It's been quite the year for Enphase Energy (NASDAQ: ENPH), which has seen its shares more than double since the start of 2018. Stretch back just a bit further and the stock has gained over 400% since the beginning of 2017.

Fueling the gains is an order-of-magnitude improvement in profitability. The business reported an operating loss of $3.4 million through the first nine months of 2018, a nearly 11-fold improvement from the $37.2 million operating loss coughed up in the year-ago period. A focus on selling higher-margin products and reducing general overhead appears to have Enphase Energy on the path to profitable operations.

From where things stand now, individual investors might expect the good times to keep rolling in 2019 and beyond, although it may require a little help from new growth opportunities, which are far from certain in the unpredictable solar market.

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A strong year on multiple fronts

Enphase Energy stock certainly earned a higher valuation after the company significantly improved its bottom line. That includes a third-quarter 2018 operating loss of just $374,000. Adjust that for a $2.6 million restructuring charge taken during the period, and the microinverter leader would have had a profitable quarter.

Much of the year-over-year improvement was made possible by sharply higher product margin. In the third quarter of 2018, the company delivered gross margin of 32.4%, up from 29.9% in the previous quarter and just 21.4% in the third quarter of 2017. That's impressive considering total revenue grew just 1%, which means the business is capturing benefits from shifting its product mix, rather than scaling sales volumes.

That's not to say the business is incapable of scaling. President and CEO Badri Kothandaraman told shareholders to expect fourth-quarter 2018 revenue of around $85 million, which represents a sharp increase from the $78 million in sales reported during the most recent quarter. The growth in revenue reflects the initial shipments of microinverters to SunPower. The two companies signed a deal earlier this year, with Enphase Energy emerging as the exclusive module-level microinverter supplier for the solar leader. That alone will be an important near-term catalyst for the business, but it's not the only opportunity for growth.

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What's ahead for Enphase Energy in 2019?

Enphase Energy expects the supply agreement with SunPower to generate around $65 million in annualized revenue -- at a gross margin in the neighborhood of 34% -- by the end of 2019. That represents a healthy 20% increase in full-year revenue compared to levels achieved in recent years. And it's only one source of growth investors can look forward to.