Its most ferocious critics call it Harvard's half-billion dollar mistake, Milder types ask whether Harvard should be in the energy business. Some argue it is necessary, sensible and will pay for itself in time.

The Medical Area Total Energy Project (MATEP) has always been somewhat controversial. In the 1960s and '70s when it was conceived, critics argued that the power plant, which provides energy for Harvard's teaching hospitals, was expensive and inefficient. In the 1980's the Massachusetts Department of Environmental Protection held long, tortuous hearings over whether Harvard should be allowed to set up an array of diesel electricity generators in the Longwood Medical Area.

Finally, nine years ago, the generators went on-line, and MATEP's goal--that of providing a number of utilities simultaneously and using the waste from one process as fuel for the next, dubbed "cogeneration"--was fulfilled.

The MATEP plant, which has 105 employees and runs 24 hours a day, supplies 95 percent of the electricity for the Medical School, the School of Public Health, the Dana-Farber Cancer Institute and four of Harvard's teaching hospitals--Beth Israel, Children's the Deaconess and Brigham & Women's.

The plant charges the same rates as Boston Edison, which also runs power lines to the hospitals, so that if MATEP were to fall, electrical power would not be interrupted.

Usually hospitals and other large facilities which require stable power sources build their own generators and plants.

But because there are a half-dozen hospitals, and several schools in the Longwood area that are all Harvard-affiliated, Harvard decided to build a single large plant rather than create many smaller ones.

MATEP is the successor to the Harvard Powerhouse, which was built in 1906 but failed to meet the demands for chilled water, steam and electricity required by Harvard's hospitals, which were expanding in number and size.

It is hard to determine how much MATEP actually costs--estimates range from $300 million from the plant's own managers to $500 million from its critics--since the project was paid for with educational and health bonds.

Today MATEP, which is housed in a brick and glass building that blends in with the Longwood urban landscape, is worth about $200 million, according to one of its administrators.

The plant has two steam turbine generators, three boilers, six diesel generators, seven chillers and seven cooling towers.

It can generate enough electricity to run a good-sized town.

There are other power plants in Boston, mostnotably Boston Edison's, but none which produce"total energy" -- chilled water, steam andelectricity--at the same time.

MATEP's cogeneration, principally its recaptureof hot exhaust form the diesel engines, allows itto produce 30 percent more energy perequivalentgallon of oil than normal electric power plants.

Nationally, the Mayo Clinic in Minnesota has asimilar plant, although it is smaller thanHarvard's .

MATEP itself is a very profitable endeavor. Ittakes in about $48 million a year from its ratepayers, about $10 million of which constituteprofits, a MATEP official says.