Solar Irvine: The numbers
Gianna Lum, Ben Leffel and I recently published an Op-Ed in the OC Register describing that the solar energy resource exists to power all of our City’s residents’ energy needs right from our own home’s rooftops. (Note: the OC Register article had a typo sneak through, and where it’s written EJ, exajoule, the unit is PJ, petajoule. The calculation and match between solar and need is correct.)
Here is an outline of the numbers that go behind assessing the match between Irvine residents’ energy usage and the solar resource on rooftops.
Usage
First, let’s tackle electrical energy. We base the calculation using Irvine’s population as reported by census data to be 277,453 from 2017. This has shifted up 1.8% in recent reports, but this is a small change that does not affect our conclusions. Orange County has a total annual electrical energy usage of 6745 GWh (California Energy Commission). Given OC’s population of 3.19 million people, this works out to 2,114 kWh per year per person.
So, the electrical energy need for one year is approximately 2.1 PJ.
Next, let’s estimate natural gas energy usage. California natural gas utilities report residential per day natural gas usage at 1160 MMcf/d for the entire state (pg. 31, California Gas Report). This amounts to the following kWh/yr per capita:
And, multiplying by the population of Irvine and converting to petajoules, as above, we get 3.2 PJ per year for all of Irvine’s natural gas energy usage.
Last, but not least, is personal vehicle transportation usage of residents. The Federal Highway Administration reports 8572 miles/year/person for California, and we take Irvine to be typically Californian. A typical electric car requires 28 kWh per 100 miles.
And, again multiplying by the population of Irvine and converting to petajoules, we get 2.4 PJ per year for all of Irvine’s residential vehicle usage.
The total annual energy need for the City of Irvine’s residents is then 7.7 PJ.
Solar Resource
The rate of solar energy averaged over all year, all weather, and over day and night, in Irvine is 233 Watts per square meter, according to the National Renewable Energy Laboratory of the Department of Energy. The number of homes in Irvine is approximately 101,592 (City-Data.com). Average home size in the U.S. is reported to be 201 square meters (2164 sq. ft.). Let’s say all homes in Irvine are two stories, a conservative assumption, so that half that area is then covered by a roof. Total residential roof area in Irvine is then 10.2 square kilometers (2,520 acres!). This provides the following amount of energy per year, using typical photovoltaic efficiencies of 15%:
which is more than enough to power all energy needs. This actually leaves about 32% of roof space for mounting area and hardware.
