How much CO2 does solar electricity save?

The calculations below are complicated. The results are found on my page What must we do to reduce the rate of warming. Please feel free to modify the results if you believe I have made an error in the calculations. I believe such changes will not change the conclusions on the above-mentioned page.

The dark red area are the best for solar panels. The legend for the above map, divided by 365 days in a year, indicates that for a dark red area (El Paso, Texas) a panel surface with a 1 kilowatt rating will produce 6.57kilowatt hours per day , a slightly red yellow area (southern Italy, St. Louis), 4.10 kWh per day, a yellow area (Cleveland, Lyon, France), 3.56 kWh per day and a mid-green area (London), 2.74 kWh per day. I checked these numbers for El Paso, Texas. In El Paso, summer kilowatt-hours were 7.82, in winter only 4.32. That is 55% of the summer amount; for Boston, the kilowatt-hours per day were 22% higher in July and 33% lower than they early average in December, so again the lowest month was 55% of the highest. I will use this ratio in subsequent calculations.

This page considers first: solar panel emissions; then rooftop solar panels without batteries; then roof top solar panels with batteries; and finally, utility scale solar power.

CO2 emissions from solar panels and their cost per kWh:

Online you will find varying statements of the amount of CO2 per kWh from the manufacture of a 1 kilowatt solar panel. ( 1 kW of solar panels equals 2.5 average residential panels of 2 square meters): 60, 50, 40, and one at 6.

I finally located the old unharmonized data and the method for harmonization¹: Lifetime 30 years, 75% efficiency for rooftop mounted, monocrystalline nameplate panel efficiency 14% (average overlifetime 13%), solar strength 1,700 (4.65 per day), harmonized result 45 grams CO2 per kWh (median result 60 g/kWh).

This old result needs to be updated: The CO2 value per panel should be increased by 15% compared to the above reference, because China uses far more coal than the Europe manufactures did, ² and China accounted for nearly 78% of all panels. Coal emits double the CO2 of natural gas and 30% more CO2 than oil.³ On the other hand, the efficiency of new monocrystalline solar panels has increased from 14% in the study to 22.4% , so the amount of CO2 used must be reduced by 37.5%. The net effect is a 28% reduction, or about 32 grams of CO2 per kWh where the annual solar strength is 1,700 kWh (4.65 kWh per day).

All panels are believed to have a 25-30 year life, so a panel that produces more kWh in its lifetime obviously has a lower CO2 emissions rate per kWh. Thus, in El Paso, Texas, a panel has embedded CO2 of about 22 grams per kWh, while the one in England has about 58 grams per kWh.

According to Forbes good panels are at least $2.40 a watt installed, so that comes to $2,400 per KW, so for El Paso we divide by 30, 365 and 6.57 kwh per day, or a cost of 3.3 cents per kWh, and assuming some maintenance cost, about 4 cents a kWh. For London the calculation gives 9 cents a kWh.

CO2 savings using Rooftop Panels without batteries:

For the calculations that follow I assume that home energy use while the sun shines is 30% more than when it doesn’t shine. Most homes do not use as much energy when people are asleep. This varies by location and season⁴, and of course on some days there is no sun at all, but this assumption will do for these ballpark calculations. The energy coming over the electrical grid is always on, i.e 24 kWh per kW. Subtracting the solar hours from 24 to calculate grid hours and reducing the grid hours result by 30% there are for El Paso 12.2 hours of grid use; for St.Louis, 13.93; for Cleveland 14.3; for London 14.8.

Assuming that the grid will take any excess electricity generated by the panels, the entire amount of solar electricity generated by panels results in fossil fuel saved. Thus, using yearly averages, to calculate the CO2 savings for solar panels in El Paso, we have (6.57 hours solar x 28 CO2 emitted per kWh + 12.20 hours x 330 grid CO2 emitted) or 4026 units of CO2. If the energy was all from the grid, the Co2 emitted would be 18,77 x 330 or 6194. The CO2 savings are about 32% of CO2 emitted. This is better than no savings, but only a moderate contribution to slowing global warming.

For London the same calculation yields a savings of CO2 of only 13%. For southern Europe and the eastern United states, interpolating, the savings of CO2 may be about 20 percent.

If the grid cannot accept surplus electricity from the panels (for example if the utility does not allow it or has reached its limits on accepting power) the savings will be less than calculated above— because some of the electricity generated by the solar panels will be wasted.

CO2 savings using Rooftop Panels with battery support .

