West Australian taxpayers are forking out almost $5,000 dollars a day in electricity costs to run the new Perth Children's Hospital before it has even opened its doors to patients.

And the cost will increase from next month when the first patients move in, with health officials predicting electricity consumption will jump by another 15 per cent once the hospital is fully commissioned.

Health Minister Roger Cook insisted the power costs were manageable and said the hospital's new location on the QEII Medical Centre site would deliver cost savings in other areas, including shared catering and laundry services.

"The costs associated with the hospital once it's up and running will be good costs to bear, because at least we will have the kids in the hospital and it will be open," he said.

The average daily electricity usage at the new and so far patient-free $1.2 billion hospital (47,772 kWh) is already more than one-and-a-half times as much as at Princess Margaret Hospital (29,163kWh) when it is full of patients.

Mr Cook defended the costs, saying it was a state of the art facility and the increased costs were to be expected.

"We know it's got a bigger footprint than Princess Margaret Hospital (PMH)," he said.

"It's got more adult lounge areas, it's got double the number of operating theatres and there will be costs associated with that," he said.

"But what we do know is that this will be a world-class facility.

"While there are some elements of extra cost associated with this aspect of it, we do have savings in other areas associated with its current site."

The Minister said the hospital incorporated other tenants, such as the Telethon Kids Institute, that were not currently housed within PMH.

Opposition predicts big jump in power use

While the first patients will be treated at the new hospital from mid-May, the remaining patients at PMH will not be transferred across until June when its emergency department opens.

The new hospital will accept the last patients from Princess Margaret Hospital in June. ( ABC News: David Weber )

WA's chief health officer last month gave the all-clear for the hospital to open following nearly three years of delays after latest testing showed the drinking water to be finally clear of lead contamination.

Opposition Leader Mike Nahan was not surprised by the high power costs but predicted once the hospital was fully operational, electricity consumption would increase by much more than the predicted 15 per cent.

"When they go in there, it's fully occupied. All the equipment, it's all electronic — I think 50 per cent," Dr Nahan said.

"A 50 per cent increase from a dormant hospital is a reasonable expectation, that's what happened with Fiona Stanley Hospital."

Perth Children's Hospital timetable