Wednesday, 23 July 2014

Electric bike vs. shower - which uses more energy?

Suppose you commute to work and you normally go by bike. Do you have a shower when you get there? If so you might save energy and carbon emissions by using an electric bike and not using the shower. From my calculations, a 2 minute electric shower uses more energy than an electric bike, if your journey is up to 8 miles.


Carbon emissions from riding an electric bike versus using muscle power and taking a 2-minute shower afterwards, either electric or with hot water heated by a gas boiler

First, how much electricity do you need for the bike? Judging by the specs [1] 1.4 kWh/100km is typical and this is close to one user's report which comes to 1.3 kWh [2] so let's call it 1.4 kWh/100km. At that rate a 5-mile (8 km) journey uses 0.11 kWh.

By the way, a litre of diesel contains about 10 kWh of fuel so 1.4 kWh/100km is 0.14 litres/100km or 2000 mpg.

Now suppose that if you cycle in on your normal bike, using your own muscle power only, you are sweaty enough when you get in you need a shower. Just a quick one - say 2 minutes. A typical electric shower uses 7.2 kW [3] so 2 minutes comes to 0.24 kWh. That is about twice as much as you would have used with the electric bike.

If it isn't an electric shower then you probably use more energy because electric showers dribble compared to normal showers - that 7.2 kW can only heat enough to supply at most 5 litres/minute. A 2 minute shower at 8 l/minute shower heated by a 90% efficient gas system would use 0.43 kWh. On the other hand the carbon emissions would be lower.

However, what about the embodied energy and carbon emissions? Electric bikes are heavier and making them uses energy and generates carbon emissions too. A study by the European Cyclists Federation [4] found that the carbon emissions associated with making and maintaining a normal bike come to 5g/km ridden whereas for a pedelec (an electric assisted bike) it comes to 7g/km, an extra 2 g/km. This means our 5 mile (8km) commute on the electric bike emits an extra 16g CO2e.

The chart and table below show the figures and what happens on longer bike rides. There is still a good saving on the 2 minute electric shower for a 5-7 mile commute.

ActivityEnergy (kWh)Carbon emissions (g CO2e) 
including extra embodied carbon for the bike
Electric bike 5 miles (8km)0.1171
Electric bike 7 miles (11 km)0.1598
Electric bike 8 miles (13 km)0.18116
Electric shower 2 minutes0.24119
Gas heated shower 2 minutes0.4379

If you commute 200 times in the year both ways the electric bike saves 19 kg CO2 compared to 2 minute electric shower but nothing to speak of against a gas heated shower. For a longer ride the gas heated shower is definitely better but even at 8 miles the electric bike uses less electricity than an electric shower.

Well you may argue with my assumptions here. The electricity used by the bike depends on hills and how heavy you are so I may be low or high there. I haven't taken into consideration the extra clothes washing but that is probably minor provided you wait for a full load. Perhaps 2 minutes is a rather quick shower, especially if you have long hair to wash. If you take longer that increases the bike savings. However, you could always have a Navy shower - wet yourself then turn the water off while you soap, then rinse off. That would pretty much wipe out the electric bike savings leaving you no excuse not to use muscle power.


[1] Electric Bike Review - I looked at the Motiv Spark, Prodecotech Storm 500, Pedego Classic Comfort Cruiser and Evelio Auroro. Judging by the battery spec and the range (using the lower figure for the range) they come to about 1.1 kWh/100km. Divide by 80% to allow for charging ineffeciency and you get 1.4 kWh/100km
[2] Energy Efficient Electric Bikes (first comment, not article body) (NyceWheels)
[3] From my analysis of showers for the Household Electricity Survey
[4] New study compares bicycling’s CO2 emissions to other modes (BikePortland.org). This reports 5g/km for production and maintenance of a bicycle and 7g/km for a pedelec, assuming 2400 km/year for 8 years in both cases.

No comments:

Post a Comment