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| Cost of bath/shower etc. in Cambridge. The assumptions are discussed below. |
22p for a shower may not sound like much, but if you do that every day, that comes to £80/year. Two baths a week would cost you £37/year.
| Activity | Volume of water (litres) | Temperature (C) | Energy cost | Water cost | Total cost | Number per year | £/year |
|---|---|---|---|---|---|---|---|
| Bath | 88 | 41 | 15.1p | 20.5p | 35.6p | 104 | £37.03 |
| Shower (7 min at 8 l/min) | 56 | 39 | 8.9p | 13.1p | 21.9p | 365 | £80.11 |
| Shower (4 min at 6 l/min) | 24 | 39 | 3.8p | 5.6p | 9.4p | 365 | £34.33 |
| Basin wash | 4 | 41 | 0.7p | 0.9p | 1.6p | 365 | £5.91 |
Incidentally, although we do use energy in treating water this is much less than the cost of heating it. Treating 1m3of water takes 1.22 kWh [3]. Heating it up to 40C takes 37 kWh.
In practice, of course your water use may differ from the assumptions I have used:
Heating system
I assume you use gas and your boiler is 85% efficient. The cost of gas per unit is 4.4p + 5% VAT as on my last bill. If you have an older boiler that is less efficient, then costs will rise.
Size of Bath
The average is apparently 88 litres[2] and this is the figure I used in the charts. However, they do vary a lot. I estimated mine for my book at 110 litres: 25% bigger, so costing 25% more. Unfortunately, baths are specified as the volume to the overflow level - of course we don't actually fill them anywhere near that level.
Shower duration and flow rate
A typical mixer shower flows at 8 litres/minute. With a low flow shower head you can reduce this to 4 or 5 litres/minute. Also I have assumed a 7 minute shower which is the average reported in [2]. Some people take 10 minutes or even more. However, before I cut my hair I used to take about 4 minutes - now I take even less time. So I have used 4 minutes as the short shower comparison.
Shower duration and flow rate
A typical mixer shower flows at 8 litres/minute. With a low flow shower head you can reduce this to 4 or 5 litres/minute. Also I have assumed a 7 minute shower which is the average reported in [2]. Some people take 10 minutes or even more. However, before I cut my hair I used to take about 4 minutes - now I take even less time. So I have used 4 minutes as the short shower comparison.
Basin wash
This is the most difficult to estimate because it depends a lot on how much water you waste waiting for the hot water to come through. I expect you only use about 1-2 litres in the basin but you could easily have wasted 2-3 litres on top.
Water inlet temperature
I have assumed your cold water comes in at 14°C. This is based on monitoring by the Energy Saving Trust in 2008 [1]. For homes without a cold water tank the average was 13.4°C and with a tank it was 16.2°C. This is because the tank absorbs heat from the house and warms the cold water a little. There is also some seasonal variation.
Temperature of shower/bath
I have assumed 39°C for a shower and 41°C for a bath or a basin wash [2].
Water charges
I have checked the volumetric charge (ignoring standing charges) assuming you have a domestic metered supply. The cost varies somewhat around the country - in Cambridge, adding fresh water and sewerage charges, the total charge for 2015/2016 is £2.33 per m3. In London it is a little cheaper: £2.06 and in Yorkshire a little more: £2.75.
Cambridge water http://www.cambridge-water.co.uk/customers/charges-scheme
Anglian sewerage http://www.anglianwater.co.uk/household/your-account/bills-and-payments/tariffs/standard-rates/
[1] Measurement of Domestic Hot Water Consumption in Dwellings (Energy Saving Trust) 2008
[2] How much energy could be saved by changing everyday household behaviours (DECC/CAR) 2012
[3] Energy and Carbon Emissions: The way we live today (Nicola Terry) 2011
[3] Energy and Carbon Emissions: The way we live today (Nicola Terry) 2011
Isn't that interesting? Wd be great to know if this is true for the whole country, or just arid parts like Cambridge where you might expect high water costs.
ReplyDeleteDoes this mean there's a stronger economic argument for rainwater harvesting than solar water heating? (There's scope for a follow-up on this.)
The cost of water supply is a lot to do with the cost of treatment and pumping rather than sourcing the water. In my post I mention London water is cheaper and Yorkshire water more expensive. I can't find prices for South West water but North West water is also more expensive. I believe London and Cambridge both use mainly water from aquifers and this probably needs less treatment than water from reservoirs. There is also a significant standing charge which is not relevant to my calculations.
DeleteVery useful. Makes me focus as well on the cost of an electric shower which many people (including me) have. With the cost of electricity about 3x higher than gas the last time I looked then an electric shower ends up being about twice the cost of a gas one in your two examples. It will make me get out quicker. Thanks.
ReplyDeleteElectric showers tend to have lower flow rates so that the water can be heated fast enough. Typically they are 3-5 litres/minute and 7-10 kW. An electric show 4 minutes at 4 l/min (8 kW) costs 11.4p, only a bit more than a mixer shower with gas for the same duration but 6l/min. For the electric shower 65% of the cost is heating. This is based on electricity at 14p/unit.
DeleteI believe that an electric shower costs less. Nobody has confirmed this before, though. I am wondering if the boilers that continuously heat up the water on the go are efficient enough. Replacing the water heater will save a lot of money for a larger family, but the investment will only return after a few years, I guess.
ReplyDeleteAn electric shower will cost substantially more. The cost per kwh of electricity is around four times that of gas.
DeleteElectricity costs more but electric showers often have lower flow rates which will offset this to some extent. They simply cannot heat the water fast enough otherwise. Which is more expensive depends on how powerful your shower is.
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ReplyDeleteHi, could you publish the formula you used to calculate the energy requirement to raise the water temp from 14C to 39/41C? Thanks - great post
ReplyDeleteThe specific heat capacity of water is 4.2 kJ/litre/degreeC. To heat 1 litre by 25C would be 4.2*25 kJ = 105 kJ. To convert this to kWh, divide by 3600 (seconds in an hour)
DeleteHow much does it cost to have a shower per KW using gas in 2022
ReplyDeleteJust calculated my long shower used 6kwh at 7 pence a kwh standard rate which works out at approx 42 pence for a 25 min shower. Hope this helps.
DeleteEdit... 15 min shower not 25!
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