Thousands of passengers remained in limbo on Saturday, and airlines warned that severe disruption would last for days as they scrambled to relocate planes and crews and get travellers to their destinations.
Friday's travel mayhem raised concerns about Britain's ability to withstand disasters or attacks on critical infrastructure.
Inconvenienced passengers, angry airlines and concerned politicians all want answers about how one seemingly accidental fire could shut down Europe's busiest air hub.
"This is a huge embarrassment for Heathrow Airport," said Toby Harris, a Labour Party politician.
Energy Secretary Ed Miliband said he'd asked the National Energy System Operator, which oversees UK gas and electricity networks, to urgently investigate the fire: "to understand any wider lessons to be learned on energy resilience for critical national infrastructure".
It is expected to report initial findings within six weeks.
"The government is determined to do everything it can to prevent a repeat of what happened at Heathrow," Miliband said.
Heathrow said it had "hundreds of additional colleagues on hand in our terminals and we have added flights to today's schedule to facilitate an extra 10,000 passengers".
It advised passengers to check with their airline before going to the airport.
British Airways, Heathrow's biggest airline, said it expected to operate about 85 per cent of its 600 scheduled flights at the airport on Saturday.
It said that "to recover an operation of our size after such a significant incident is extremely complex."
While many passengers managed to resume stalled journeys, others remained in limbo.
Laura Fritschie from Kansas City was on vacation with her family in Ireland when she learned that her father had died. On Saturday she was stranded at Heathrow after her BA flight to Chicago was cancelled at the last minute.
"I'm very frustrated," she said.
"This was my first big vacation with my kids since my husband died, and ... now this. So I just want to go home."
More than 1300 flights were cancelled and some 200,000 people were stranded on Friday after an overnight fire at a substation 3.2 kilometres away from the airport cut power to Heathrow, and to more than 60,000 properties.
Residents in west London described hearing a large explosion and then seeing a fireball and clouds of smoke when the blaze ripped through the substation.
The fire was brought under control after seven hours, but the airport was shut for almost 18.
Police said they do not consider the fire suspicious, and the London Fire Brigade said its investigation would focus on the electrical distribution equipment at the substation.
Heathrow chief executive Thomas Woldbye said he was "proud" of the way airport and airline staff had responded.
"Remember, the situation was not created at Heathrow Airport," he told the BBC.
"The airport didn't shut for days. We shut for hours."
He said Heathrow's backup power supply, designed for emergencies, worked as expected, but it wasn't enough to run the whole airport, which uses as much energy as a small city.
"That's how most airports operate," said Woldbye, who insisted "the same would happen in other airports" faced with a similar blaze.
Willie Walsh, who heads aviation trade organization IATA, said the episode "begs some serious questions."
"How is it that critical infrastructure – of national and global importance – is totally dependent on a single power source without an alternative? If that is the case, as it seems, then it is a clear planning failure by the airport," he said.
Walsh said: "Heathrow has very little incentive to improve" because airlines, not the airport, have to pay the cost of looking after disrupted passengers.
Heathrow is one of the world's busiest airports for international travel and saw 83.9 million passengers last year.
