Mayfield Fund is one of several well-known Silicon Valley venture capital firms dancing around the green sector. Its M.O. so far has been to invest in relatively capital-efficient plays in lighting and energy efficiency.

Today, it finally staked a claim in solar — the fastest growing segment for green investment — by leading a $21.5 million fifth round of funding for solar panel financing firm SolarCity.

Essentially, the company is the solar industry’s middleman, making rooftop solar systems more affordable for both residential and commercial consumers. It doesn’t make the solar panels, but it installs them, maintains them and retains ownership of them, selling the power generated back to its customers.

This allows the company to offer generous financing deals: no upfront costs for installation, and only monthly lease and electricity payments over the course of the panels’ lives. This brings the total cost of installing a solar system down from as high as $25,000 to the low four figures.

This business model isn’t too much of a departure for Mayfield. It may be new to solar, but SolarCity still falls neatly into the capital-efficient category, and it’s already profitable, making a quick, lucrative exit all the more likely.

“The way Mayfield sees the solar ecosystem, too much money has gone into early-stage companies developing the next best solar cell,” says Navin Chaddha, managing director at Mayfield. “We decided to invest downstream, with a company that already has customers and is science agnostic.”

SolarCity says it will use its new funding (which it didn’t expect to raise), to accelerate its geographical expansion in the U.S. (it’s only operating in five states today) and to fuel its acquisition strategy. Already this year, the company bought up the assets of Building Solutions, maker of software used to evaluate building energy efficiency.

Partnerships are also playing a key role in SolarCity’s expansion. In March, the company started offering its panel installation services through 92 Home Depot locations — a tandem program that may soon go national, raising visibility of the SolarCity brand.

The most important partnership in the company’s arsenal is with Pacific Gas & Electric, which is funneling $60 million in tax equity into the company to install about 1,000 solar systems on roofs in its coverage area.

PG&E is the first utility to turn to tax equity to fund alternative energy projects. Basically, it set up a new entity called Pacific Venture Capital to invest the money, which will produce returns in the form of clean energy tax credits. Last month it used the same tactic to invest $100 million in SolarCity’s prime competitor SunRun for the installation of 3,500 rooftop systems.

SunRun, which provides similar installation and financing services for residential customers, is a year younger than SolarCity, but already claims to have the bulk of the existing market share. Chaddha says that this isn’t the case if you look at actual deployments instead of the companies’ pipelines. On top of that, SolarCity was the first to pioneer a tax equity deal with PG&E, opening up new doors in the market.

But that’s not the biggest difference between the two companies. While SunRun hands out a lot of the installation and maintenance work to local contractors, everyone involved in SolarCity systems actually works for the company — making it a tighter-run, vertically integrated operation, Chaddha says.

“Our feeling is, SolarCity is a one-stop shop for all of our customers’ solar needs,” he says, giving Apple as an example of how vertically integrated companies have better success. “If you look at the other companies, one person does the panels, another does the installation, someone else follows up. We want to do it all. We want there to be one company responsible for providing quality service and controlling the experience.”

Still, even if SolarCity is in the lead (its brand recognition is boosted by listing Tesla Motors’ CEO Elon Musk as its chairman), SunRun is growing big in its review mirror and is not to be discounted. Lately, the company is pouring more resources into developing software to help installers that could give it an edge going forward.

In the meantime, neither company is hurting for cash. SunRun has raised more than $275 million, including $90 million in tax equity from U.S. Bancorp, and counts Foundation Capital, Sequoia Capital and Foundation Capital among its investors.

SolarCity’s recent round of funding included Draper Fisher Jurvetson, DBL Investors and Generation Capital — boosting its total raised to around $169 million. Private share trading marketplace SharesPost pegs its valuation somewhere between $375 million and $443.8 million.

Mayfield’s other green investments include cPower, a company that manages energy demand and supply issues for utilities; LatticePower, which makes materials for LED manufacturing; and Inphi, a maker of high-speed electronic components that aid in energy efficiency.