Dunedin Hospital is urging people not to phone or visit on Wednesday morning unless it's an emergency while its backup power supply is tested.
Mains power will be off between 7am-10am while the hospital runs off an emergency generator.
The aim is to ensure all essential services continue during a power cut.
The Otago District Health Board says the exercise will limit the use of lifts, telephones and computers.
Inpatient clinical services and emergency services will still operate.