PARIS, Feb 22 - France's Engie said on Thursday it booked a 714 million euro ($774 million) impairment charge last year as a result of issues with onshore wind turbines in the United States and a fall in some U.S. long-term energy prices.

Engie said in its annual results that the operational issue concerned wind turbines produced by Germany's Nordex, while the price drop was in the U.S. Southwest Power Pool (SPP), which runs from Montana in the north to New Mexico in the south.

"It's a very local issue, it's company-related, the bulk of it is one market and one portfolio of assets ... The project started a few years ago, we had to fix it and now it's properly done," Engie CFO Pierre-François Riolacci told analysts.

"We have one wind farm on which the Nordex turbines don't work, so we have a rate of availability of about 30%, which is very low compared to what we should have, so it renders the operation uneconomic ... We have assessed the loss of value linked to this asset, given it is not at a normal level of functioning," Riolacci added on a separate media call.

Nordex did not immediately respond to a Reuters request for comment on Riolacci's comments.

Engie was not the only energy company suffering from price assumptions that did not materialise in the SPP market, he said.

The fall in long-term power prices had a positive overall impact on the "fair value" of related virtual Power Purchase Agreement (PPA) contracts totalling about 300 million euros, Engie's results statement showed.

"We have restructured all our PPAs in this portfolio, so we are now in control of operations, and so we decided to take this hit," Riolacci said.

Shares in Engie were up 2.7% at 1111 GMT. ($1 = 0.9220 euros) (Reporting by America Hernandez, Editing by Alexander Smith)