world in which more customers are producing their own power from renewable sources - Peco now has 2,321 customer-generators, about 10 times as many as it had five years ago. Utilities increasingly will have to manage two-way transactions and energy flows with their customers.

The Jensens' saga began when they got their first payment in 2011 for excess power produced from their new solar system.

Peco is required to pay net-metering customers the same price for which it sells power. The utility's price varies each quarter, but it tends to be 9 to 10 cents per kilowatt hour. From 2008 to 2012, it paid almost $196,000.

Peco based its payments on the average price of power over the previous year. But the Jensens dug deep into the PUC's regulations and found that Peco was supposed to make the payments based on a weighted average price. A weighted price would reflect the rates in effect during the months when the excess power was actually delivered to the grid.

But Peco denied it needed to use the more complicated formula, which would require a separate calculation for each customer based on his or her unique energy production.

The utility initially offered to give the Jensens a small payment to make the matter go away, but the Jensens declined.

"The whole thing," Mari Jensen said, "has been about making Peco follow the law."

She took her grievance to the PUC, whose staff agreed that she seemed to be interpreting the regulations correctly. But they told her she needed to file a formal complaint.

What she didn't realize was that by doing so, she was launching a legal proceeding in which the Jensens would act as their own attorney through the PUC's arcane process. And Peco opposed them at every turn.

"How much did they pay a lawyer to put this together?" Arthur Jensen, a refinery engineer, asked as he hoisted a half-inch-thick filing submitted by Peco's legal department.

The PUC's archives are littered with the dismissed cases of thousands of aggrieved utility customers, and the Jensens' case was nearly undone last year when she neglected to sign a filing. By the time she resubmitted it, a deadline had passed. Administrative Law Judge Christopher P. Pell dismissed the entire case because of the "untimely" filing.

But Jensen persisted, and asked the PUC to reinstate the case. In December, the commission ruled that it was "clear" the regulations required utilities to use the weighted average price - that the Jensens had gotten it right.

The case was not over, though. Peco asked for six months to calculate the refunds. It also said it would cost $500,000 to devise an automated payment system for net-metering customers, which triggered a new round of exasperated indignation from the Jensens, who ran the calculations using a conventional spreadsheet.

The Jensens are aghast at the hours they, the PUC, and Peco spent adjudicating something they thought was obvious.

"I cannot have imagined," she wrote the PUC, "the extent to which this simple request has been blown out of proportion."