Owners of PV systems know first-hand that passing clouds cause rapid changes in the amount of energy produced from second to second. The impact of this short term power "variability" on system owners is usually seen rather than felt because during times of low production, electricity from the grid picks up the slack. This is good for PV system owners, but makes it difficult for grid operators, who are charged with providing reliable energy for their customers.
One way for utilities to manage variability is to ramp up other power plants, but that increases the costs and complexity of managing the electric power distribution system. Fortunately, recent studies have found that renewable energy sources that are distributed across a wide geographic range greatly reduce the effects of weather-related variability to the overall electric system. This means that more "dispersed" residential and commercial PV is good for utilities.
Joy Hughes, Founder, Solar Gardens Institute http://www.solargardens.org
CEO, Solar Panel Hosting LLC http://www.solarpanelhosting.com