One sign of the long-term viability of solar power is how quickly corporate America has been putting solar panels on unused rooftop space. A recent report from the Solar Energy Industries Association says that Wal-Mart (WMT 0.83%), Costco, Kohl's, and Apple (AAPL 1.66%) are the top four companies installing solar in the U.S. and they're just getting started.

Wal-Mart led the list last year with 65 MW and if you need any evidence that solar saves money you don't need to go any further than that. Wal-Mart is no charity case and if power from the sun weren't saving the company money it wouldn't do it. Over the past year, another 24.4 MW was put up at Wal-Mart stores and distribution centers, which was second only to Apple. 


Cumulative Installed-2012 

Cumulative Installed-2013 


65 MW

89.4 MW


38.9 MW

47.1 MW


36.5 MW

44.72 MW


<3.3 MW

40.7 MW


21.5 MW

35.1 MW

Source: Solar Energy Industries Association

The retail stores are using their massive rooftop space to expand into solar. Wal-Mart partnered with SolarCity (SCTY.DL) earlier this year to install solar power on another 60 stores in California, part of the company's goal to have solar on 130 stores, or 75% of its stores in the state.  

Source: Walmart

Apple is taking a very different approach, building utility scale projects next to its data centers. SunPower (SPWR 5.04%) built 40 MW of solar next to its Maiden, N.C. data center and is building another 20 MW  at its Reno, NV data center with the help of NV Energy. As you can see above, Apple is late to the solar game, but it's catching up in a big way. 

Image courtesy of WCNC-TV. 

Corporate America goes solar
The reason Corporate America is taking notice of solar is the financial benefit of going solar. A recent report from GTM Research and SEIA says that solar system costs are down 40% since the start of 2011 and 50% since the start of 2010. Over the last year alone, the cost to install a commercial installation is down 14.7% to $3.71 per watt. Assuming a 10% return on investment and a 20% capacity factor, that's a cost of electricity of 8.5 cents per kW-hr, below grid prices. And that doesn't include tax benefits. 

SolarCity and SunPower have been two of the biggest winners in corporate solar installations this year and that's something I'd expect to see in the future as well. SolarCity is a dominant player in residential solar and can translate some of those efficiencies to the commercial market. SunPower is one of the largest utility scale installers and with the most efficient panels on the market it is can squeeze more power out of a commercial rooftop than any other company.

The fact that Wal-Mart and Apple are going solar is a sign of things to come for Corporate America and SolarCity and SunPower are two partners investors should watch as the industry grows.