UPDATE 1-Saudi Electricity Q3 net up 15 pct, costs growth down

* Q3 net profit up 15 percent, turnover up 9.4 percent

* 500 million riyals booked for depreciation in Q3

* Costs growth reduced mainly on new power plants

(Adds detail, CEO comment, background)

By Souhail Karam

RIYADH, Oct 19 (Reuters) - Saudi Electricity 5110.SE posted a 15.4 percent rise in third-quarter net profit, as seasonal peak demand increased revenue and new and more efficient plants helped it tame cost growth.

The Gulf's largest utility by market value made 1.73 billion riyals ($461 million) in the three months to Sept. 30. The rise was below the average 18.5 percent rise analysts had forecast in a Reuters survey last month. [ID:nL4471694]

The third quarter, which coincides with the summer season, typically marks the best quarter for the state-controlled company as soaring temperatures boost power consumption mainly for air-conditioning in the desert kingdom.

Chief Executive Ali Saleh al-Barrak could not immediately give a turnover figure for the third quarter, but based on the company's previous quarterly financial statements and Reuters calculations, third-quarter turnover rose 9.4 percent to about 7.9 billion riyals.

"Demand of course peaks every third quarter of the year," Barrak said. "We managed to lower the pace of costs growth mainly because of the start of new plants that are more efficient both on the consumption of fuel and cost less to maintain."

The company did however have to deduct 500 million riyals from its operating profit to pay for the depreciation of the new power plants, he said.

The utility is carrying out projects worth 75 billion riyals to be completed within three years to meet domestic power demand that is rising by an annual 8 percent. (Editing by David Holmes) ($1=3.750 Saudi riyals)