The solar sector has seen its share of pain. In fact, many observers believe that the industry would not be able to operate without tax assistance and subsidies for the end-users buying the solar panels. So what do you make of the recent gains in the sector again? It seemed almost insane that Citigroup decided to issue bullish outlooks on the solar sector back in February of this year, when solar stocks were still so low, but it may be getting the last laugh now.

24/7 Wall St. is evaluating five key solar stocks. These have exhibited serious changes of late, and their shares have all recovered handily from their lows. Some may have much more upside, even if their incredible strength of late merits profit taking or outright selling in the days or weeks ahead.

We are focusing on shares of the following: First Solar Inc. (NASDAQ: FSLR), GT Advanced Technologies Inc. (NASDAQ: GTAT), SolarCity Corp. (NASDAQ: SCTY), SunEdison Inc. (NYSE: SUNE) and SunPower Corp. (NASDAQ: SPWR). SunEdison reported earnings on Wednesday morning and SolarCity is on deck to report. This outlook and evaluation is not meant to get in front of any reports.

We also caution investors that the solar trade is one that can fade for many reasons. Financial loss is not unavoidable, and we would assume that many of the green energy companies and clean-tech companies may have serious holes in their business plans.

First Solar Inc. (NASDAQ: FSLR) shares have somehow managed to come all the way back to more than $60.00. This is nearly triple from the lows of the last year. What has driven this interest so much of late was when First Solar managed to dazzle investors with its earnings. Some of the huge upside may have simply been in the timing of the revenue recognition. It seems odd that the stock jumped so much when you consider that sales guidance for year was effectively trimmed by about $200 million while it simultaneously raised its earnings guidance.

Note that even if analysts have ratcheted up their price targets, they continue to doubt First Solar’s ability to deliver constant rewards. The consensus price target is down to just over $46, versus a $62 handle now. That does not sound promising on the surface. Still, any Fist Solar bull would simply point out that the analysts were wrong when they were so bullish and they are were wrong when they remained so bearish. Is 15 times forward earnings too much to pay? That remains to be seen.

GT Advanced Technologies Inc. (NASDAQ: GTAT) may be a solar overlap now more than a solar pure-play, but it is back in focus now that Apple reached a pact with GT to supply sapphire glass for Apple products. Apple may be looking to work around the Gorilla Glass from Corning, but the gains if the project works out as well as the stock indicated could be a good omen for GT.

GT’s stock even managed to hit a new 52-week high above $10 this week on the Apple news. The question to ask is whether the Apple deal will be enough of a bang for the company to make leaps in the solar operations as well. The GT HiCz systems produce low-cost monocrystalline ingots for high-efficiency solar cells. Its DSS furnaces are also called the solar industry standard by the company itself, with more than 3,200 units shipped worldwide. After a 20% gain to $10.10, the stock is up about 300% from its 52-week low, and the consensus analyst price target was down around $7.60 on last look.

SolarCity Corp. (NASDAQ: SCTY) is the solar energy player that may simply be the beneficiary of a trend. This outfit sells and installs solar panel systems on houses and business locations rather than makes solar panels. In theory, it stands to benefit as the price of solar modules drops in the future as it makes installing new systems cheaper. One issue we have is that SolarCity’s profitability is hard to get to, even if it is a company still controlled by Tesla’s Elon Musk.

SolarCity has been public less than a year and shares are very close to all-time highs. The latest close of $62.77 compares to a 52-week range of $9.20 to $65.30. It also comes with a value of almost $5 billion, and the consensus analyst price target is closer to $50 for the stock. The one consideration here is that SolarCity is expected to lose money for this year and the years ahead, even on expected sales growth of 60% or more in 2014.

SunEdison Inc. (NYSE: SUNE) almost brings back too much pain to trust as a serious and sustainable turnaround in the solar sector. This is the old MEMC outfit, which makes and sells silicon wafers to semiconductor and solar energy companies. The company now offers home solar solutions and intelligent energy solutions, and it announced plans to spin off its chip-related operations in August.

SunEdison shares hit a new multiyear high of almost $11 this week after news of a $160 million PV project in Chile. The company also ended its most recent quarter with some $640 million in cash and a growing backlog in solar. Sales growth appears to have resumed, and 2014 is expected to be a year of profits. Trading at $10.38, its consensus analyst price target is $11.31, and the market cap is about $2.4 billion.

SunPower Corp. (NASDAQ: SPWR) looked as though it might at one point simply disappear. Total S.A. (NYSE: TOT) is the majority owner of it, and we were a bit surprised that the oil giant did not scoop up the rest of the company when the stock looked a skydiver without a parachute. That was then. Now things are good enough again that SunPower is acquiring Greenbiotics so that it can prosper off of solar cleaning systems to make the solar more efficient.

SunPower shares are back up above $34, after falling to a low of under $10, and then under $4 at the absolute lows over the last year. Now SunPower’s recovery has been nearly a ten-bagger from the lows. Its market cap is back up to more than $4.1 billion. Total could even turn around and make a handy profit on its investment at this point of it wanted to.

For investors who want solar stock exposure but find individual solar stocks too challenging for an all-or-none bet, there is a more diverse sector play via the Guggenheim Solar ETF (NYSEMKT: TAN). Volume has risen of late and the exchange traded fund just hit a new multiyear high. One issue to consider here is that the holdings are not all American, and some of the Asian or Chinese companies may not have even been heard of. All five solar stocks above are a part of this ETF. As a testament of how strong the solar sector has been this year, this ETF is up a whopping 164% so far in 2013.

Another ETF that gives solar exposure as a part of the broader green energy and renewable energy sectors is the PowerShares WilderHill Clean Energy (NYSEMKT: PBW). This ETF has its heaviest weighting in solar energy companies under green and renewable energy, but it at least includes wind, biofuels, power optimization and other aspects of the sector. This ETF recently hit a new high not seen since 2011, but it remains well under the former glory days. We would point out that this ETF is up about 68% so far in 2013.

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