Welcome to this edition of Net Zero by Narsi, In which I provide insights and updates on decarbonization and climate action, trying my best to make it interesting and useful - and wishing I don't sound pedagogic and conceited.
Until 2000, there were not too many rooftops worldwide with solar panels - perhaps a couple of tens of thousands - and even these were mostly concentrated in Germany and Japan.
In 2026, there are about 100 million residential rooftops worldwide sporting solar panels.
A phenomenal growth.
There is little doubt that having solar panels on your rooftop provides an excellent avenue to cut your personal carbon emissions.
But by how much do the solar panels on your rooftop decrease your electricity carbon footprint?
Interestingly, the answer to this question is a bit more nuanced than you think, and in some cases, even surprising.
I went into a deep dive into this, and it was quite a bit of fun.
I hope this somewhat longish article - the result of the deep dive - is useful to you in getting a perspective on how carbon footprint reduction from the use of renewable sources could vary quite a bit, depending on...
OK, let me maintain a bit of suspense and start the story.
Basics and variables
The following are the variables involved in determining how much you reduce your carbon footprint through the use of rooftop solar panels, and also by what percentage:
- Amount of power used by you: Obvious!
- Amount of solar power you are consuming: What % of your electricity are you replacing with solar power? The answer will depend on how much rooftop area you have and how many kilowatts of solar panels you are installing on it. (1 KW of solar panels requires about 100 sq ft)
- Carbon footprint of grid power: We do not think much about the carbon footprint of our grid power. We simply assume that it must have a very high carbon footprint - else, why go solar? It is not such a cut-and-dried case, as you will see!
- Carbon footprint of solar power: The carbon footprint of solar power itself. Well, surprise, surprise, it is not zero, as some of you might have thought, especially as it is promoted as a no-emissions source!
Let’s consider each of the above in a bit more detail.
Amount of electricity used by you - Et
- This is the total amount of electricity used by you per month. Let us denote this by Et (in kWh)
- Of course, before installing solar panels, 100% of Et was from the grid.
Amount of solar power consumed - Es
- How much of your electricity is replaced by solar power? Depending on your rooftop area and also depending on your use of battery storage, you could be using solar power all the way from something like 10% of your total power consumption to 100%.
- Let’s denote this by Es (in kWh)
Carbon footprint of solar power - Cs
- Solar power might be advertised as a zero-carbon emissions power source, but technically, its carbon emissions are not zero. Why?
- When calculating the emissions of any product, one needs to consider its life cycle emissions, and not just emissions while it generates electricity.
- For a solar panel, its lifecycle emissions come to about 35 grams of CO2/kWh. This is the sum of emissions that add up all the way from producing polysilicon to wafers to ingots to cells and then finally, the solar panels. A large portion of the lifecycle emissions from solar power thus are from the manufacturing stage of its business value chain.
- Let’s denote the carbon footprint of solar power by Cs (=35 grams of CO2/kWh)
Carbon footprint of your grid power - Cg
- Here is where you are likely to come across a lot of surprises. Commonly, grid power is spoken synonymously with fossil-based electricity (coal or natural gas).
- But, the power that you use from the grid could actually be generated from a variety of fuels: Coal, natural gas, diesel (rare, only in remote locations and islands), nuclear, hydro power, geothermal, biomass power, wind power, or solar itself.
- Depending on the region you live in, the actual mix of sources that generate electricity could be quite different.
- For instance
- - France generates about 70% of its power from nuclear energy (and about 25% from renewable sources such as solar, wind and hydropower, with only about 5% generated from coal and natural gas!).
- - In the US, the picture is quite different. Just over 40% of its power is generated from natural gas, with nuclear supplying about 20%, coal about 15%, hydro about 5%, and wind and solar together about 17%.
- - Japan gets about 10%^ of its power from wind & solar, about 35% from coal, about 10% frm hydro, about 10% from nuclear, about 30% from natural gas.
- - India gets about 70% of its electricity from coal, 2-3% from natural gas, about 15% from solar and wind, 3% from nuclear, about 10% from hydro.
- You can see the significant diversity of energy sources for power generation across different countries.
- Each of the above sources has its own carbon footprint, and thus the carbon footprint of your grid power is a weighted average of the carbon footprints of the various sources that are used to generate the electricity.
- Let’s denote the carbon footprint of your grid by Cg (grams/kWh)
How much CO2 does each of the sources emit when they are used for electricity generation? The following metrics can be used (Grams CO2 emitted/kWh):
- Coal: 1000
- Natural gas: 500-600
- Solar, wind & hydro power: 15-50
- LCA carbon emissions for geothermal & biomass power can vary significantly depending on a variety of factors, all the way from 25-200 grams/kWh
- Nuclear: 15 grams/kWh
Wow, that's a lot of emissions difference between the various fuels.
