Sempra Energy's (SRE) - Get Report CFO attributed the company's earnings beat to the commodities trading segment.

Sempra reported fourth-quarter earnings of $319 million, or $1.30 per share, compared with $289 million or $1.10 per share during the same period last year.

Sempra's quarterly revenue fell to $2.29 billion from $3.11 billion last year.

Analysts were expecting Sempra to report EPS of 91 cents and revenue of $2.55 billion, according to Thomson Reuters.

Sempra reaffirmed its yearly EPS guidance of between $4.35 and $4.60 per share.

Sempra Energy and its subsidiaries generate electricity and transport natural gas in North America, Europe and Asia. Sempra's subsidiaries include San Diego Gas & Electric and Southern California Gas.

"Our trading business performance was well ahead of pace in the quarter," said CFO Mark Snell. "This surprised some analysts."

Sempra enjoyed a favorably low tax rate during the fourth quarter because of debt on its balance sheet that was underwritten in Mexico but is denominated in U.S. dollars.

The Mexican peso weakened last fall when the price of oil tumbled from its summer highs. The weakening peso inflated Sempra's Mexican debt, which resulted in a beneficial tax event that lowered its effective tax rate. This benefit was partially offset by a hedge on the peso that went the wrong direction, according to Snell.

Snell said that Sempra is largely unaffected by high or low natural gas prices since its utilities assets are all located in California. California's utility rate structure is decoupled from commodity prices in order to encourage energy conservation rather than energy use.

"Adjustments in commodity prices are straight pass-throughs to our customers," Snell said.

However, Snell said that Sempra's power generation segment and its commodity trading segment can potentially benefit when natural gas prices are higher.

"Every dollar rise or fall in the price of natural gas will equate roughly to 5 cents per share by the end of 2009," Snell said.

Sempra was recently moving 5% higher at $41.12 per share.

Among Sempra's peer group,

El Paso

( EP) was recently falling nearly 1% at $7.03 per share, and

Southern

(SO) - Get Report

was rising 3.7% at $30.82 per share.