SolarCity, the big rooftop solar panel installer rumored to be preparing for an IPO, has just landed an $81 million round led by the green VC supergroup Silver Lake Kraftwerk in its first investment.

SolarCity, which ranked first in residential rooftop solar installments in the U.S. last year and second-place in commercial rooftop PV, said it plans to use the money for sales and technology development, as well as potential acquisitions.

The San Mateo, Calif.-based company has previously raised just more than $200 million since its 2006 founding, and has installed or is installing solar panels that it owns and manages for customers at about 30,000 sites.

It has also lined up hundreds of millions in solar funds from investors including Walmart, Citi ($40 million), Google ($280 million) and Bank of America Merrill Lynch ($350 million) to install solar on up to 120,000 U.S. military homes.

Wednesday’s investment was joined by Valor Equity Partners, DBL Investors and Shea Ventures, the venture arm of builder Shea Homes. Individual investors included Nicholas Pritzker, a partner at Tao Ventures, and Tesla Motors CEO Elon Musk, who is the cousin of SolarCity founder and CEO Lyndon Rive and the startup’s chairman.

It’s also the first investment for Silver Lake Kraftwerk, the partnership formed last year by billionaire George Soros, Washington insider Cathy Zoi and Foundation Capital partner Adam Grosser. The supergroup has set the goal of raising $1.25 billion to invest in green technology, but had raised only about $206 million as of December, according to government filings.

SolarCity is rumored to be seeking an IPO, with news reports saying the startup could be seeking a market valuation of about $1.5 billion. That might be hard to achieve, however, in a market where top-line publicly traded solar companies like SunPower are seeing market valuations fall to less than that amidst falling solar panel prices and economic uncertainty. Of course, cheaper solar panels also help installers like SolarCity.

The economics of the third-party solar financing model are simple. First, the customer should pay nothing, or even see an immediate benefit from reduced power bills. Second, the company can manage thousands of rooftops as a unified asset, and bundle the tax credits, utility rebates and other financial incentives that make solar power affordable today in a way that allows them to attract cheap capital to finance the projects.

That fundamental premise is being tested out in the residential market by competitors including SunRun and Sungevity, along with new entrant Clean Power Finance, which landed a $75 million fund from Google to finance residential solar projects in September. Other firms such as Vivint, OneRoof, Solar Universe, Ontility, SunPower, SolarCraft, Sun Edison, CentroSolar, and Suntech’s BriteLease program also work in the market.

SolarCity’s rise hasn’t come without its share of problems. Earlier this month, solar rival SunPower sued SolarCity and five former SunPower employees who left to work for the startup, accusing them of stealing confidential information.

Tags: california, efficiency, finance, first solar, google, ppa, solar, solar panels, solar power, solarcity, sungevity, sunpower, sunrun, suntech, utilities