Residential solar systems usually don’t have much interaction with the meter attached to the side of the home. Mostly, they spin the meter’s dial backwards when they’re cranking out more power than the house is using, and get read once a month to calculate the bill.

Smart meters can enable two-way communication to deliver more up-to-date information on how that digital dial is spinning -- although we’ve seen reports of early problems with smart meters running twice as fast, instead of backwards, when they’re not programmed to realize they’re attached to a solar power generating home.

Of course, that’s a simple thing to fix. Because most U.S. customers see solar’s payoff as power bill reductions, rather than direct payment for the solar power they generate, simple net-metering-enabled smart meters may suit their purposes, and the utility’s purposes, just fine. Some utilities install a second meter as part of a solar net metering or feed-in tariff deal with their customers as well.

But for third-party residential solar companies like SolarCity, SunPower, Sunrun, Borrego Solar, Clean Power Finance, OneRoof Energy and others, keeping a revenue-grade tally on actual solar power generated is critical. That’s because these companies have promised their customers -- and their investors -- that they’ll keep close track of every dime of power generated, to make sure their complex lease or power-purchase arrangements are paying out properly over time.

That’s the market Itron is tackling with its cellular-linked solar power meters. While the big smart meter maker announced their availability just this week, they’ve actually been out in the market for some time now (PDF) under the name of SmartSynch, the company Itron (Nasdaq: ITRI) bought for $100 million in March. And while no customers have been formally named, Campbell McCool, marketing manager, told me in a Monday interview that one buyer of the Itron cell-linked solar meters is Sunrun.

Sunrun, which has raised about $145 million in VC investment and has amassed the financing for more than $1 billion in solar systems, hasn’t publicized the meter vendor it uses for its solar systems. But the company’s customer website states that its "meters are based on cell phone technology, and while they collect data constantly, they only communicate with [... the company] four times a day, rather than every 15 minutes.”

Those readings are uploaded onto web portals to show customers just how much money they’re saving with their financed solar system from day to day, along with other data and updates. Most of the other third-party solar aggregators offer similar services, as do smart meters from utilities like Pacific Gas & Electric, though these measure solar power’s net effect on household bills, not direct generation.

But third-party solar needs revenue-grade meters, to provide auditable data to the various financing parties involved, McCool said. It would be nice if they could also monitor system performance, to make sure their asset isn’t being degraded by dirt on the panels or shade from overgrown trees, for example. Itron's system comes integrated with meter data management, network management, back-end IT integration and all the other supports of a modern AMI network, McCool said.

Third-party aggregators also need to be able to connect with individual meters, installed in an unpredictable pattern across different utility service areas, he said. Cellular works well for that, while the mesh networking used by the majority of the smart meters deployed in the U.S. to date requires lots of meters in proximity to one another to pass each others’ messages along to central collection points -- a fact that’s led key mesh smart grid players, like Silver Spring Networks, to add more cellular capabilities to their technology.

Of course, Itron isn’t the only one using cellular to connect solar smart meters. About 60,000 to 80,000 of these meters were sold last year, McCool said, and Itron competes with various vendors on that front. Commercial and industrial meters have been using cellular for the past decade or so, as an improvement on the “plain old telephone line” (POTS) systems that were the first two-way communicating meters. Utility-scale projects are fully integrated with grid generation and transmission operation systems, but aggregated rooftop solar could also be a significant grid resource -- if it can be monitored and managed effectively.

That could open the door to a big new growth market for smart meter players like Landis+Gyr, Trilliant, Grid Net that are using 3G and 4G cellular to connect to meters. At the same time, communications technology giants like Verizon (NYSE: VZ), AT&T (NYSE: T), Sprint (NYSE: S), Qualcomm (Nasdaq: QCOMM) and others in the United States, along with coequals in Europe and Asia, have been beefing up their smart grid offerings.

Just how big might the solar metering market grow to be? McCool wouldn’t take a guess, but he did highlight the fact that third-party solar financing now leads the residential markets in California, Arizona, and Colorado. GTM Research’s U.S. Solar Market Insight: 2nd Quarter 2012 report finds that in the California market, the average installed price of a third-party-owned system dropped to $5.64 per watt in the second quarter, compared to $5.84 per watt for directly owned solar systems -- a first in the industry.

There’s also a lot of financing available for the third-party solar boom. Bloomberg New Energy Finance estimated earlier this month that the market for solar-backed securities could reach $4.6 billion by year’s end, combining roughly $1.16 billion in third-party solar financing projects and about $3.6 billion for solar projects being financed this year.

There are a myriad of other approaches to making residential rooftop solar a smarter part of the grid. Microinverter maker Enphase delivers data to homeowner and company via powerline carrier and cellular or broadband connections. Other companies offer home solar energy management portals, and for the well-heeled green enthusiast, there are battery systems to store solar power for use at night, or during blackouts -- or, perhaps, to stabilize the local grid.

Tags: batteries, borrego, california, clean power finance, finance, general electric, google, itron, landis+gyr, oneroof energy, pv, silver spring networks, smart grid, smartsynch, solar