Fire crews and electricity experts from SP Energy Networks scrambled to the scene at Home Bargains in Willow Street, Oswestry, in the early hours of Monday.
Max Smith, an energy engineer in charge, said: "It was a slow leak that had gone unnoticed for some time. When the height of the water got up to a point, it caused an explosion."
Mr Smith, who lives in Gobowen and was on the scene within 10 minutes, said at one point up to 1,200 power customers in Oswestry lost their power.
"We had all bar 18 back on supply in one hour and 20 minutes following the incident at around 4am."
After two Shropshire Fire & Rescue Service crews had done their bit, a digger team was on the scene as the energy suppliers worked furiously to find and cut affected cables.
Some 18 businesses in Bailey Street were expected to be off supply until 2pm or 3pm until their part of the network can be reset.
But Mr Smith said a lot of damage had been caused to the Home Bargains store's own internal system. It means that the store could be out of action for a while longer.
Some of the shops on Bailey Street had signs up to tell customers that they were closed.
But the lack of power did not deter family jewellers RJ Christian, where Miss June Williams placed a big candle on the counter and was determined to continue.
In a separate incident, a crash on the A5 just outside Oswestry town centre caused tailbacks in rush hour.
The accident happened between the Queens Head crossroads and the British Ironwork Centre at Whitehall at around 7.30am.