These calculations assume that the objective is to have complete grid independence on days when the sun shines. This is a much more expensive alternative that does not have a payback, but it saves more CO2 than solar panels used without batteries. In El Paso, to store enough electricity to get through the average winter night, at least twice as many panels must be purchased, while in London, five times as many panels must be purchased.

Enough batteries must be purchased to replace the draw of a battery free system from the grid. As an example of costs, a 13 kWh Tesla Powerwall battery and system in 2024 installed will cost about $12,000 per battery, so for two batteries per kW, $24,000. Tesla only warrants 70% capacity for the 10 year battery life. (The average American home uses 30 kWh per day , so 3 batteries would be required and more panels.) Dividing by 365, 10 and 19 hours of electricity used per day, we get a about 35 cents per kWh for the batteries in El Paso, perhaps 70 cents in London. The panels cost about 8 cents per kWh in El Paso, 20 cents per kWh in London. Total costs are approximately 43 cents per kWh in El Paso, 90 cents per kWh in London.

Lithium Ion Phosphate Batteries are the most economical a battery with the lowest CO2 emissions. It takes about 2,043,000 grams of CO2 to produce a 12 kWh battery. (See long analysis here: https://yourenergyanswers.com/environmental-impact-solar-batteries/.) I make no additions to this for the CO2 embedded in the inverter and other electrical components.

The calculations depend upon the system and the number of batteries. I am using a Tesla Powerwall as an example. Using the above numbers , since El Paso needs on average 12 kWh of storage per day per kilowatt-day use, but 30% more in winter, we need 2 batteries per system, so the system has caused the emission of roughly 4,086,000 grams of CO in production. The battery is good for 10 years, so we divide this by 10 and 365 for grams of CO2 per day and by 19 for KWh used per day, which equals 93 grams of CO2 per kWh.

In El Paso we have a subtotal solar system emissions 93 + 2 x 22 = 137 grams of CO2/per kWh. However, grid energy is still needed, as there are days without sun. El Paso has 43 days of precipitation per year and 25 very cloudy days. Arguably, therefore, there are 65 days where 330 grams of fossil fuel emissions occur. However, the insolation data takes these into account. So arguably we should increase the solar incidence on days withoug rain or heavy clouds by 365/300 = 21%, which reduces the number of panels needed but not the number of batteries. So we have system CO2 use of 93 grams per kWh for the battery and 36 grams for the panels for a total of 129 grams. Therefore, in El Paso the annual CO2 emissions per kWh are (65 days x 330 grid CO2 emissions plus 300 days x 127 solar system emissions) / 365 = or 163 grams of CO2 per kWh. That is a saving of 49% compared to natural gas.

CO2 savings with Utility Scale solar power.

An extensive review of utility scale solar is here⁵:

Utility solar power is produced with normal panels, or by focusing the sun with lenses or mirrors, and collecting the energy either with solar panels or with with molten salts that produce steam to drive turbines. Installations can store the electricity in batteries or by means of the heated salts.

Many recent versions of these thermal plants with day-long storage produce about 35 grams of CO2 per kWh, which is much better than rooftop solar in full sun locations. This obviously does not count the days with no sun. And the storage technology has to be much cheaper than lithium phosphate batteries.

Above: Solar Farm of 250 Acres in Datong, China, Image source: Forbes.

The 1 square kilometer (250 acres) Chinese array of panels pictured above, apparently without battery backup, is rated at 100 megawatts⁶. Hence it is 10,000 square meters per megawatt, which produces 6.57 average mWh per day, and would provide 59,951 mWh of electricity over 25 years. Hence roughly 16,700 square meters are required per 100,000 mWh.

Below: Gemasolar power plant

For a mirror molten salt system, the Gemasolar plant near Seville is interesting: https://www.renewable-technology.com/projects/gemasolar-concentrated-solar-power-seville/. Using heliostats, it has 24 hour storage, produces 19.9 megawatts for each of 24 hours on 270 days a year, and saves about 30,000 tons of CO2 per year. I calculate the stated savings as 171 grams per kWh, which suggests emissions of about 160 grams of CO2, if the comparison was combined cycle gas. This is just over a 50% savings. The Gemasolar plant cost $358 million dollars In equivalent 2020 US$, which works out to about $ 013 per kWh. It occupies 1.85 square kilometers. So we have 19.9 x 24 x270 x 25 or 3,223,800 mWh over an estimated 25 year lifetime., or 57,000 square meters per 100,000 megawatt hour