How much greenhouse gas does a solar power system replace?
The short answer is:
It’s really impossible to say for sure, but a good estimate is little under one tonne per kilowatt of nameplate capacity.
So a 3 kilowatt system will replace around 2.5 tonnes of greenhouse gases, possibly more. It depends on what your annual output is, and what the local mix of power sources is in your province/state/country.
The real answer is:
There is no simple answer to this question. The whole area of “carbon offsets” is full of claims that are hard to substantiate, and back-of-the-napkin math that doesn’t stand up.
The great thing about a rooftop solar power system, is that it introduces new electricity generation into the power grid. As more and more systems come online, they will replace the power that we currently generate from burning fossil fuels.
With tree planting, and other carbon offsetting techniques, you still burn just as much carbon as before. You just try to make up for it by doing something else as well. Clearly that’s not a long term solution to anything. But a solar power array actually reduces the amount of carbon going into the atmosphere.
Really? When you think about it, it’s hard to imagine that somewhere in Ontario there’s a power station operator ratcheting down their output by 3 kilowatts, because the sun is shining on our roof. But it’s not just our roof. As of April 6, 2010 there were over 8,400 microFIT solar installations either connected or in the planning stages. Once they all come online they will add over 76 megawatts of capacity to the grid. You can bet that will have an effect on the amount of fossil fuels Ontario needs to burn each day.
So there is a real decrease in greenhouse gas emissions that comes from even a small rooftop solar array. But how much?
The answer varies depending where you are. Because the mix of power sources varies from place to place. Fortunately, in Ontario, we have some very detailed information courtesy of the Independent Electricity System Operator, the organization that is responsible for running Ontario’s electricity market.
Bullfrog Power has done a lot of analysis of the numbers, so it can tell its customers how much greenhouse gas each kilowatt hour of electricity is worth. Bullfrog’s customers want to know this, because they have agreed to pay Bullfrog above-market rates for their electricity, on the assumption that Bullfrog will buy enough electricity from green suppliers in the Ontario market, to equal the usage of its customers. It’s a very, very, very, very indirect way of supporting green electricity generation. If you’ve got extra money to spend on extra-expensive electricity, put it into rooftop solar instead. It will have a bigger effect, and you’ll make some money too.
Anyway, because Bullfrog’s customers want to know how much carbon all their extra-expensive kilowatt hours are worth, Bullfrog has published all the details of their emissions calculations on their website. You can read details about their entire methodology, and download the spreadsheet with the raw numbers for Ontario.
Bullfrog does two different calculations. One calculation, called “footprint”, gives the overall emission levels from all sources in the entire grid. The other calculation, called “system”, figures out the emission levels of just the “on margin” power source, that is, the last power source to come online in the grid.
The last power source that came online is important for our purposes, because the whole environmental premise of having a rooftop solar array is that it will reduce the need for other power sources to come online during the day to meet demand. So knowing the emissions of the last power source that did come online will tell us how many emissions we have replaced.
According to Bullfrog, in 2010, one megawatt hour of electricity from the “on margin” power source is worth:
- 0.676 tonnes of carbon dioxide
- 1.908 kilograms of sulphur dioxide
- 0.877 kilograms of nitrogen oxide
For our solar array, that means one year of electricity (10,300 watts x 365 days = 3.76 MWh) will eliminate 2.54 tonnes of carbon dioxide, 7.17 kilograms of sulphur dioxide, and 3.3 kilograms of nitrogen oxide.
Is that a lot?
A few years ago, the government of Canada ran a program called the One Tonne Challenge, aimed at getting people to reduce the amount of annual emissions they were responsible for. The literature from that program claimed that the average Canadian was responsible for five tonnes of greenhouse gases each year.
So by that measure, two-and-a-half tonnes of carbon dioxide is a really, really, really significant reduction — fully half of a person’s individual contribution to climate change!
To put it another way, the Conference Board of Canada says that in 2005, we were approximately 9 tonnes per person above our Kyoto targets*. So if everyone in Canada reduced their emissions by 2.5 tonnes, that would bring us over a quarter of the way to meeting Kyoto all by itself.
*The Conference Board data says that Canada’s 2005 per capita emissions were 22.6 tonnes, 54% above our 1990 levels. The Kyoto target is 6% below 1990 levels. So some simple math tells us that our Kyoto target is approximately 13.8 tonnes per capita, approximately 9 tonnes below 2005 levels.