Now, let’s calculate by how much you reduce your carbon footprint using solar power
To recap the variables:
- Total amount of electricity used by you (per month): Et (in kWh)
- Total amount of solar electricity used (per month): Es (in kWh)
- Carbon footprint of grid power: Cg
- Carbon footprint of solar power: Cs
The formula:
- Your electricity carbon footprint before solar panels (when you were relying fully on grid power): Cg*Et
- The carbon footprint after solar panel installation : Cg*(Et-Es) + Cs*Es = Et*Cg - Es*(Cg-Cs)
- Your carbon footprint reduction = Es*(Cg-Cs)
- % of carbon footprint reduction = Es*(Cg-Cs)/(Cg*Et)
Using the above formula, let’s see how much CO2 reduction you can have under different scenarios, in a practical example.
The following assumptions are applicable in all the scenarios:
- In this hypothetical example, your total residential electricity consumption before solar (Et) is 3000 units (kWh)/month
- For this example, let’s say solar power from your rooftop covers about 20% of your total consumption = 600 units (kWh)/month (Es)
- Solar power carbon footprint = 35 grams CO2/kWh (Cs)
Scenarios
What varies across different scenarios is the carbon footprint of the grid power (Eg).
Let's estimate the carbon footprint reduction under the different scenarios.
When your grid is entirely powered by coal: 19.3% reduction
- Had your grid power been mainly coal-based, its carbon footprint is 1000 g CO2/kWh (Cg)
- Your total monthly carbon footprint before solar = 1*3000 = 3000 Kg
- Your total carbon footprint reduction = Es*(Cg-Cs) = 600*(1000-35) = 600*965=579000 g = 579 Kg CO2/month
- % carbon footprint reduction: 579/3000 = 19.3%
When your grid is entirely powered by natural gas: 18.6% reduction
- Your grid is mainly powered by natural gas power plants, so carbon footprint = 500 g CO2/kWh
- Your total monthly carbon footprint before solar = 0.5*3000 = 1500 Kg
- Your total carbon footprint reduction = Es*(Cg-Cs) = 600*(500-35) = 600*465 = 279000g = 279 Kg CO2/month
- % carbon footprint reduction: 279/1500 = 18.6%
When your grid is entirely powered by hydro-power: An increase of 8% in your carbon footprint!
- Let’s say your grid is mainly powered by hydro power plants, whose LCA CO2 emissions are 25 g CO2/kWh (Cg)
- Your total monthly carbon footprint before solar = 75 Kg
- Your total carbon footprint reduction = Es*(Cg-Cs) = 600*(25-35) grams = 600*(-10)=-6 Kg CO2/month
- % carbon footprint reduction = -6/75 = -8%, it is actually a small increase in emissions, of about 4%!
When your grid is entirely powered by nuclear based: An increase of 27% in carbon footprint!
- Let’s say your grid is mainly powered by nuclear power plants, whose LCA CO2 emissions are only 15 g CO2/kWh (Cg)
- Your total monthly carbon footprint before solar = 45 Kg
- Your total carbon footprint reduction = Es*(Cg-Cs) = 600*(15-35) grams = 600*(-20)=-12 Kg CO2/month
- % carbon footprint reduction = -12/45 = -27%, wow, it is an increase of 27%!
So, here’s the summary of carbon footprint reduction under different scenarios for the above example
All data are monthly.
- Total electricity consumed (kWh): 3000
- Total amount of solar power consumed, out of the above (kWh): 600
Whew! While for a grid powered by natural gas or coal, your rooftop solar is contributing to some decent amount of carbon footprint reduction, if the grid is powered entirely by nuclear or hydro power, you are actually increasing your carbon footprint by replacing grid power with rooftop solar power!
Who would have thought!
Well, all right, the above is somewhat theoretical because there are very few large grids in the world powered only by one source/fuel. But still, it shows how your carbon footprint reduction could vary vastly depending on the dominant fuel being used for electricity generation in your region. And solar power does not ALWAYS mean carbon footprint reduction.
Let's get more realistic
Let's look at a far more realistic comparison. By taking the actual carbon footprint of grid power of various countries and estimating how much solar power could reduce this.
So we are getting more practical, OK?
The portfolio of feedstock/sources used for power generation differs from one country to another - and even from one region to another for some countries - and so will its grid carbon footprint. So, let's see how much your rooftop solar could reduce your carbon footprint for a unit of power it generates, for different countries.
(Solar power carbon footprint taken = 35 g CO2/kWh)
Well, there are some dramatic scenes being played out in the above table too, aren't there!
Massive variations of carbon footprint reduction across countries when a unit of solar power replaces a unit of grid power. Reductions from -133% (an increase!) to an exceptional 96%. Significant variations even within each country for different states (reductions of 73 to 96% across the USA).
So, the possibility that in some select cases you could actually be increasing your carbon footprint through the use rooftop solar to replace some amount of grid power is not entirely theoretical. Thankfully though, this will only be for a few countries or regions.
Now, as some of my learned friends will surely point out (especially for surprising cases such as Norway), a direct comparison of solar and hydro/nuclear carbon footprint per kWh is not the entire picture. As there is a pan-European grid, perhaps generating solar could free up hyrdo power to be sent to countries powered by high-carbon sources - a valid argument in spirit. Another argument could be that solar power could be generated during those small periods (typically peak periods) when Norway uses fossil fuels like coal - not sure about this, as Norway uses very little coal, apart from hydro it uses wind power to a large extent, and to smaller extent, natural gas and biofuels.
