One of the main problems with renewable energy in its current state is the need for backup power generation to level out production. Since the wind doesn't always blow and the sun doesn't always shine, backup power, usually natural-gas-driven generation, is needed to smooth out the lumps.

This is something most people know about renewable energy, but how much does it really cost?

NV Energy and Navigant Consulting did a study with the backing of the Department of Energy to find out how much this backup costs. In the case studied in Nevada, the analysis showed backup generation costs utilities $3 to $8 per MWhr, or $0.003 to $0.008 per kWhr. This is something that should be considered when looking at how much it costs to put renewable energy on the grid.

The number will become more significant as manufacturers and utility-scale installers add more projects to their pipelines, something manufacturers have focused more on recently.

  • First Solar (Nasdaq: FSLR) ended the third quarter of 2011 with 2.7 GW of projects in the pipeline and had added 654 MW in the first three quarters of 2011.
  • SunPower (Nasdaq: SPWR) recently began construction on the 250-MW California Valley Solar Ranch, which will be owned by NRG Energy (NYSE: NRG) and feed electricity to Pacific Gas & Electric.
  • LDK Solar (NYSE: LDK) is getting into the utility and power plant business with a recent loan from China Development Bank to build two power plants in California totaling 8 MW.

With solar module sales slumping, manufacturers are turning to power plants as a way to create demand for their product. But as these plants get larger, the impact on the grid grows, something utilities need to deal with.

Solutions to a solar storage problem
Other solutions are beginning to arise, but few are ready for prime time. Pumped hydro and compressed air storage have been used on large scales in locations where it's feasible. A123 Systems (Nasdaq: AONE) and Ener1 are working with utilities to test batteries for energy storage on a somewhat smaller scale right now.

But until a bigger solution is developed, utilities will have to use fossil fuel backup power with renewable energy, and that costs money for everyone involved.

Interested in reading more about renewable energy stocks? Add your favorites to My Watchlist, and My Watchlist will find all of our Foolish analysis on these stocks.

Want a stock idea we think will outperform the market? Our analysts have selected a stock that they believe is poised for tremendous growth in 2012. Find out which company in our new free report "The Motley Fool's Top Stock for 2012." Thousands have already requested access and it'll only be available for a limited time. Simply click here. It's free.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.