Thursday, 25 August 2016

Electricity is getting cleaner - should you heat or drive with electricity now?

Increasing renewable electricity (and less coal) in our mix is having a real impact on the carbon emissions from electricity. I used to say that electricity cost (roughly) three times as much as gas and generated three times as much carbon emissions - but now electricity is only twice as much as gas for carbon emissions. This has implications where you have a choice of fuels. For example, heat pumps for heating will now generate less carbon emissions than gas even if their performance is well below average. Also it increases the benefits of switching to electricity for cars. However, decisions made today are often based on out of date carbon factors.

Carbon emissions factors for grid electricity. DUKES is most up to date - DEFRA lags by 2 years (other differences are described below). Natural gas is added for comparison, based on gross calorific value (as appropriate for a condensing boiler)

Generation trends during 2012-2015 from DUKES table 5.5. Coal is rapidly replaced by renewables. Also the overall demand has reduced slightly from 363 TWh in 2012 to 339 in 3015.

DUKES figures show a 33% drop in carbon emissions from electricity since 2012
The top chart above shows changing carbon emissions from electricity over the last 7 years, compared with gas. The DUKES data series is the most up to date. It shows the actual power station emissions in the given year averaged over the total generation including nuclear and renewables. This shows a startling drop of 33% between 2012 and 2015. The carbon emissions fluctuate somewhat year to year due to changes in availability of nuclear power stations and the relative price of gas and oil as well as renewables generation. However, there is a clear trend downwards well beyond the normal variation. The second chart shows the rise in renewables and drop in coal use that has produced this very encouraging drop in emissions.

DEFRA figures, used for company accounts, lag behind DUKES by 2 years
In practice the DUKES carbon factors are rarely used. For example, companies that have to report carbon accounts use the DEFRA factors. These differ from DUKES in a number of ways, the most important being:
  • You are supposed to include the scope 3 emissions for transmission and distribution - this adds about 7%
  • Prior to 2013 the DEFRA figure was based on a 5 year rolling average
  • Since 2013, the DEFRA figure is based on one-year data but with a lag of two years (because they have to be published in time for the year ahead).
For 2015, electricity was 2.7 times the emissions from mains gas according to DEFRA but the latest DUKES data shows it was only 1.9 times as bad - even allowing for an additional 7% for transmission and distribution. The DEFRA figures are way out of date.

If we switched over immediately to electricity for heating and driving the grid could not cope.
These changes in electricity generation emissions have implications for our decisions on how to heat our homes and how to drive our cars. However, if we all switch over to electricity instead of gas or petrol/diesel we would not have enough generation capacity to handle the extra demand. Fortunately, that is very unlikely and the National Grid expects demand to increase only slowly.

Building regulations are based on out of date figures from 2012
In fact, unless building regulations catch up we may not switch over to heating by electricity at all. Building regulations require that new homes conform to target carbon emissions levels but the factors they use were last released back in 2012. At that time their assumptions found electricity emissions were 2.4 times those for gas. Heating systems installed now should last 25 years by which time electricity should be cleaner still. Building regulations should factor in these future expectations, to avoid locking us into fossil fuel technology for decades to come.

Factors for electric cars are hard to find
With electric cars the story is different and it is hard to get sensible figures at all. Pure electric cars are often advertised as having zero emissions which is only true if you consider emissions at the point of use - the actual emissions are at the power station. DEFRA gives emissions factors for other kinds of car including hybrids but not pure electric cars. For companies that charge their vehicles on their own premises, the electricity required will come on their bill but when vehicles are charged away from home this is usually not even measured. By my best estimates based on the experience of friends and DUKES carbon factors, an electric car has lower emissions than a similar diesel car (see Counting the benefits of electric cars) but this would not be true using DEFRA electricity factors.

Fuel price is not the driving factor either for cars or heating systems - so let's remember carbon factors .
So far this post has been all about carbon factors and has hardly mentioned prices. I have been criticised a number of times of being less green than I should be because I spell out the financial implications of green technology - so I have resolved in future to look at cost per unit carbon savings instead of payback times. However in this case neither approach applies. The choice of heating system installed in new homes is driven more by regulation than by price and the cost for electric cars is already similar to diesel if the lifetime costs are taken into account (also discussed in Counting the benefits of electric cars). So, let's just welcome the reductions in greenhouse gas emissions from electricity and factor these into our choices from now on.

Data sources
DUKES factors (Digest of UK Energy Statistics)
DEFRA factors (Government emission conversion factors for greenhouse gas company reporting)
Building Regulations factors from SAP 2012


  1. Hi Nicola,
    Thanks for this great blog.
    I remember reading that electric cars were better than the average fossil fuel car in David McKay's Without the Hot Air Book. But I couldn't remember what he said exactly. I've just looked this up and the key lines seem to be: "electric vehicles can deliver transport at an energy cost
    of roughly 15 kWh per 100 km. That’s five times better than our baseline
    fossil-car, and significantly better than any hybrid cars." I've always assumed this would outweigh the fact the UK grid has higher carbon emission / kWh than petrol itself. I haven't been through the detail that informs David's calculations and I assume much of it is out of date now but that hasn't stopped me using it when trying to defend using electric vehicles even now! I don't know the accuracy with which life cycle emissions for electricity versus petrol/diesel can be calculated but I've assumed that as petrol is heavy and transported around using fossil fuel powered vehicles (ships/trucks and then the car itself) that this would be a big inefficiency compared to the grid's lifecycle footprint. Does that sound right?
    All the best

    Link for Without the Hot Air E-car section

    1. The main reason that electric cars generate less carbon emissions than conventional is that electric motors are much more efficient than engines. MacKay says engines waste 75% of the fuel energy as heat and that sounds about right to me. Motors waste very little - less than 10% I think.

      I doubt petrol transportation is that significant as petrol and diesel are very energy dense (which is what makes them a convenient transport fuel in other ways).

  2. About 5 years ago the best diesel engines (e.g. golf blue motion) were still better than pure electrics on the UK grid carbon intensity, but the improved carbon intensity covered here means this is no longer true (almost certainly - I haven't actually redone all the numbers but it was quite close back then - a 33% intensity improvement will have made most EV's best than even the best diesels) A tesla may not be yet because it's a dirty great heavy car so has relatively high kWh/100km numbers.

    Clearly this situation will only improve, and if you are charging from your own PV a significant portion of the time then of course that makes your personal carbon intensity much lower. (UK solar being around 90gCO2/kWh - i.e about twice as good as gas, but still surprisingly high).

    Yes electric motors are 90-95% efficient. Petrol engines 20-25%, Diesel engines 25-35% (from memory, but I think those are about right).