Stagg has lived in her two-story rowhouse on the western edge of Center City since 1980 - "I was a pioneer," she says - and had a very good feel for her heating costs.
This bill didn't feel right.
As director of institutional research for Penn's Wharton School, she knows numbers. So it didn't look right either.
"I'm a single person without the responsibility of a household. People could lose their houses over this," she tells me in her office at Penn.
"I'm not disputing that I may owe something, but no one has an idea of when the meter started failing." Actually, I learned PGW does.
Diminutive and intense, Stagg prepared a bushel full of questions, a few of which I selected to keep the focus as tight as possible.
First was how is it possible she was billed for only half the gas she used? Second was more important: If she was being billed for the missing half of her usage over four years, and the total she paid in that period was about $3,000, how could she be billed for almost $12,000, which is four times that amount? And why was she billed $177.42 for late charges when she was just now notified of the back charges?
I wanted to know, given modern technology, how PGW could be so far off with its billing, and for so long.
The answers are twofold - and both involve human error.
When a "new" meter was installed in Stagg's home in 1996, explains PGW vice president Doug Oliver, the installer mistakenly set the meter at half speed instead of full speed. She got the right amount of gas, but PGW was aware of only half of it.
So her billing was wrong since 1996, but the error wasn't discovered until a new meter was installed in May.
The original error, Oliver says, was the result of "a PGW programing error" in the early '90s that affected fewer than 35 of PGW's 515,000 meters.
Meters installed since 2000 make that error impossible, he says.
As to the astonishing amount demanded from Stagg, although the numbers were right, Oliver says, "the letter was inaccurate.
"When you look at the letter, it's scary," he says.
The letter says "the re-bill represents the usage that you were underbilled over the last 48 months and the usage on the new metering equipment. The total bill is $11,828.18 for this time period. We have reviewed this bill carefully and can assure you it is correct."
It was not correct. At least its English was not correct.
The actual amount Stagg owes is about $2,200 and someone from PGW should have called Stagg with that information by the time you read this.
Oliver says PGW can't simply write off the debt because that money would have to be paid by other PGW customers and that wouldn't be right. Future letters will be written more clearly, he says.
Under usual procedure, customers who are being back-billed for four years get four years to get caught up, but "we are adjusting our policy to allow six years for our customers" to get even, Oliver says.
The errant late charge, will that be removed? "I imagine it will be," Oliver says, adding "nobody should have to feel the way she feels, through no fault of her own."
On Twitter: @ StuBykofsky