Another day, another downpour is the new saying in the ech2o office. Still, every cloud has a silver lining and for me it’s the fact that the dismal weather means I haven’t yet had to brave the cold shower experience and instead can resort to scientific calculations as to whether electric showers or gas showers are better for the environment. So, good for me, less good for some of you as you battle with the seeming inconsequentials of a one hundredth of a kg of CO2 here or there.

To start at the beginning, in the UK for every kWh of grid electricity used in a building, 0.525kgCO2e have been produced whilst for every kWh of gas used in a building, 0.184 kgCO2e result1.Carbon Trust Conversion Factors Document. So far so straightforward.

But then things start to get complicated. We have to put the gas through a boiler to heat the hot water so that starts to increase the amount of CO2 produced for every kWh of heat. A gas condensing boiler works at 90% efficiency, which means that 90% of the gas burnt is turned into useful heat, resulting in a CO2 load of 0.204 kgCO2/kWh of heat. However, that is still 2.5 more times carbon efficient using a shower where the water has been heated by a gas condensing boiler rather than using an electric shower. Of course these calculations (and the ones below) assume the same amount of time in each shower.

But what if you are still heating your water using a non condensing boiler? There are some boilers out there (mostly permanent pilot ones) that are working at 60% efficiency at best, though thankfully they are few and far between. But there are a lot of D rated boilers working at 78-80% efficiency. The CO2 emissions from a boiler at 78% efficiency are 0.236kgCO2/kWh. That is worse than a condensing boiler obviously but is still 2.2 times more carbon efficient than using an electric shower.

But now we need to consider flow rates as in the paragraphs above we are not completely comparing like to like. If you want a decent hot shower from an electric shower I think you are struggling to get much more than a 5 litres/minute flow rate. Whereas most showers heated by gas will be delivering a flow rate between 6 – 15 litres/minute. Comparing a shower heated by gas to an electric shower with a flow rate of 5 litres/minute, then if the flow rate from your gas heated shower (condensing boiler) is less than 12.5 litres/minute it is more carbon efficient than an electric shower, and for a D rated boiler the equivalent figure is less than 11.5 litres/minute.

I could go on as there are plenty more variables to consider, and you cannot imagine how much fun I have just had doing these sums, but I will save them for a future blog.

And, I promise, no maths next month!


  1. Although the figures are now stated in kgCO2e, for ease (if not scientific accuracy) I shall continue to refer to emissions as CO2.
  2. Although the flow rate can be as low as 1.2 litres/minute (see my Jan blog!) to a ridiculous 25+ litres/minute.
June 2012 – Gas or electric showers? The definitive answer…