Where Did All the Solar Go? Calculating Total U.S. Solar Energy Production

Michael Mendelsohn's picture

Good data can be hard to come by. Let's take solar production data. According to the Energy Information Administration (EIA), solar electricity production facilities— including photovoltaic (PV) and concentrating solar power (CSP) — produced a total of roughly 1,800 GWh in 2011 [1]. That's a 50% increase over 2010.  Even so, as a percent of total energy produced, the number is so small, in Figure 1, it's easy to confuse solar energy production with the horizontal axis.

Figure 1: Non-hydro renewable energy generation according to the EIA

This charts shows estimated generation as a percent of the total for multiple renewable energy systems from 1998 to 2012. Bioenergy, Geothermal, and Solar remain relatively flat while Wind increases significantly beginning in 2007. Solar is the smallest percentage and lies nearly flat on the bottom axis.

Source: NREL, adapted from EIA Electric Power Monthly

Problem is — EIA misses the majority of solar energy produced. The agency's numbers only capture facilities over 1 MW, and even then, likely miss production as system owners may not know of their obligation to report this information [2].

It's difficult to say what percent of total installed solar capacity is represented by systems greater than 1 MW. Reviewing data available from Solar Energy Industries Association (SEIA), I calculate only 32% of the PV capacity installed from 2008 thru 2011 was developed for the utility markets, which seemingly represent the majority of systems larger than 1 MW [3]. 

Table 1 represents an educated guess at how much solar electricity was actually produced in the United States in 2011.  According to SEIA, about 2,100 MW of PV capacity was installed by the end of 2010.[4]  An additional 1,855 MW was installed during 2011, but mostly in the last two quarters of the year.  Accordingly, the adjustment factor column in Table 1 represents the remaining portion of the year for potential production. As a measure of conservatism, installations were assumed to be made on the last day of each quarter.  The analysis also assumes a 17% capacity factor for PV and 20% for CSP, relatively conservative values [5].  Finally, 503 MW of CSP installations were operating in the United States in 2011 (none were added during the year).  Even under the conservative assumptions applied, 2011 solar generation was calculated at 4,737 GWh, about 2.6 times the EIA value.

Table 1: Estimated U.S. Solar Production
  Capacity Installed (MW) Assumed Capacity Factor Adjustment Factor Projected 2011 (GWh)
PV thru 2010 2,099 17% 100% 3,126
PV installed in 2011:
Q1 276 17% 75% 308
Q2 331 17% 50% 246
Q3 473 17% 25% 176
Q4 776 17% 0%
CSP 503 20% 100% 881
Total 4,458 18% 69% 4,737

Of course, the actual production of distributed solar systems is generally not publicly available. Actual energy production and system capacity factor are tied not only to system location, but also how the systems are wired, oriented, and maintained. NREL's PVDAQ system tracks a small subset of systems, but much more data is needed to fully understand production trends over time.

In fact, accurate production data will be an increasingly critical component, not just to better understand actual solar production, but to mitigate investment risk. A favorite topic of mine is securitization, the process of pooling projects and enabling investment through a liquid, tradable security. The process is similar to how a mutual fund pools lots of stocks into a single tradable product enabling investment by individuals.  Solar securitization could allow pension funds and other money managers to invest in the industry with the benefit of a diversified portfolio and a tradable, market-priced product—important criteria to investors who don't have the resources or risk capacity to evaluate and invest in projects individually.  But pension funds and other investors need data, and lots of it, to understand risk in aggregate—of system production, customer default, and other facets—if they are to invest in a security of this type.

Accordingly, NREL is endeavoring to improve the datasets available to the investment community.  If you are interested in the effort, in either the design of the datasets or in providing your data (in an aggregated or otherwise fully confidential manner), please let us know by providing a comment or contacting us directly.


[1]  EIA, Electric Power Monthly, April 2012.  See https://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_1_1_a

[2] Email correspondence with Christopher Namovicz, May 16, 2012. EIA collects data for Electric Power Monthly through Form EIA-923, a mandatory report for all electric power plants and CHP plants that meet the following criteria: 1) have a total generator nameplate capacity (sum for generators at a single site) of 1 megawatt (MW) or greater; and 2) where the generator(s), or the facility in which the generator(s) resides, is connected to the local or regional electric power grid and has the ability to draw power from the grid or deliver power to the grid. 

[3] See “U.S. Solar Industry Year in Review 2009.” SEIA 2010. See also “U.S. Solar Market Insight, 2011 Year in Review”, SEIA, 2012.

[4] “U.S. Solar Market Insight, 2011 Year-in-Review”. SEIA 2012.

[5] PV capacity factor represents an average of state-specific values per data from NREL's PVWatts program.  See https://pvwatts.nrel.gov/.  CSP capacity factor source based on actual 2011 production.  Source: Email correspondence with Christopher Namovicz, May 23, 2012.  Mr. Namovicz also indicated 2011 actual PV capacity factors for facilities tracked by EIA were approximately 24%, far higher than the 17% applied in this analysis.  However, the systems tracked by EIA are most likely to employ tracking technologies to increase system capacity factor.  CSP capacity factors can be significantly higher than the 2011 value (20%) applied, ranging from 25% for facilities without thermal storage to over 50% for facilities with thermal storage.  See Mendelsohn, et al. (2012), “Utility-Scale Concentrating Solar Power and Photovoltaic Projects: A Technology and Market Overview”, NREL, April, 2012.