It's getting really intricate, so I leave the Norway verdict to experts who know a lot more than me. But surely, you will agree that the above table gives all of us something to debate!
Some notes on the above table:
- France: 40-50 g/kWh, the grid has a low carbon footprint due to the dominance of nuclear power in its mix, about 70% of the grid power is from nuclear. By the way, some reports from within France claim that their grid has a carbon footprint of only about 20 g CO2/kWh, though these did not provide a break-up. I estimated the number here based on a weighted average of their power generation sources and their corresponding carbon footprints.
- Norway: a super low 15 g/kWh, as the country relies almost entirely on renewable energy sources, dominated by hydro power. Now, some of you might wonder how Norway could have a grid power carbon intensity of 15 g/kWh when its main source, hydro power, has a carbon footprint of 25 g/kWh. The reason is because the Norwegian estimates give a very low estimate for the country's hydro power carbon footprint, less than 5 g/kWh. I'm going to leave this at this!
- India: 730 g/kWh, dominated by coal power, 70% of electricity from coal
Of course, there are always assumptions, notes and caveats!
- While nuclear or hydro power has a lower carbon footprint than solar right now, the future could be different as solar panels get more efficient and also are exploring the use of materials other than silicon (perovskite, for instance) as the main semiconductor. But the carbon footprints from nuclear and hydro power could also decrease in future owing to new technologies and processes.
- The other aspect that has not been considered in the above comparison are the losses associated with transmission and distribution of power through the grid, losses that are eliminated from rooftop solar.
And there are, of course, a few other benefits from solar that are not fully captured in the above analysis:
- A decrease in the roof temperature (and the temperature of the rooms below the roof) owing to having solar panels on top
- If rooftop solar power is used along with batteries to fully or partially replace diesel genset based backup power generation, this could result in significant CO2 savings - savings that are possible from any other power source that need the grid to reach the destination. But then, we need to factor in the carbon footprint of batteries - might not be a pleasant number, beware!
- Also, the carbon footprint of solar power has been taken as a constant 35 g CO2/kWh of solar power generated, but this itself could change from one place to another. In really sunny countries such as India where a panel could generate between 4.5-5 kWh/kW per day, whereas in Germany, this could be 3-3.5 kWh/kW per day, almost 30% lower than that in India. This of course will mean a higher carbon footprint for solar power generated in Germany compared to that for India, though it might not change the big picture dramatically.
Conclusion
While carbon footprint reduction from rooftop solar is indeed considerable for most regions worldwide
- There are considerable differences from one region to another
- For a small set of regions, the reduction might be much smaller than for most, and
- In a very few select regions, there could even be an increase in carbon footprint from the use of rooftop solar. But not to worry, these regions will be so few and far between (even perhaps on a map!), that they should not make much of a dent in the overall rooftop solar low-carbon business case.
So, what's the point of this entire post?
OK, so a house with rooftop solar panels in Norway might actually be resulting in an increase in carbon footprint.
So what! Especially given that Norway's total annual electricity consumption is just about 130 TWh (130*10^9 kWh), compared to a global total of 30,000 TWh - that's less than 0.5%.
Put another way, it is likely that solar power will contribute to significant CO2 emissions reductions for perhaps 99% of the grid power users worldwide.
Well, while the above is most likely the case, I should point out that hydro power and nuclear, the two sources that have considerably less carbon footprint over their lifetime, contribute about 7000 TWh per year globally, out of the total of 30000 TWh. Now that's almost 25%! Unit for unit, these sources emit (with a lifecycle analysis) much less CO2 than does solar. Something to much about, wouldn't you agree?
So, I would submit the following to be the points of this post:
- For folks in countries like India - or USA and Australia or China - you can go to be bed singing a happy tune.
- Folks who are putting up solar panels in places where the grid has a relatively low carbon footprint should feel quite happy too - you are still contributing to a low carbon world - but also appreciate that there are other sources such as hydro and nuclear that have been already doing a good job in this context for years.
- Investors keen on significant reductions in emissions reductions through investments in solar should also consider the current grid carbon footprint as part of their overall impact analyses.
I told you, I always have a point when I put in a post at Net Zero by Narsi - or I ensure I manufacture one.
References
Environmental lifecycle assessment of electricity from solar PV panel systems - Garvin Heath , Jose Bilbao
Lifecycle Carbon Footprint Assessment of Solar PV Systems in US & China - Jie Gao, Holly Emerson , Luana Marangon Lima
Notes
On the worldwide total number of rooftops with solar panels - an International Energy Agency (IEA) report from 2022 mentioned there were 25 million rooftops with solar and expected 100 million residential rooftops to sport solar panels by 2030. While there seem to be no authentic data on the total number of residential rooftop solar installations worldwide in 2026, I did some estimates based on individual contributions from various countries, and this seems indicate somewhere around 100 million residential rooftop installations right now, mid 2026. (Some parallel estimates I ran also suggest there could be about 2 billion residential rooftops worldwide, if that's of any interest to you :-)).