Enphase, who make the microinverters I use and monitor their output, reported to me
Enphase Energy maximizes your solar energy production and keeps you informed about your system. Your monthly energy report shows how your system performed and how much you contributed to offsetting the global carbon footprint.
Week Peak Power Energy Produced 12/01/2015 – 12/07/2015 925 W 24.7 kWh 12/08/2015 – 12/14/2015 887 W 23.3 kWh 12/15/2015 – 12/21/2015 928 W 22.5 kWh 12/22/2015 – 12/28/2015 1.05 kW 26.1 kWh 12/29/2015 – 12/31/2015 873 W 13.4 kWh December 2015 Total: 110 kWh Previous Month Total: 140 kWh Year to Date: 1.00 MWh
For more details on these production results, please visit your Enphase® system.
Your Carbon Offset for this month: 168 lbs
You have offset the equivalent of: 2 Trees
The total energy generated seems correct, but the carbon offset is clearly wrong. PG&E reports emission factors in the range 0.391–0.641 lbs CO2/kWh (or 0.177–0.291 g/Wh) for their electricity generation, depending on the year. [https://www.pge.com/includes/docs/pdfs/shared/environment/calculator/pge_ghg_emission_factor_info_sheet.pdf]
That means that my generating 110kWh reduces CO2 emissions by 43–70.5 lbs (19.5–32kg), not 168 lbs.
Enphase’s computation using 1.53 lbs CO2/kWh is quite high—not worst case (lignite coal is 2.17 lbs CO2/kWh [https://www.eia.gov/tools/faqs/faq.cfm?id=74&t=11]), but much higher than the US average of 1.222 lbs CO2/kWh used by carbonfund.org [https://www.carbonfund.org/how-we-calculate].
Of course, household solar energy production in the summer mainly affects peaking, not baseload power generation, and that averages about 0.689551 g/Wh across the US [http://www.epa.gov/energy/ghg-equivalencies-calculator-calculations-and-references]. With that number, 110kWh would be 75.85kg CO2 (167 lb), so that is what Enphase is reporting—the largest number that they could conceivably justify.
But in the winter, mid-day is not peak power usage—early evening is in California, so it really isn’t justifiable to use the national, year-round average peaking generator CO2 emissions for California winter solar generation. The annual non-CO2 emissions for non-baseload power in California is only 1.01887 lbs/kWh (0.462 g/Wh) [http://www.epa.gov/sites/production/files/2015-10/documents/egrid2012_ghgoutputrates_0.pdf], which is the largest number that Enphase could honestly claim in the summer (probably still high for PG&E, which uses a smaller proportion of fossil fuel generation than most California utilities).
Enphase is not the worst offender in inflating carbon offsets—the “Cool Campus Challenge” at UC had some really ludicrous estimates of how much CO2 would be reduced by various changes in habit. For example, they seemed to assume that turning off a computer monitor when not in use would save 200lbs of CO2 a year, which for PG&E electricity would be 300–500kWh/year, or 36–58W. But most monitors drop into a power-save mode after being idle for half an hour, taking less than 1W, so a more honest estimate of how much CO2 would be saved is around 15kWh, or 6–10 lbs CO2. Their estimates of 50lbs of CO2 reduction for unplugging vampire loads was similarly ludicrous—most of the power supplies in my house meet at least Level IV efficiency standards, with “vampire” loads under 0.1W. Unplugging all of them would save maybe 1w, or 3–6 lbs CO2.
I wish that people who provided carbon offset calculations would be more honest about them. It does no one any good to think that small symbolic gestures do much to reduce their carbon footprint.
One thing that is a little more than symbolic—closing my laptop so that it sleeps reduces its power consumption by 20W, so closing it for 12 hours a day saves 88kWh/year, or about 34–56 lbs of CO2.
But almost anything I do with electricity reduction is mainly symbolic, as my biggest carbon footprint is usually for heating my house (and hot water) with natural gas: about 433 therm/year or 2300 kg CO2 (5100 lbs) [http://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator].
I expect this year to make a couple of family trips, for a total flight distance of around 14000km, adding another 2500 kg CO2 this year [https://en.wikipedia.org/wiki/Environmental_impact_of_aviation#Greenhouse_gas_emissions_per_passenger_kilometre] I may do another 2000 km of Amtrak train rides (about 70–90 g/km [European figures for diesel traction from http://www.uic.org/com/IMG/pdf/iea-uic_2012final-lr.pdf]) for another 140–180 kg CO2.
Even as a bicyclist without a car, transportation contributes a lot to my carbon footprint—more than I expected.