Solar stocks have had a rough time in 2018, evidenced by the fact that the Invesco Solar ETF has lost more than 25% of its value on the year. However, just because solar stocks had a down year doesn't mean their prospects aren't bright. In fact, these three fool.com contributors think this year's sell-off could be a great opportunity for investors with a long-term mind-set to take a closer look at the sector.

Three solar stocks that caught their eye are Brookfield Renewable Partners (NYSE:BEP), SolarEdge Technologies (NASDAQ:SEDG), and SunPower (NASDAQ:SPWR). Here's why they think you should put them on your watch list.

A brightening future in solar

Matt DiLallo (Brookfield Renewable Partners): Renewable power generator Brookfield Renewable Partners is currently a global leader in generating hydroelectric power, which comprises about 76% of its portfolio. However, the company has been investing to develop wind and solar platforms in recent years.

Brookfield Renewable made its first investments in solar last year when it acquired stakes in TerraForm Power (NASDAQ:TERP) and its sibling, TerraForm Global. As a result of those deals, the company now generates about 4% of its earnings from solar. While that's a small percentage, Brookfield's solar power business is expanding at a fast pace, with profits from that segment skyrocketing 600% last quarter due to its increasing stake in TerraForm Power. That company currently gets about 37% of its earnings from solar, which should expand as it buys additional assets from developers in the coming years.

On top of participating alongside TerraForm Power's growing solar business, Brookfield Renewable has further solar-powered upside from its relationship with Brookfield Asset Management (NYSE:BAM). Earlier this year, the latter formed a joint venture with a leading logistics and industrial facility manager in China to install 300 megawatts of rooftop solar projects over the next three years as part of a 1-gigawatt development pipeline, which is enough to power 750,000 households. That deal will enable Brookfield Renewable to expand its footprint in China as well as increase its exposure to the fast-growing solar market.

That combination of solar growth from its interest in TerraForm as well as its investments in China make Brookfield Renewable an emerging solar stock to watch in the coming years.

This year's stock at last year's prices

John Bromels (SolarEdge Technologies): What the stock market giveth, the stock market taketh away -- at least, that's what happened in the case of solar energy system component manufacturer SolarEdge Technologies this year. After huge run-ups in the company's stock price on stellar earnings reports in February and May, the second half of the year saw SolarEdge's shares knocked back down to about $36 a share, roughly where they were trading a year ago.

What went wrong? Your guess is as good as mine. SolarEdge posted some fantastic third-quarter 2018 numbers, in which revenue increased 42% over the prior-year quarter to $236.6 million. Net earnings jumped even higher, up 62.9% to $45.6 million. The company even posted strong guidance, so it doesn't seem to be a case of market concerns about its future.

Of course, even 42% revenue growth looks unimpressive when compared to the 60%-80% revenue growth that the company had been posting in the prior few quarters, so maybe that's why investors were selling off the stock. It's also possible that investors have concerns about SolarEdge's recent acquisitions of lithium-ion battery supplier Kokam or uninterruptible power supply component manufacturer Gamatronic. But those purchases should actually make the company more stable, allowing SolarEdge to offer one-stop shopping for residential and commercial solar installation components that handle power generation, storage, and consumption. Of course, a customer would still need to purchase the panels themselves separately.

With shares stumbling this far down, the company is trading at a P/E ratio of just 12.8, which seems awfully cheap for a company with the growth potential that SolarEdge possesses. It's a top solar stock to watch right now.

The "do-it-all" solar company

Travis Hoium (SunPower): The solar industry has had an up and down year, with high demand in the first half of the year but tariffs hitting the industry, followed by plunging demand in China midyear that caused a huge drop in demand and solar panel prices. But near the end of the year, demand in everything from residential solar to large-scale solar plants began to rise again.

As the year ends, SunPower is a great company to watch because it's one of the few companies with exposure to residential, commercial, and utility-scale solar power plants, generating about one-third of its revenue from each segment.

What we've seen in the last few quarters is strengthening demand for residential and commercial solar, which are less susceptible to tariff costs than utility solar, and the addition of energy storage in both markets. Margins haven't been very strong in either market because of SunPower's relatively high-cost solar panels, but the company's demand and margin trend can tell us a lot about how residential and commercial solar markets are heading.

On the utility side of the business, SunPower is now a gigawatt-scale manufacturer of its P-Series solar panels, which is a commodity-type solar panel but has slightly higher efficiency than most competitors. If SunPower sees strong demand for its solar panels, we can deduce that installers are valuing efficiency over costs alone. However, if SunPower continues to struggle in power plants, we may see confirmation that costs are more important than any technology advantage in power plants.

The final thing to watch with SunPower is the impact of a tariff exemption the company got this fall from the Trump administration. High-efficiency solar panels manufactured in Asia will escape tariffs for the next three years and that could save the company as much as $100 million in 2019 alone.

Unlike any other solar company, SunPower can give investors a glimpse of what's going on in any segment of the market.