Alit subway signal can indicate danger ahead, but yesterday it was the darkened signals that warned of one of the greatest dangers facing New York City. Signals went out along the 1, 2, and 3 lines, lights dimmed around Times Square and parts of Brooklyn, baggage conveyors stopped at La Guardia airport, and residents of Queens were asked to conserve electricity. The city’s grid should be able to function seamlessly even when the weather gets hot. That it didn’t even under relatively good conditions is a sign that serious trouble looms in the future.

Yesterday’s outages were not caused by a shortage of electricity. Con Ed has calculated there will be enough juice to meet the city’s demands this summer. The Independent System Operator, which monitors electricity usage statewide, reports that it successfully implemented conservation measures to reduce demand by about 600 megawatts. Rather, “isolated equipment” failures led to the disruptions. Stranded subway passengers might not care whether the problem was a lack of electricity or an unreliable way to deliver it, but at least the current troubles are, in theory at least, relatively easy to fix. That is, until you try to build a new sub-station and run into local community opposition. Nor has the mayor been aggressive enough either in promoting infrastructure improvements over the complaints of neighbors or in pressuring Con Ed, which is responsible for maintaining the city’s grid.

Fifty years ago, the city’s grid was the envy of the world for its reliability. Now, outages like yesterday’s are increasingly the norm, and one can’t walk a dog in Chelsea without worrying it will get electrocuted. City Hall has effectively given Con Ed a pass by not advocating alternative energy suppliers that would force Con Ed to compete on reliability.The mayor named a former Con Ed employee, Gilbert Quiniones, as head of the mayor’s energy task force and effective city energy tsar instead of bringing in an impartial outsider.

As serious as yesterday’s problems looked, the real danger lies in the power supply itself. There was enough of it yesterday, but come 2010 there won’t be. A report released earlier this year by the New York Building Congress added up projected population growth and economic expansion and subtracted aging generators that are expected to go offline in the next 20 years and realized the city will need a net gain of between 2,400 and 3,000 megawatts in generating capacity over the next two decades.The city needs to build a total of between 6,000 and 7,000 megawatts capacity.

The New York City Energy Policy Task Force convened by Mayor Bloomberg in 2003 reached much the same conclusion in its 2004 report. At that time, the task force estimated the city would need about 3,780 megawatts of new electricity sources just by 2008 — 665 megawatts to accommodate demand growth, 2,115 megawatts to replace possible plant retirements, and 1,000 megawatts of extra supply to reduce prices. A recent progress report from the task force has identified about 2,153 megawatts of new capacity now online or nearly so, including new generators in the city and new transmission lines to bring power in from elsewhere.

That’s less than it appears, however. While it looks like the city is making progress toward its 2008 projection, most of that capacity, including the 1,125 megawatts in new-plant capacity the mayor touted when the progress report was released, comes from plants that were already in the works when the task force formed. Future plants in the pipeline are few. In 2004, the task force identified a 1,100 megawatt Brooklyn waterfront plant proposed byTransGas Energy as an important possible source for meeting the 2008 need, but the Bloomberg administration has done everything it can to block the proposal, even after the plant’s developer proposed building the facility underground to minimize the aesthetic impact on the surrounding redevelopment the mayor is planning.

The mayor’s decision to block that proposal will come back to haunt his reputation.Not only has City Hall deprived New Yorkers of that capacity, but the ferocity with which Mr. Bloomberg and his economic development adviser, Daniel Doctoroff, have opposed the project threatens to scare off other potential investors in new generation projects.The whole industry has seen how the TransGas project has been treated, the company’s chief executive, Adam Victor, warns. “Michael Bloomberg’s legacy will be that he fiddled while New York City’s energy infrastructure collapsed,” Mr.Victor says, and he has a point.

Little more than three weeks have elapsed since the mayor heralded the task force’s report as good news that the city is making progress in solving its energy problem. The commission’s announcement that the city will have sufficient supply at least until 2012, a calculation based on a rosy interpretation of numbers that have led others, like NYISO and the Building Congress, to estimate a crunch by 2010, came as cold comfort to commuters stuck for an hour in steamy subway cars yesterday.