Renewable energy supporters often ask why we can't simply get all our electricity from rooftop solar, backyard wind and energy efficiency. Why do large energy companies keeping pushing for centralized generation?

GTM asked this of a panel on distributed renewables at the sixth annual VerdeXchange conference, a California-centric event bringing together greentech policymakers, advocates and entrepreneurs.

“California has a mature and built-out distribution and generation infrastructure base,” explained Nicholas Chaset, Special Advisor for DG, CHP and Energy Storage to Governor Brown. “We are working toward an increasingly distributed energy future, but the goal isn’t meeting the state’s energy needs with 100 percent localized resources. The goal is trying to decarbonize our system as completely as possible.”

Localized resources and efficiency, Chaset said, are preferable. “Our 2020 goals of zero net energy in our commercial and residential spaces are going to require an incredible reliance on localized renewables and efficiency.” To enable that transition over the next twenty years, the state is making infrastructure investments in efficiency and localized renewables as opposed to conventional central station resources.

But at present, Chaset said, “there are concerns about cost and about load balancing. Even when you account for transmission upgrades and environmental mitigation, central station renewables are more cost-effective than end-load renewables. And there is the absolute need for power quickly. In a load pocket like LA, it would be very difficult to meet that need with only distributed generation. You need some centralized generation to balance load and supply.”

The utility representatives agreed, explaining that grid stability was their most important objective when considering massive amount of renewables on the grid.

"The real problem," said James Kelly, a former Southern California Edison executive focused on transmission, "is keeping the grid stable by simultaneously matching demand and supply. Most renewable resources are fundamentally intermittent. Not biomass or geothermal, but the most plentiful are intermittent. And they are really intermittent. The grid operator can’t match supply and demand without an incredibly smart system.”

Most renewable resources, Kelly added, don’t provide “the other electrical characteristics necessary to keep the lights on, like VARS, inertia, and frequency regulation, so you would get very poor quality or ineffective power until the technology is in place to maintain grid stability.”

However, it's not just distributed renewables that worry these companies -- it's the level of penetration.

“We tend to see that idea [100 percent renewables] as utopia,” said NRG Energy Services VP David Stickler, “because so many technological advances have to happen to achieve it. There are great technologies coming. Large-scale storage, for instance. As an investor-owned utility, we make investments in storage technology. But we don’t have that technology yet.”

There is an infrastructure that has yet to be built, Stickler said, involving communications and data analysis. “We call it ancillary services. Frequency response and other such things have to be replaced by something to keep the grid stable.”

As an investor-owned utility, NRG Energy sees utopia out there somewhere, said Stickler.

“If we continue to make conscious, strategic investments to build out the infrastructure, there are certain technologies now -- bridge technologies, like our ultra-low-emissions, behind-the-meter, distributed natural gas technologies -- which fill the void for twenty years or so. We can build all those infrastructure systems on a very reliable source today, so when storage comes or when the Bloom Box or fuel cells can be reliably deployed, we are already, as a system, ready.”

"We don’t know what energy delivery will look like in twenty years, but we’re willing to go down that path and work very hard to get there,” he said.

From last year's VerdeXchange conference, a session on Transmission, Grid Integration, and Storage of Renewables: