A gleaming new data center sits less than half a mile from the electric substation where a fire plunged Heathrow Airport into darkness last week. The data center’s own power was also cut that day. But no one who relied on it would have noticed, thanks to a bank of batteries and backup generators designed to kick in instantly.

Meanwhile it took officials at Europe’s busiest airport close to 18 hours to bring its terminals and runways back into operation, causing global travel delays and underscoring the vulnerability of Britain’s infrastructure.

It is a striking contrast that energy experts say can be explained largely by one word: Money.

“The data center industry is relatively young. They are more attuned to the cost of a catastrophic failure,” said Simon Gallagher, the managing director at UK Networks Services, which advises clients on the resilience of their electricity networks. He said most of the world’s airports — including Heathrow — have not been willing to make the big investments necessary to build total backup systems.

Even at an airport the size of Heathrow, which officials have described as equivalent in power use to a small city, it is possible to create backup systems robust enough to maintain normal operations during a catastrophic power failure, Mr. Gallagher and other engineering experts said.

But it could cost as much as $100 million and would likely take years to put in place. So far, most airports have chosen not to make the investment.

“It comes down to a cost-benefit analysis,” Mr. Gallagher said. “At the minute, there seems to be an assumption that it would cost too much.”

The Airport

Heathrow officials were quick to point out after Friday’s incident that the airport has backup power in place for its most critical systems: runway lights and the tower’s traffic control safety systems. If a plane had needed to land that day, it could have done so safely.

But the airport had no way to power the rest of the sprawling and complicated facility: the vast terminals, filled with shops and restaurants, moving walkways and escalators. Cut from the grid, there was no power to move bags to the claim area, or for ticket counters or bathrooms.

First opened at the end of World War II, Heathrow has been expanded and updated over the decades. The result has been a patchwork of older and newer electrical cables and systems carrying an ever-increasing demand for power.

“The grid is old,” said Najmedin Meshkati, an engineering professor at the University of Southern California. “For aviation, for the grid and for other safety critical systems, the older that they get the more important maintenance becomes.”

What Heathrow does not have are backup generators that could supply the 40 megawatts of power required at peak times to maintain normal operations.

Instead, on Friday, engineers at the airport had to manually reconfigure switches at another substation to temporarily reroute available power to Heathrow. That took hours, and because the airport’s systems had been sitting without power, it took even more time to boot them back up, followed by rounds of testing.

The Substation

The airport’s primary power source is the Hyde North substation about a mile away, owned and operated by National Grid Electricity Distribution, the private power company responsible for the area.

Two of the substation’s transformers were taken offline by the fire. The cause is still under investigation, but the police said Tuesday they had found “no evidence” of suspicious activity.

John Pettigrew, the chief executive of National Grid, told The Financial Times that there was “no lack of capacity” in the area after the fire. Energy experts said that is correct: The places where there is an actual lack of power tend to be developing countries and war zones.

The challenge, though, was making use of the area’s ample power once Heathrow’s connection to Hyde North was severed. Thomas Woldbye, the chief executive of the airport, told the BBC that he was proud of the employees who worked through Friday to switch their systems to use power from two nearby substations.

But he said that Heathrow would now assess whether to install “a different level of resilience if we cannot trust that the grid around us is working the way it should.” Heathrow did not respond to requests for comment for this story.

The Data Center

The airport’s leaders might want to examine their corporate neighbor just to the north.

The Union Park data facility, run by Ark Data Centres, is a six minute walk from the Hyde North substation. Inside, computers run 24 hours a day, powering the cloud services and artificial intelligence that are at the heart of modern banking, commerce, research and government operations.

Huw Owen, the company’s chief executive, said its electrical supply was interrupted when the fire broke out. But sophisticated sensors detected the loss of power and instantly shifted to batteries that operate much like an uninterruptible power supply system for a personal computer. That gave the facility’s generators time to spin up, and they soon took over.

“It’s a well-rehearsed, well-known process,” Mr. Owen said in an interview. “It’s this mind-set that resilience and keeping everything powered is absolutely front and center of our world.” Mr. Owen said the company installed the costly generator backup system despite expectations it might never be needed. A permitting application prepared for the company in December described the possibility of a power outage as “extremely rare.”

“It would require a catastrophic regional failure on the grid, or at the supplying power station, and would likely impact not only the site but the surrounding London area,” the summary notes. “As a result, the grid connection is considered to be highly reliable as demonstrated in the grid reliability letter provided with the application (calculated as 99.999605%).”

The Decision

Prime Minister Keir Starmer told the BBC after the fire, “I don’t want to see an airport as important as Heathrow going down in the way it did on Friday.”

But how to avoid it in future?

The challenge in making electrical upgrades to places like Heathrow is determining how to pay for it when high energy costs are straining consumer budgets. In the past, airport investment has often been passed on to customers in the form of higher ticket prices on airlines.

Mr. Gallagher, the consultant on electrical network resilience, noted that new airports in places like Dubai were built with the kind of backups that could keep terminals open. And a few older airports, like Schiphol in Amsterdam, have upgraded their facilities with large generators.

But if Heathrow’s management wants to follow suit, experts say, they will need to accept that it requires a large investment to prevent a crisis that may not happen again for many years.

“It’s a hell of a lot easier to build it from Day 1 than it is to try and retrofit stuff,” Mr. Owen said of Heathrow and other old airports. “They are as capable of instigating resilience at those sites as I am, but they’re now going to have to retrofit, whereas I built it from Day 1.”