When vacationers at one hotel in Rehoboth have had enough of the beach for the day, head back to their rooms for a shower and then go to the resort's rooftop restaurant, they will be using the sun not once but three times.

The first time, it will be for a tan, like thousands of others visiting Washington's closest ocean beach. But for anyone staying at the Henlopen Hotel, located at the north end of the boardwalk, the sun also heats water for the bathrooms, linen laundry and Horizon Room restaurant dishwasher.

The $105,000 system, installed by Cra-Sol Enterprises Inc. of Wilmington, Del., just in time for this season, is an attempt to deal with skyrocketing energy costs. The hotel's electric bill, including water heating, had climbed so high that the restaurant was forced to close four years ago. Switching from electric to gas stoves helped, and so did installing a computerized energy-monitoring system last year, but the big factor in keeping consumption down is the solar system.

The first monthly electric bill issued to the hotel since the system started in late May was for $7,412. That may sound like a lot, but the important figure for the 90-room hotel was the 70,500-kilowatt-hour tally, down more than two-fifths from the whirling-meter days of five years ago and down more than 8 percent from the same month last year when occupancy was lower and the restaurant was closed.

So far, the hotel's bill has not actually been cut, however. When Sara Brownlowe, the hotel's manager, compared the first bill with the one for the same month in the 1981 season, she found it was $1,900 higher -- but she said it would have been much worse without the solar system. David Cubitt, the maintenance supervisor, pointed out that the first three weeks after Memorial Day, just as the system was being put into use, were poor ones for solar energy.

"We had a bad stretch of weather when we put it in," he said. "We had only 16 hours of sunshine in 21 days." Even so, Cubitt did not wait for the storage tank to heat up before putting the system to work. "We're gaining a little bit every day," he noted.

The system consists of 63 plastic-covered collectors, each measuring 4-feet-by-10-feet, with black copper pipes located at the bottom of shallow V-shaped aluminum troughs that increase the efficiency of the panels.

Ordinary water is used in the collectors but kept separate from the city water that flows through the taps. There is no problem with freezing--the bane of some plain-water solar systems--because the hotel is closed from mid-October until April 1.

Solar-heated water is drawn from the 12,000-gallon storage tank, which rises next to the hotel like a two-story aluminum-sided silo, and sent through a counter-flow heat exchanger to warm the incoming city water from about 68 degrees to 98 degrees or more on a good day. This means that the expensive electric-resistance backup heater needs to add only another 20 to 30 degrees, which should effectively cut the bill in half.

Installing the collectors on the hotel, built in the early 1970s, was fairly simple for a retrofit job. The collector racks were bolted to the building's superstructure. The panels were hoisted to the roof by two men on top of the hotel and three on the ground, one with guywires "so the collectors didn't go into the building," explained Cra-Sol President Robert L. Craven.

A construction crane set the storage tank on its concrete pad, and the plumbing was run through the laundry chutes, which are in the same place on each floor. Only a short distance of piping is visible on the second and eighth floors.

However, before construction could take place, the hotel owners found themselves facing an unusual wrinkle in the question of solar-access rights: a neighbor's complaint that the collectors were shading one of the suites in the Henlopen condominium, which is part of the building complex but operated independently.

One condo owner tried to fight the variance the hotel sought for the collectors, claiming that the panels themselves would cast a shadow on his windows, cut the sunlight entering them in winter and raise his heating bill. Cubitt recounted how he had to go up on the roof on a January day, when the wind-chill factor lowered the temperature to 37 degrees below, to track the shadow from a hand-held pole representing the collectors.

"It never hit the window, only the balcony, only for four minutes. And his drapes were closed, he would have no heat gain anyway," Cubitt said. But just to be sure, the plans were changed to move the collectors back 10 feet, and the variance was granted.

In addition to the solar system, the Henlopen employs a load-leveling device that prevents the building's instantaneous peak demand from rising above 300 kilowatts and sending the electric bill higher. It works by selectively shutting off the water heaters on individual floors until the peak demand falls back to a safe distance from that mark. Cubitt and Brownlowe point out that the demand charge is already 30 percent greater than the usage bill -- but it could be much worse.

In addition, each floor has an "Energy Cruncher" duty-cycling device that cuts off air conditioners for nine minutes out of every 15. Together, the solar system and the two electronic energy watchers have managed to keep peak demand at no worse than about 267 kilowatts, compared with 590 kilowatts.

"We're doing all this because the power company [Delmarva Power and Light Co.] is putting the squeeze on us," Cubitt said. Pointing to the load leveler, he added, "We live and die on this thing right here."