Con Ed is guesstimating this month’s electricity usage while meter-reading workers remain locked out due to failed union negotiations — and customers tell The Post they’re being overcharged as a result.

“I opened up my mail and thought it was my rent bill,” fumed one angry Upper East Side resident, whose electricity charges jumped from $72 last month to $235 this billing cycle.

His bill — the first since Con Ed locked out rank-and-file workers during a contract negotiation impasse — includes a footnote: “We could not read your meter because of a work stoppage.”

“People can’t afford to put food on the table and Con Ed is guessing bills,” the man said.

Sonja Leobold, 72, bought a smaller air conditioner for her East Village apartment, hoping her bill for this summer would be lower than the $120 she was charged for the same month last year. Instead, she was charged $180 — nearly double last month’s $96.

Mike Ferguson, 50, is in a similar situation.

“I’d like to pay for what I use,” said Ferguson, who installed a more efficient window unit in his West Village apartment — but still saw his bill double compared to last year. “I’d like to have them read the meter and to pay what’s accurate.”

Con Ed locked nearly 8,500 workers out of their jobs on July 1 when contract negotiations with the union went sour.

Utility officials and union representatives met twice yesterday, but once again failed to reach an agreement.

A Con Ed spokesman said customers who think they’ve been overcharged should take a picture of their meters and e-mail it to customerservice@coned.com for further review.

pedro.oliveira@nypost.com