General Electric's "net-zero energy" home of the future – which is planned for 2015, but exists only on paper right now – relies on a rooftop solar array to give the home enough energy to spin back the electricity meter (see GE Unveils Net Zero Energy Home Strategy).

But that blueprint also includes a battery. Without it, solar may be too unstable an energy source to incorporate into utility grids in a big way.

Silent Power has a solar inverter-battery rig it says can accomplish that balancing task. In fact, the Baxter, Minn.-based startup is involved with GE in at least a few smart grid projects seeking stimulus funding to install those systems across a large number of homes, CEO Todd Headlee said last week at the Clean Energy Venture Summit in Austin, Texas.

Silent Power has raised $4 million in angel financing, and is seeking to raise a $7 million series A round to "attack the utility space," with commercialization planned for 2011, Headlee said.

As for GE, "We've partnered with them on a couple of proposals," he said, though he wouldn't give additional details on the projects the two were planning (see Green Light post). 

The problem with utility-scale energy storage has so far been its price. Beyond huge, capital-intensive and site-limited systems like pumped hydro and compressed air energy storage, batteries, flywheels, fuel cells and other distributed energy storage technologies are still too expensive for most utilities to justify (see Grid Energy Storage: Big Market, Tough to Tackle).

Silent Energy says it can drastically undercut those costs by maxing out the federal tax rebate available for solar system installations – and getting utilities to foot part of the remaining bill.

A Silent Power system should add about $5,000 to the cost of a typical solar installation, Headlee said. The 30-percent federal tax credit shaves that by 30 percent, leaving about $3,500 remaining – or, as Silent Power likes to frame it, about $350 per kilowatt-hour in storage capacity. That's cheaper than commercially available battery storage system today, though it's a target price for many.

Batteries count for the solar credit when linked to an inverter system, Headlee said. If Congress passes a bill proposed by Sen. Ron Wyden (D.-Ore.) that offers an energy storage equivalent tax incentives to wind and solar, that could open up the pool of financial support, he said (see Energy Bill Could Boost Storage Technologies).

Granted, the batteries won't last more than a few hours, he said. But that's enough for utilities to smooth the difference between the time that solar panels get their best output, in the mid-afternoon, and the slightly later peak demand times that are typical for utilities during hot summer afternoons as people get home from work and start turning stuff on, he said.

That demand response capacity could be worth something to utilities – perhaps, even a $3,500 rebate or incentive to homeowners to make Silent Power's system as cheap as a regular solar panel array, he suggested.

A solar-battery system would require inverter systems that can switch instantly between the two – something Silent Power's system can do, Headlee said. About 200 homes with Silent Power's units should give a utility 1 megawatt of load-shifting capacity, he estimated.

Beyond that, batteries can help smooth the big swings in solar panel generation when clouds pass overhead, he said. Spread out to neighborhood scale, such drastic ups and downs in home power demand could spell trouble for an unprepared distribution grid.

The market is huge, Headlee said. American Electric Power, for one, says it will need 1 gigawatt of storage, or 2.6 percent of generation capacity, by 2020, he noted, and California may require 4 gigawatts of storage to reach its goal of getting a third of its power from renewable resources by 2020. About 20 gigawatts of new storage would represent a $10 billion market, he said (see Green Light post).

Beyond its work with GE, Silent Power has been talking with California utilities including San Diego Gas & Electric, Southern California Edison and Sacramento Municipal Utility District, he said.

Most of California's utilities are asking the DOE for money to build grid storage, but those projects have concentrated on larger-scale storage, like a 30-megawatt lithium-ion battery Southern California Edison wants A123 Systems to build, or Pacific Gas & Electric's plan for a compressed air energy storage system (see PG&E Wants DOE Dollars for Underground Air Energy Storage and SoCal Edison Wants A123's Biggest Grid Battery Ever).

But SDG&E's proposed $212 million microgrid project, which is also seeking DOE stimulus funding, is likely to require smaller distributed energy storage systems, Headlee noted (see Balance Energy Wants to Build Microgrids, Starting With San Diego).

For now, the company plans to use lead-acid batteries with an estimated five to seven year lifespan, he said. But it's looking into lithium-ion and advanced lead-acid batteries as well, he said.

A few venture capitalists briefed on Silent Power's plans at last week's conference in Austin noted that the company might be a bit early to the market. Systems for allowing utilities to control large numbers of home-based batteries are in their infancy, though the millions of smart meters being deployed could help extend those networks, Headlee said.

In fact, smart grid startup GridPoint started out in 2003 with a similar business plan, then switched to software after finding the market too small, he said (see GridPoint Gets $120M, Buys V2Green).

But then, utilities have been running demand response programs for years, with factories and other large power users turning down their use at peak demand times, Headlee said (see Is Demand Response Doomed?)

And utilities expecting hundreds of megawatts of rooftop solar panels to be installed in their service territories may feel under the gun to put storage in place to make it an asset, rather than a grid-destabilizing liability, he said.

Interact with smart grid industry visionaries from North American utilities, innovative hardware and software vendors and leading industry consortiums at The Networked Grid on November 4 in San Francisco.