There are various dashboards (for example, here) that show the current energy generation mix and the overall CO2 emissions (gCO2/kWh).

In the UK renewables can make up a large proportion of the total, but it is never 100% - there is always some fossil fuel generation (from natural gas and occasionally coal).

Naively, I feel I should charge my electric car when the CO2 emissions are low (typically on days when wind generation makes up a greater proportion of the total energy mix).

And a recent article in the New Scientist (Smart Scheduling for Big Computing Tasks Cuts Emissions Up to a Third) also concluded that CO2 emissions are lower if you plug in when renewable sources account for a greater proportion of the energy mix.

On the other hand, plugging my car into the grid presumably increases the load, which will be supplied by non-renewable generation (since renewables don't yet cover 100% of demand).

So, it seems to me that maybe it's worth avoiding times when coal-fired power stations contribute to the mix, but otherwise it perhaps doesn't matter whether gas supplies 10% or 90% of the total: the power needed to charge my car will always come from burning more gas.

The question: can I reduce my CO2 emissions by charging the car when renewable energy generation makes up a larger proportion of the mix? (Knowing that renewables never contribute 100% and the shortfall is covered by burning fossil fuels).

  • 1
    $\begingroup$ So do you have your own pv panels or wind turbine? That would make a difference. $\endgroup$
    – Solar Mike
    Jan 26, 2022 at 8:45
  • $\begingroup$ Related (from Sustainability.SE): Is electricity used at night more likely to be renewable? $\endgroup$
    – LShaver
    Jan 26, 2022 at 17:19
  • $\begingroup$ Based in the UK. I've edited my question to try to make it clearer. $\endgroup$
    – Gransden
    Jan 27, 2022 at 8:19

4 Answers 4


tl;dr: Very windy days might be worse but the data isn't great. Charging in the late afternoon is your best bet to reduce marginal emissions.

From the question:

On the other hand, plugging my car into the grid presumably increases the load, which will be supplied by non-renewable generation (since renewables don't yet cover 100% of demand).

What you have described is the marginal energy -- basically, which power source will be adjusted in response to a change in demand. This is determined by a complicated mix of physics, engineering, market forces, and, of course, weather, however by analyzing historical data it's possible to determine how this varies with time, without needing to understand all the complicated factors that influence it.

The paper "Marginal greenhouse gas emissions displacement of wind power in Great Britain" provides an analysis of the marginal emissions factors considering both time of day and month of the year.

The charts below show the marginal emissions factors (MEF) for 2009 (in red) through 2014 (in black). You can see that as wind power capacity is added to the grid over time, the overall MEF has dropped, but there are still daily and seasonal variations.

Diurnal and seasonal marginal emissions factors in the UK, 2009 to 2014

Specifically, it would be best to charge during the late afternoon in December, when the MEF is lowest.

Another interesting result from the paper is that as the wind output to the grid in any moment increases, the MEF also increases:

Marginal emissions factor as a function of wind output in the UK, 2009 to 2014

This actually indicates that charging could be best when there's a medium amount of wind -- not on the windiest days. The paper speculates that the reason for this trend is that high wind output correlates with high system demand, meaning that most gas plants are already online, and a higher proportion of marginal energy is actually coming from coal plants.

However, this paper is from 2014, and there have been significant changes to the grid makeup since then. From UK Energy Brief, 2021, you can see that coal capacity ("conventional steam") has reduced by nearly half, while renewable (mostly wind) has nearly doubled.

UK electricity capacity, 1996 to 2020

Further, the UK government forecasts the long-run marginal emissions factor for the overall grid, out to 2100. "Long-run" just refers to a permanent change, more useful for long term planning and analysis. The data is available here: Green Book supplementary guidance: valuation of energy use and greenhouse gas emissions for appraisal . I put together a chart showing how the emissions factor is expected to change over time:

UK long-run marginal emissions factor over time

From this we can see that as renewable penetration on the grid increases, the timing of when power is used becomes less and less important; by 2040 or so, it won't really matter at all.

Unfortunately I wasn't able to find any updates to this research since 2014, and the National Grid's very fancy carbon intensity API doesn't account for marginal emissions at all. But there is some other related research:

  • $\begingroup$ Thank you for the reply - an interesting and educational read! $\endgroup$
    – Gransden
    Feb 15, 2022 at 19:28

TL;DR: It is a difficult juggling act to know when to plug in the car. It depends on the energy mixture of both the fossil fuels and the renewables.

First of all an interesting read would be the following question and answers, in order to give a brief intro how does electricity generation works -- if you are not familiar with.

Additionally, yes it makes sense to plug in your car on windy days (on the wind farms) if you can afford it. However, obviously that is not always possible.

IMHO, what is very important to understand is that different fossil fuel generators have different response times. In general power plants (not only fossil fuels) can be categorised as:

  • base load: these are plants that tend to operate at maximum output. They usually take a long time (a day or two to power up and shut down), and they have better fuel efficiency.
  • peaking power plants have slightly faster response times ( a couple of hours) and they are dispatched during times of the day with higher loads (e.g. summer midday with air conditioning).
  • load following power plants: these are the most flexible (and usually the least efficient). They are brought online or are adjusted constantly by the electric company grid operator. (usually this includes gas turbine, hydroelectric)

The following image presents the ramp rates (the percentage increase in power output per minute).

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Figure: generator technologies ramp rates (source: Cathreine Linsday Anderson

So, the most efficient is a difficult juggling act because it would depend on the mixture of renewable energy. From what I gather you live in the UK, where solar hasn't really caught up compared to other countries. However, Wind has the benefit that it may be producing all day long, compared to solar that has a more predictable pattern (barring cloudy skies).

If solar is the main mixture, then you probably would benefit the environment to plug in the car during the time that the solar is producing (that would take the load of the load following power plants). However, if everybody plug it in at the same time that would create a problem (this actually might be a grid management problem in a foreseeable future that includes massive fleets of electric cars).

On the other hand when wind is the primary source of renewable energy, then (as mentioned before), energy may be produced throughout the day. However, the problem is that it is unpredictable (sidenote: in my native language wind is called "ανεμος", which can be though of as the thing that does not follow a law, which shows the unpredictability).

Also, it is interesting to note that sometimes, when there is excess energy, wind farms can act as a type of "consumer" instead of producing energy. That is primarily done so that the frequency stability is maintained.

  • $\begingroup$ In the UK, I think there's a significant fourth category: a couple of large natural gas CHP plants that are turned on and off in response to process heat demand unrelated to the electricity market, and need to dump their "waste" electricity to the grid. $\endgroup$ Jan 27, 2022 at 12:58
  • 1
    $\begingroup$ BTW, "ανεμος" turns up in English in the word "anemometer". $\endgroup$ Jan 27, 2022 at 13:47

Plug your car when energy prices are at lowest....The cheapest electricity can be found during “off-peak” hours when demand is low. For example, on the East Coast, summer off-peak hours might be from 8pm until 3am, summer nighttime temps tend to be lower


You are assuming a very unlikely scenario, a network so tightly organized that there is not enough wiggle room to accept your tiny draw on the network.

A more plausible assumption is that the system has built-in resilience, that is already set to take small fluctuations, only the really huge peaks in the network will trigger auxiliary generators to kick in.

Thus your original idea makes sense and you better charge your car on windy days.


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