Wishaw General in North Lanarkshire experienced the power cut between 08:45 and 16:00.
It meant one emergency patient was redirected to Hairmyres Hospital. NHS Lanarkshire said IT systems were also affected.
The hospital was powered by back-up generators until mains power was restored.
NHS Lanarkshire chief executive Calum Campbell said: "In the interest of patient safety and as a precautionary measure, we took the decision to reschedule a small number of surgical procedures based on clinical priority.
"The power interruption impacted on our diagnostic and IT systems, which have now been fully restored.
"As a result, some outpatient clinics have been affected and one emergency patient was redirected to Hairmyres Hospital.
"This is in line with our business and emergency continuity plans which worked to good effect."
He added: "All patients affected by the interruption have been rescheduled and we would like to apologise for any inconvenience this will have caused."

Paterson, 59, who exaggerated or invented cancer risks in patients, was convicted of 17 counts of wounding with intent last month.
Health Secretary Jeremy Hunt said his malpractice was "profoundly shocking".
Baroness Lorely Burt, former Lib Dem MP for Solihull, called for any inquiry to be held in public.
Paterson, from Altrincham, Greater Manchester, was convicted on 28 April following a nine-week trial which found he intentionally and unlawfully wounded 10 patients.
He is due to be sentenced this month.
Mr Hunt said Paterson "totally neglected" his duty of care for the patients he treated at two privately run West Midlands hospitals.
"As a result I have agreed that, if returned to government, we will hold a comprehensive and focused inquiry to ensure that any lessons are learnt in the interests of ensuring patients are protected in future.
"We will take any testimony from those affected, their families, and others who may wish to come forward."
Baroness Burt said: "I'm sure the patients harmed by Paterson are all very grateful that the health secretary has announced an inquiry into the case, after they battled alone for years to get justice.
"It's just a shame he never intervened on behalf of the private patients who had to go it alone through obstacle after obstacle."
She said she hoped it would reveal why Paterson was allowed to operate for so long with impunity, but called for the inquiry to be held in public so a "bright light be shone into the way the NHS and private practice is being run which allows such things to happen".
Shadow health secretary Jonathan Ashworth said Labour would support an inquiry into Paterson's "reprehensible actions".
"Not only have his actions cost the public purse dearly, but they have devastated the lives of innocent people who deserved better," he said.
"This unacceptable practice must never be repeated again."
Victims have claimed Paterson had a "God complex" and jurors were told he performed "extensive, life-changing operations for no medically-justifiable reason".
Ian Paterson: Surgeon was 'psychopathic like Shipman'
Patient 'violated' by breast surgery
The Heart of England NHS Trust has paid out almost Â£18m in damages and legal costs to hundreds of his former patients. These cases did not form part of the criminal proceedings.
An independent report into his work in 2013, by lawyer Sir Ian Kennedy, found concerns dated back to 2003 but were not dealt with for four years.
Later this year, 350 private patients who had unnecessary operations will seek compensation at the High Court, solicitors have said.
Some former patients have been calling for a public inquiry into what he did and complained this week that they have been "airbrushed" out.
1,207
patients given a mastectomy, including "cleavage-sparing" operations
675 mastectomy patients have since died
68 of his surviving patients have seen their cancer return
A hospital was forced to reschedule operations after losing power for more than seven hours.
[NEXT_CONCEPT]
The Conservatives have pledged to hold an inquiry into the actions of disgraced breast surgeon Ian Paterson, if the party is returned to power.