Parts of Ninewells Hospital were plunged into darkness on Thursday after a power cut.
Patients said the power went off in the hospital at around 5am and again later in the morning.
Some areas had no lighting and lifts were also reportedly out of use for a period.
One patient said torches were handed out.
NHS Tayside says there was some disruption to services on Thursday while the issue was fixed.
A spokeswoman confirmed the backup generators started as required but power was unable to connect to the electrical system.
Patient says power at Ninewells went off twice
Firefighters were also called out briefly due to the power cut causing an alarm to go off.
David Alexander – who was staying at the hospital before being discharged on Thursday – said: “There was one power cut just after 5am until 6.45am.
“Then it went off from the back of 7am until just before 9am.
“Lifts were inactive from the back of 10am as they were trying to sort the problem and wards were supplied with torches.
“A head nurse said something like this last happened around 23 years ago.”
The fire service confirmed it had attended an alarm activation at 5.24am but had no further involvement.
A spokeswoman for NHS Tayside said: “There was a brief electrical outage in the early hours of this morning which affected areas in the south block at Ninewells Hospital.
“Power was restored with a temporary solution put in place at that time.
“A permanent upgrade to an electrical unit was undertaken this morning and this has ensured all appropriate care to inpatients and no impact on admissions today.
‘Some disruption to small number of services’ after Ninewells power cut
“There was some disruption to a small number of outpatient services at Ninewells today while the upgrade was carried out, however, staff have managed this with individual patients affected.
“For unplanned outages such as the one which happened in the early hours, there are service-level business continuity plans which our staff invoke as necessary to ensure patient safety.
“A small number of areas did invoke their contingency plans for a short period with staff responding quickly and professionally.
“As the power outage was resolved, staff have returned to normal business with no ongoing impact.
“We would like to thank all staff for their response which demonstrated their professionalism and dedication to their patients.”