Nikola Tesla's alternating current technology could transmit power efficiently over great distances, eliminating the need for a power station every mile or so down the road. The 1895 debut of Tesla's George Westinghouse-financed Niagara Falls alternating current hydroelectric plant irrevocably demonstrated AC power's practical superiority, and every major consumer electrical utility initiative since has been based around an alternating current infrastructure.
That doesn't mean DC power went away overnight. There were still many vested interests in the DC power industry, and many customers already had DC power appliances and lighting they weren't eager to replace. Thus, many New York residences and businesses remained on DC power decades after AC power became the norm. But the actual longevity of consumer DC power in New York is still startling.
In what year did the last of New York's DC electrical power customers to convert to AC service?
The last of the Consolidated Edison (the New York city electrical utility) DC power lines was severed and replaced in 2007 -- over a century after alternating current became the prevailing electrical technology. Moreover, there are still a number of DC power systems in place in New York -- the NYC Subway's third-rail power supply chief among them.
Con-Ed didn't begin the project to finally deprecate all DC consumer power transmission until 1998, and it took nine years to finally convert all the DC lines to AC. In many cases, those DC customers still have DC lighting and power inside their homes and shops, but Con-Ed converts the AC line-power to DC with an on-premise rectifier. In the early 20th century, many taller building were designed with DC elevator systems that are still in service. Several hotels were designed with DC power stations in their basements to run their lights and appliances.
One of the major drawbacks of DC power --besides an inability to efficient transmit power over long distances -- was a lack of easy voltage transformation. Phased AC power lines can run to a transformer that steps down 440-volt current to 220-volt appliance-grade power, and then your local panel box can step that down to to the 110-volt lighting and outlet current. DC installations required wholly separate lines from the power plant to deliver different voltage currents, and retrofitting buildings designed around these DC limitations is prohibitively expensive. Thus, New York has a startling number of buildings with DC-powered elevators and and central light systems running alongside later-installed AC electrical systems.
Thus, while the last New York DC power line was cut in 2007, DC power is alive and well in the Big Apple. That's not just some long-tenured tech, it's an immune to obsolescence example of the Truly Trivial.