ELECTRIC POWER is second only to the water supply as a necessity of urban life. The rotating blackouts that the utilities were forced to impose last Thursday were the first in this area in 21 years, and the reason was the growing power shortage. To cut the load, two power companies shut off current to some 210,000 customers in the Washington and Baltimore areas, in most cases for 15 minutes or so. A brief interruption like that is not the end of the world. But the blackouts demonstrated that, in this essential commodity, delivery is becoming less reliable.

Public policy and the regulators have to take some of the responsibility for this deterioration. No doubt some of it lies with the utilities, and there was a lot of pure bad luck. Baltimore Gas and Electric's two big reactors at Calvert Cliffs have been down for repairs and modifications for more than a year. One of Pepco's big conventional generating plants at Chalk Point was out of service with a malfunction, and in midafternoon, with the temperature at 100 degrees in Washington, another failed. But this region has come to take its power supply for granted, and the regulators, particularly in the District of Columbia, have not given enough consideration to it. They have worked hard to keep rates down and to be responsive to people trying to keep generating plants and power lines out of their neighborhoods. That's important, but it's not enough. There have been plenty of warnings of a rising shortage, and the public agencies haven't paid sufficient attention.

Demands for power have been growing faster than supply, and Thursday's experience shows that there's no longer an adequate margin of safety. If both supply and demand stay on their present tracks, these episodes will get more frequent and more serious. There will be a measure of relief when the two Calvert Cliffs reactors come back into service -- one is scheduled to begin operating again in late summer, the other at the end of the year -- and several coal and gas units are under construction. But demand for electricity in this area is now rising much faster than the utilities, or anyone else, expected.

To bring the system back into balance, either consumers must do a better job of conservation or the utilities will have to build generating capacity faster. The Thursday blackouts were, for most customers, no more than a minor nuisance. But you wouldn't want to see blackouts become a regular practice on hot afternoons, and you certainly wouldn't want to see those blackouts lengthen.