Monday, 27 July 2015

How much does your bath or shower cost?

When we have a bath or a shower we don't just pay for heating the water, we have to pay for the water too (if we have a metered supply). Taking both into consideration, I was surprised to find that the water costs more than heating it! In fact, assuming you heat by gas with a reasonably efficient system, only about 40% of the cost of your shower or bath is for the heating. In Cambridge, the total cost of an average bath is about 36p and a shower probably 9-22p - depending on the amount of time you spend in the shower and the flow rate. These figures are shown in the chart.
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.


ActivityVolume of water (litres)Temperature (C)Energy costWater costTotal costNumber per year£/year
Bath884115.1p20.5p35.6p104£37.03
Shower
(7 min at 8 l/min)
56398.9p13.1p21.9p365£80.11
Shower
(4 min at 6 l/min)
24393.8p5.6p9.4p365£34.33
Basin wash4410.7p0.9p1.6p365£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.


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.





5 comments:

  1. 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.

    Does this mean there's a stronger economic argument for rainwater harvesting than solar water heating? (There's scope for a follow-up on this.)

    ReplyDelete
    Replies
    1. 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.

      Delete
  2. Very 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.

    ReplyDelete
    Replies
    1. Electric 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.

      Delete
  3. I 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.

    ReplyDelete