A woman who went on holiday leaving gas escaping from an illegally-fitted meter endangered homes nearby, a court heard.
The judge described Pauline Williams' crime as 'startling'
Widow and foster carer Pauline Anne Williams, 61, from Neath, was ordered to do 150 hours community service at Carmarthen Crown Court.
She had denied theft but was found guilty of stealing gas worth an estimated £3,600 by a jury.
The court heard neighbours noticed the smell of gas, nearly three weeks after she left for a holiday in South Africa.
An earlier hearing heard that Williams left the illegal meter in place at her home while she was abroad.
But the connecting nuts were not tightened and gas workers found the leaking meter after the smell was reported.
A locksmith gained access and the supply was cut off - at which point the illegal meter was found in a cupboard.
At the trial she claimed that she did not know how the illegal meter came to be in her house, and suggested an odd job man had put it in when he did some work.
Sentencing her to a community service order, Judge Gerald Price QC said it was "startling and bewildering behaviour" by someone who previously had an impeccable character.
She was ordered to pay prosecution costs of more than £1,200 and defence costs, which are to be calculated.
Francis Jones, defending, said that Williams had had a clean character up until the age of 60.
She had moved to the UK from South Africa with her husband and two children, and had bought the property to renovate and run as a bed and breakfast, but that he husband had died soon afterwards.
That plan changed when she found herself alone.
She had turned to full-time child fostering instead, which would now be at risk because of her criminal conviction.
Speaking after the hearing a spokeswoman for British Gas warned the theft could have potentially fatal consequences, with homes, offices and a school nearby.
"People tampering with gas meters are not only breaking the law - they are endangering the lives of themselves, their families and their neighbours through an explosion."
"The potential for a fatal accident doesn't bear thinking about. If someone had rung the doorbell we could have had an explosion which potentially could have taken out the neighbourhood.
"A big explosion would have certainly reached the school."
She said commercial and domestic gas fraud cost the industry £300m a year.