Solar energy is now capable of powering 4.6 million American homes -- but how much are homeowners willing to pay for it? A new research report sheds light on solar system premiums, but the news isn't necessarily good for everyone. Here's what you need to know.

Rich roofs

There are a lot of reasons to both love and loathe rooftop solar. Those who love the idea are probably most interested in energy savings and/or helping the environment. Those who loathe rooftop solar systems may view them as an environmentally unsound energy source, a potential leaky roof liability, or simply an overall deadweight loss in terms of costs versus savings.

Researchers hate rhetoric, and a team at the Department of Energy Berkeley National Laboratory have been hard at work to answer a seemingly simple question that prospective solar system owners want to know: does a rooftop solar panel system actually increase home value?

Through a series of recently expanded evaluations, the answer is: sometimes yes. Using cost estimates, income estimates, and side-by-side comparisons to similar non-solar homes, owned rooftop solar systems end up in the black. By comparing the sale prices of around 4,000 solar rooftop homes across eight states and 12 years to nearly 19,000 comparable non-solar homes, researchers found that, for an average 3.6kW solar system house, buyers were generally willing to pay around $15,000 more than for a comparable non-solar home.

Sometimes yes

While this might seem like excellent news for the solar sector, and especially for residential solar heavyweights like SolarCity Corporation (NASDAQ:SCTY) and Vivint Solar, (NYSE:VSLR), the news isn't all good. The Berkeley Lab's research focuses on fully owned solar systems, and these solar power purveyors rely primarily on solar leases and power purchase agreements (PPAs) to sell their wares -- models that are much less certain to increase home value.

Solar leases have historically been an easy win and important revenue-maker for residential solar companies. It's easy money on top of product sales. From liability worries to home value appraisal guidelines to general worries about the longer-term profitability of current systems, solar leases and PPAs wrap customers up in 20- or 30-year agreements that might not seem stellar to prospective homebuyers.

On top of all this, increased home value is only part of the equation -- savings over time may add up to even more. According to energysage.com, a solar financial product comparison site, I could save $21,500 over 20 years and own my system if I take out a $0-down loan, but would save just $9,500 and rent my system if I agreed to a $0-down lease or PPA.

SolarCity Corporation, the largest residential solar power company in the U.S., built its business on the back of solar leases. With 78% of its cumulative MW installed from residential and over 300,000 cumulative customers, SolarCity has expanded residential solar more than anybody else. But while contracted customer payments now add up to around $9 billion, SolarCity Corporation introduced its own loan model around a year ago to stay competitive in an increasingly saturated market. CEO Lyndon Rive expects that, as his company continues to grow, loans will be responsible for much of SolarCity's future growth, and could account for as much as half of all new business by the end of 2015.

Just last month, Vivint Solar, became the latest major solar company to offer loans. Similar to SolarCity, Vivint built itself up using power purchase agreements (PPAs). PPAs allowed for every utility bill to be a bit cheaper, starting immediately, but failed to bring about the long-term value that a solar loan could provide.

"We're pleased that we can now offer another affordable financing solution to our customers," said Chance Allred, Senior Vice President of Sales at Vivint Solar, in a statement. "We want to provide our customers multiple products to choose from to enable them to 'go solar.' If a customer wants instant financing or wants to eventually own their own solar energy system, we can now help them do that."

While Vivint Solar, will only offer its loans in Utah to start, all indications from the sector, at large, point to loan offerings everywhere if the company expects to stay competitive with other solar players.

Value add for who?

For investors, this latest research puts solar stocks in perspective: things are looking up -- but mostly for the proactive players. When solar savings are so dependent on whether or not a company can offer the right type of financial packaging, scale isn't enough to keep SolarCity Corporation and Vivint Solar in the lead.

An average $15,000 more in home value for solar owners will go a long way to expanding solar markets, but only the companies with the best financial solar packages will stand a chance in expanding their market share.