NEW YORK ( TheStreet) -- Something quite extraordinary has been happening over the last three months: Solar stocks are back.Most survivors of the shakeout from the last few years have gained in the range of 40%-60%, with the TAN ( TAN)ETF up 58% and the KWT ( KWT)ETF 66% higher. The U.S. industry's "bell cow," First Solar ( FSLR), trails the field with a gain of 47%. The leader by far is SunPower ( SPWR), which has soared 230% over the last three months. After falling as low as $3.71 last summer, the company, which is majority owned by France's Total ( TOT), now trades at $13 per share. The reason for SunPower's relative success, according to Insider Monkey , is efficiency. Its panels are up to 21% efficient at turning solar energy into electricity. That's almost double the average for FSLR. Its balance sheet is also fairly good for a company of this type, with about half as much cash as debt. The cash balance has been rising steadily since Total moved in. Like its Chinese rivals, SunPower uses polysilicon in its panels. It generates most of its revenue from U.S. projects on a utility scale, although its Web site is filled with happy homeowners. It adds microinverters to its panels, lowering installation costs, and says it has cut costs per-watt by 25%. SunPower has enjoyed huge benefits from its link with Total, which has let it keep improving in efficiency despite continuing losses. The rest of the industry is coming back on a wave of new demand. New markets are opening, like Argentina, according to Bloomberg, and the Middle East, according to Nanowerk. As costs drop, more U.S. businesses find they can afford panels, which create power where it's used. Some, like a nursery in Ft. Worth, claim they can pay for the investment over three years from electricity savings, writes the Ft. Worth Star-Telegram. The financing market has also improved, as companies like SolarCity ( SCTY), which had its IPO in the fourth quarter of 2012, scale into the commercial market. Honda dealerships in California say that their SolarCity-financed panels deliver power for less than the cost of the state's grid, according to Marketwatch. The cost of solar power fell below that of power from the electrical grid in Hawaii some time ago. Now this "grid parity," sometimes called "crossover," has reached the mainland and begun spreading across the country. There are several ways in which you can reduce costs - panel efficiency, manufacturing costs, financing costs, installation costs, and permitting costs. It's not just about the cost-per-watt delivered from the factory, but the total installed cost-per-watt.