LEGISLATION AS COMPLEX and voluminous as the administration's energy bill is certain to have unexpected effects. One cautionary example came to light this month after the Clean Air Act was passed. Taken together, these two pieces of legislation have an interesting joint effect. The Clean Air Act requires utilities to build scrubbers into coal-fired generating plants and to cleanse the exhaust gases on their way out into the atmosphere. It's an altogether laudable purpose but, like most kinds of environmental protection, it's expensive. It raises the cost of generating electricity with coal.

One major purpose of the energy bill is, of course, to discourage utilities from using imported oil and, instead, push them toward greater use of coal. But coal-fired generation is already more expensive than nuclear power and adding the scrubber requirement widens that differential even further. Instead of pushing toward greater use of coal, economics now pushes toward greater use of nuclear reactors.

But a great cloud of uncertainties surrounds nuclear power. Utilities do not know what the future licensing requirements are going to be. The Carter administration wants to revise them. But that bill is still in the talking stage, and enactment is a long way off. For the utilities, the effect is to create a powerful incentive to postpone decisions on the giant base-load plants. Demand for electricity has been rising rapidly this year. What if the power companies get caught short, with inadequate generating capacity?

The only quick fix is a light turbine generator that runs on heating oil. The actual effect of all the current legislation may well be to increase the amount of oil burned to generate electricity in the early 1980s. Once again the snake has bitten its own tail. The tail is a very long one and, as the snake has rather plaintively pointed out on previous occasions, keeping track of it is exremely difficult. But the bite is painful and, as the doctors say, if neglected could be dangerous.

A real dilemma confronts Congress. It knows that it is trying to do too much at once, but there is no way to break the subject into pieces. To enact the energy bill with its emphasis on coal would be intolerable without the protection of the Clean Air Act. As for the nuclear-licensing procedures, uncertainty over future rules is no doubt better than trying to stick with the present ones and the endless paralyzing reviews that they invite.

The energy bill is now halfway through Congress, and it's likely to be passed before the end of the year. The nature of the process suggests that it will become more complicated, rather than less, as it moves through the Senate. Its full meaning will not be apparent until well after it has gone into effect. This year's bill will have to be followed every year or two by further energy bills - strengthening, refining and correcting - if the country is to come out anywhere near the goals that President Carter has set for 1985.