The private hospital where the intensive care patients were being treated reportedly suffered the power outage late Thursday after days of severe flooding that has already killed hundreds and submerged much of the state capital Chennai.
Tamil Nadu health secretary J Radhakrishnan confirmed the deaths at the Madras Institute of Orthopaedics and Traumatology (MIOT), but did not comment on the cause.
“At least 18 patients admitted to the MIOT hospital died,” he told reporters.
“I have asked health officials to conduct an enquiry into the matter and report back.”
Hospital officials could be reached for comment.
The Hindu newspaper quoted relatives of victims saying hospital equipment failed because of power failures.
Broadcaster NDTV said the hospital had been flooded since Tuesday, after some of the worst rains in decades left much of the city under water.
It said the hospital had sent out an SOS message Tuesday to say it was running out of food and water.
Thousands of rescuers are racing to evacuate victims of the Tamil Nadu flooding, which has claimed nearly 300 lives since November 9.
Weather officials said rainfall in Chennai had diminished since earlier in the week, but parts of the city of 4.6 million people remained submerged.
Thousands of stranded passengers had to be evacuated this week from Chennai’s international airport, whose runway was completely submerged.
Experts at the Centre for Science and Environment (CSE) in New Delhi said poor urban planning had likely exacerbated the disaster.
“We have forgotten the art of drainage. We only see land for buildings, not for water,” said CSE director general Sunita Narain.