A woman has spoken of her surprise when her smart meter quoted thousands of pounds for a day's usage of gas and electricity, due to a system error.
Jane Allen was one of many confused customers who posted the strange readings from their SSE smart meters on social media.
One customer's display showed more than £30,000 for a single day.
SSE apologised and said no customers would be charged "the extra amounts resulting from errors".
Smart meters send information on energy usage back to the supplier.
They let the customer know how much electricity or gas they are consuming each day - and how much it's costing them - in real time.
But for some customers, the readings have been somewhat higher than usual over the past week.
Jane Allen, from Portsmouth, has had her smart meter for around five months. But on Wednesday, she started to notice the strange activity.
"I was quite frightened to be honest - thinking it could be a gas leak," she said.
On Sunday her meter stated she was on £36,448.29 for the week - far beyond the daily £3.80 her family usually use.
"My worry is that they are going to try and get that sort of money out of my bank.
Ms Allen called SSE who assured her "not to worry".
"But I thought by now it would be working again," she added.
Usman Hussain, whose meter suggested he had used £9,576.98 of energy on Friday, wrote on Twitter: "Think my SSE Smart Meter for energy and gas may be having slight problems! Either that [or] a neighbour has started nicking my leccy or gas."
For the reading to be accurate, Mr Hussain would need a home bigger than Buckingham Palace - which, with 775 rooms, was billed £1m for a year.
The average annual bill for a large house in the UK is £1,486.
Analysis, by BBC business correspondent Joe Lynam
Smart meters are meant to provide us with far more accurate bills. So imagine the surprise of some SSE customers whose smart meters calculated that they had run up bills of thousands of pounds in a single day.
Five million so-called 'smart meters' have already been installed in British homes using a completely bespoke infrastructure at a cost of £11bn.
This SSE issue will be especially embarrassing for Smart Meter GB which is the body charged with promoting the roll out.
It's spending millions on advertising which mocks dressmakers and doctors guessing measurements and ailments respectively.
A spokesman for SSE told the BBC: "SSE is aware of the issues affecting a small number of our smart meters.
"The issue will be investigated as a matter of urgency and no customer will be charged the extra amounts resulting from errors with the smart meters.
"We would like to apologise to any customers if this has caused distress."
The government wants every home and business to be offered a smart meter by the end of 2020.
A spokesperson for the department for business, energy and industrial strategy, said: "Smart meters are a vital upgrade to Britain's energy system.
"The technology will bring an end to estimated billing, and give consumers real-time information about their energy use to enable them to make more efficient energy choices."