(Reuters) - Solar inverter maker Enphase Energy ( ENPH ) forecast first-quarter revenue above Wall Street expectations on Tuesday, after robust demand helped it post a better-than-expected fourth-quarter operating profit, sending its shares up about 6% after the bell.

The solar energy company forecast first-quarter revenue to be in the range of $340 million to $380 million, compared to analysts' expectations of $338.5 million, according to data compiled by LSEG.

The company said the forecast included shipments of 150 to 170 megawatt hours of IQ Batteries, which are used to store excess solar energy for later use, as well as about $50 million of safe harbor revenue, which is any sales made to customers who plan to install the inventory over more than one year.

"Stock up in after-market, but this (inclusion of 'safe harbor' revenue in Q1 guide) may be frowned upon as the Street fully digests it," GLJ Research LLC analyst Gordon L. Johnson said in a note.

In the U.S., Enphase's biggest market, revenue rose 6% sequentially due to higher microinverter sales in the quarter.

The Fremont, California-based company's adjusted net operating income came in at $120.4 million for the quarter ending Dec. 31, ahead of analysts' average estimate of $100.4 million, according to data compiled by LSEG.