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. I believe such changes will not change my 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, indicates that for a dark red area a panel surface with a 1 kilowatt rating will produce 6.57 kWh per day (El Paso, Texas) , a slightly red yellow area, 4.10 (southern Italy, St. Louis), a yellow area 3.56 (Cleveland, Lyon, France) and a mid-green area 2.74 (London). I checked these numbers for El Paso, Texas: in summer 7.82 kWh, in winter only 4.32 kWh, which 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.

CO2 emissions from solar panels:

Online you will find varying statements of the lifetime CO2 per kWh from the manufacture of a 1kW solar panel (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).

The CO2 value per panel should be increased by 15% compared to the above reference, because China uses far more coal. ², 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, panel name-plate efficiency has increased from 14% in the study to 22.4% for new, now prevelant, monocrystalline panels , so a reduction of CO2 used muat 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. This is a year-round average. In summer the CO2 per kWh is roughly 20% less, and in winter roughly 30% more.

CO2 savings using Rooftop Panels without batteries:

For the calculations that follow I assume that energy use by the grid while the sun shines is 30% more than when it doesn’t shine. This varies by location and season⁴, and of course on some days there is no sun at all, but this assumption will suffice here for these ballpark calculation. The energy available from the grid is 24 kWh per kW. Subtracting the solar hours from 24 and reducing the result by 30% we for El Paso we have 12.2 hours of grid use; for St.Louis, 13.93; for Cleveland 14.3; for London 14.8.

There are two scenarios: 1) The grid has flexible fossil fuel use, such that it can use the surplus solar energy generated in summer and supply the energy needed to fill the solar deficit in winter.. In this scenario, the entire amount of solar electricity generated by paneos results in fossil fuel saved. So, using yearly averages, for El Paso, for the percentage relative percentage of CO2 emissions we have (6.57 hours solar x 28 CO2 emitted per kWh + 12.20 hours x 330 grid CO2 emitted) divided by all grid 6.57 +12.20 x 330) =68%. In other words, a savings of 32% of CO2 emitted. This is better than no savings, but only a very small contribution to slowing global warming.

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

2) If the grid cannot accept surplus electricity from the panels — for example if the utility has reached its limits on accepting non-baseload power — the savings will be less than calculated above— unless the system much less than a home needs and there is never wasted energy when the sun shines.

CO2 savings using Rooftop Panels with battery support .

This can be a much more expensive alternative, as more than 3 times as many panels must be purchased to get through the average winter night in El Paso, five times as many in London, as well as roughly 20 kWh of batteries per kW solar faceplate, costing about $200 per kilowatt hour plus installation. As noted above, panels produce only 55% as much energy in winter, but usage is about 75% of summer on average in the USA or Western Europe (a greater percentage in snow country). For the environment, the best CO2 savings will occur if 1.3kW of panels are used rather than 1.0 kW for average conditions, but 1.5 is better, so that there is no need to draw on the grid on winter days with sub-par sun, (and this will have the additional advantage of not stressing the panels at the mid-day peak in summer. To store the greater summer load, the batteries need to be sized for summer, rather than average conditions, so 1.2 times as many batteries should be used, but 1.5 is better because there is a sharp peak in amperage about noon.

In summary, for the best CO2 savings we must take the calculation for panels and batteries needed based on average conditions and multiply by 1.5.

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 12 kWh battery. (See long analysis here: https://yourenergyanswers.com/environmental-impact-solar-batteries/.)

The recommended maximum charge is 80% and deep discharge is to be avoided. Therefore, for El Paso 1storage is recommended. This will emit roughly 4,086,000 grams of CO in production. Although 5,000 cycles is possible under the best conditions with the best batteries, considering power storage degradation, fluctuating charging and system losses, a calculation using 3000 cycles is reasonable. 5,000 cycles are possible under the best conditions, but 3,000 cycles is more likely, which gives an allowance for inverter and other losses as well. So we have 681 grams of CO2 per cycle = per day. Multiplying by 1.5 we have 57 grams per 12kWh, and dividing by 80% we have 71 grams of CO2 per 12 kWh storage under average of the year conditions.

In El Paso we need average storage of 12kWh, and 1.5 solar panels (for a 1kW system) So we have 71 x 1 battery + 28 x 1.5 = 99 grams of CO2.

Thus, in El Paso, the total CO2 emissions for a 1kW on average system is roughly 99 grams of CO2 per kWh.

However, the system will not always operate. El Paso has 43 days of precipitation per year and 25 very cloudy days. Arguably, therefore, there are 65 days with 330 grams of fossil fuel. However the sun availability data takes these into account. So we should increase the solar incidence by 365/300 = 21%, which reduces the panel CO2 use per kWh to 22 grams, the system to 93 grams. Therefore the annual CO2 emissions per kWh are (65 x 330 plus 300 x 93) / 365 = 42330 or 135 grams of CO2 per kWh. That is a saving of 59%.

In southern England, in winter, it will take roughly twice as many panels to charge the batteries in winter on days with son and 50% more battery storage. It seems there has been no sun to speak of for one-half of all days (and stronger sun when there is, which is why I have only doubled the panels instead of tripling them. A rough guess is (182 x 330 + 212 x 200) / 365 = 281 or a savings of CO2 of 15%. This is slightly better than no batteries, but not worth the investment

Because more panels are need that operate less, we have approximately 160 grams of CO2 embedded per kWh for a battery solar system. The panels have no almost no sun for half the year, so an average of 245 grams of CO2 per kWh is used altogether, or a savings of about 25%. Without subsidies such a system is uneconomic.

Except , in locations off the grid for minor uses, complete solar independence is uneconomic, and would cause more emissions of CO2 than does partial independence or fossil fuels— because a great number of panels and batteries are required.

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 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