The FINANCIAL -- Hourly electric power generation data illustrate the variability of wind and solar generation relative to the comparatively smooth output from other generation sources.

The California ISO, which operates the electric power system for most of California, provides these hourly data for generation sources that are eligible renewable resources under California's Renewable Portfolio Standard.

Power produced by geothermal, biomass, biogas, and small hydro generators can be easily dispatched, meaning it can be either increased, decreased, or controlled to remain fairly constant. For wind and solar, however, generation is largely determined by the availability of the resource. Wind resources, for example, are often more available at night. Solar power is not generated until the sun comes up, and can increase and decrease quickly as the sun rises, sets, or is blocked by cloud cover.

However, much of California's utility-scale solar capacity is based on concentrating solar technology, which embodies enough thermal storage to continue generating power for a few hours after the sun goes down. The variability of wind and solar power generation leads to challenges in power system operations and planning. The contribution towards total generation from eligible renewable resources also varies seasonally. For the dates charted above, the amount of renewable generation as a percentage of the total generation within the CAISO ranges from 8% on Oct 18, 2011 to 14% on May 9, 2011.

As a State, California is trying to achieve a 33% Renewable Portfolio Standard by 2020, with eligible resources that include wind, solar, geothermal, biomass, biogas, and "small hydro", which translates roughly as generators under 30 MW. The State of California also has a significant amount of "large" hydroelectric generating capacity from generators above 30 MW, which are not eligible resources under the RPS.

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