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Redefining PV Capacity

Effective metrics give solar its due credit.

Fortnightly Magazine - February 2009

the PV Capacity Workshop during the Solar Power 2007 conference 8 for a mix of utility, government and solar industry representatives.

After discussing issues directly and indirectly related to measuring capacity credit—the monetary value of capacity, emergency planning, capacity planning, ancillary services, the cost of PV, planning for future penetration of PV, the question of ownership, what happens at very high penetration levels—the workshop focused on capacity calculation methodology, ending with a straw poll on metric appropriateness and preferences.

Effective load-carrying capacity was the preferred method overall, followed by the solar load control and minimum buffer energy storage metrics (combined because of their operational similarity). There was a clear distinction, however, between utility and solar industry preferences, with utilities preferring the more familiar effective load-carrying capacity, while the solar industry preferred the methodologies exploiting control-storage synergies and eliminating the notion of risk associated with non-dispatchable PV generation.

This choice is consistent with agreed-upon methodology for other non-dispatchable resources. For example, the utility-wind industry 9 relationship shows a fair degree of acceptance for effective load-carrying capacity.

Appropriate capacity credits for PV might determine whether PV power generation will continue to be considered an energy-only resource. But research demonstrates that metrics can quantify the effective capacity of PV, providing the information utilities need to integrate significantly more solar energy into their resource portfolios.



1. Perez, R. (2006), Integration of PV in Demand Response Programs, NREL subcontract # AEK-5-55057-01 Final Report, and Proc. New Energies Symposium, Albany Nanotech, June 2006.

2 . Perez R., B. Collins, R. Margolis, T. Hoff, C. Herig J. Williams and S. Letendre, “Solution to the Summer Blackouts -How dispersed solar power generating systems can help prevent the next major outage,” Solar Today 19,4, July/August 2005 Issue, pp. 32-35.

3. T. Hoff, “Calculating Photovoltaics’ Value: A Utility Perspective,” IEEE Transactions on Energy Conversion 3 : 491-495 (September 1988).

4 . Garver, L. L., “Effective Load carrying Capability of Generating Units,” IEEE Transactions, Power Apparatus and Systems . Vol. Pas-85, no. 8, 1966.

5. T. Hoff, R. Perez, M. Taylor, and J.P. Ross, Photovoltaic Capacity Valuation Methods , Solar Electric Power Association Report No. 02-08, 2008.

6. Hoff, T., R. Perez, and R. Margolis, “Maximizing the Value of Customer-Sited PV Systems Using Storage and Controls,” Solar Energy 81, 7, pp. 940-946, 2007.

7. George, R., S. Wilcox, M. Anderberg, and R. Perez, National Solar Radiation Database (NSRDB) - 10 Km Gridded Hourly Solar Database , Proc. ASES Annual meeting, Cleveland, OH, 2007.

8. Photovoltaic Capacity Workshop, Solar Power 2007 Conference, Long Beach, California, Solar Electric Power Association (SEPA).

9. Milligan, Michael and Kevin Porter, “The Capacity Value of Wind in the United States: Methods and Implementation,” The Electricity Journal , March 2006, Vol. 19, Issue 2.