Sales of the new iPhone could add between a quarter and a half percentage point to fourth quarter annualized growth in the U.S., according to J.P. Morgan's chief economist, Michael Feroli in a note to clients on Monday.

Such an impact would be significant.

"Calculated using the so-called retail control method, sales of iPhone 5 could boost annualized GDP growth by $3.2 billion, or $12.8 billion at an annual rate," Feroli wrote. That 0.33 percentage-point boost, he added, "would limit the downside risk to our Q4 GDP growth protection, which remains 2.0 percent."

Feroli laid out his math. J.P. Morgan's analysts expect Apple to sell around 8 million iPhone 5s in the fourth quarter. They expect the sales price to be about $600.

With about $200 in discounted import component costs, the government can factor in $400 per phone into its measure of gross domestic product for the fourth quarter.

Feroli said the estimate of between a quarter to a half point of annualized GDP "seems fairly large, and for that reason should be treated skeptically." But, he added, "we think the recent evidence is consistent with this projection."

Feroli said that when the last iPhone was launched in October 2011, sales significantly outperformed expectations.

"Given the iPhone 5 launch is expected to be much larger, we think the estimate mentioned ... is reasonable," Feroli wrote.

According to a recent Reuters poll of Wall Street dealers and economists, U.S. GDP was seen at 2.0 percent on average in 2013, down slightly from estimates this summer.

(Reporting By Tim Reid; editing by Todd Eastham)

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