How Much Can You Save By 'Letting It Mellow'?

Let's get down to the business of doing one's business.

I came across an old post over on The Simple Dollar, calculating how much the average American would save by following the mantra, “If it's yellow, let it mellow. If it's brown, flush it down”. The answer: $7.66 per year, per person (which was £4.95 back in 2010).

£4.95 for an entire year??

I started reasoning that US water must cost a lot less than our own or that prices had gone up dramatically in the last couple of years. Sorry to gross some of you out, but I had to run the numbers for the UK!

The Figures

Right-o, so in Bristol, including water and sewerage we pay £2.95 per m³ of water used. The national average varies hugely and is surprisingly difficult to pin down, so apologies for the local rate!

Dual-flush toilets use around six litres for the full flush, or four litres for a reduced flush. 

The average person uses the toilet six to eight times a day, so let's take a middling value of seven. 

In the UK, one study found people go for a “number two” between 1.2 and 1.5 per day. I'll go for 1.3 as the higher estimate was for vegans. 

 

The Calculations

So, the average person needs the big six litre flush an average of 1.3 times per day, giving an average of 7.8 litres used per day.

If they're not following the mellow yellow plan, they will also need 5.7 little four litre flushes a day (7 times needing the loo minus the 1.3 already used). 5.7 x 4 litres = 22.8 litres.

The total amount of water flushed away every day would be 30.6 litres per person (22.8 + 7.8). This is 11,169 litres per year, or 11.169m³.

In Bristol, this would cost you £32.95 per person every year (£2.95 per m³). Admittedly, the person would have to be using their home toilet all year.

Right, now what about the frugalista, conscientiously following the “If it's yellow, let it mellow” policy?

Well, you'd still have the same number of “big” flushes, but no little flushes. You'd be using 7.8 litres of water per day (1.3 x 6 litres), or 2847 litres of water per year (2.847m³). This equates to £8.40 per person annually (at £2.95 per cubic metre), or about a quarter of the “normal” person's usage above.

The difference (£32.95 - £8.40) is £24.55 per year per person (or $39.50, if you want to compare to Trent's $7.66 from 2010 in the USA).

Results

So the boyfriend and I are effectively saving ourselves £50 per year by following the mellow yellow method. This is a lot more than Trent estimated for the average American in 2010 (£4.95 per person), but it's still not as high as I'd hoped it would be! Still, every little helps! That saving would pay for an MOT or pay the energy bills for another month and a half!

Another big consideration is that you use four times less water than someone flushing every time, and in an age where water conservation is becoming increasingly vital, this may be the more important factor. The Boyfriend found it incredible that so much fresh drinking water was being wasted in a time where droughts are more widespread than ever.

One flaw in the calculation is that most people don't use their own loo all year. People go to work or school for several hours a day! But Trent had this problem in his calculation too so I have not tried to factor it out. 

I guess there are two main questions to think about:

1. How far would you go to save £25 (or £50 in our case)?
2. Does the huge amount of water saved by using mellow yellow outweight the small financial gains? (Each person saves an average of 8322 litres a year). 
   
   
   Any thoughts? Is £25 higher or lower than you had expected?