Rainwater harvesting - How do you calculate 'pay off' period?

Eellogofusciouhipoppokunu

Hi,

I was wondering if anyone knows if there is a calculator where I could see how long it would take for a rainwater harvesting system to pay back its initial investment?

We moved house a couple of years ago and the four of us (2 adults, 2 kids) in our old house used an average of 60 cubic meters of water over a 6 month billing period. In our new house we have my parents living with us in a 'granny annexe' but sharing the water. Our useage is now 118 cubic meters over a 6 month period, just about double what it was.

Our major use of water will be in showering, washing clothes, flushing toilets. We have rainwater butts for watering the garden.

I know that generally rainwater harvesting has such a long payback that it's often not worth the effort, but with a family of 4 adults and 2 kids (well 5 adults now that our eldest has turned 16) I think that the figures may start to add up, but need a calculator to be sure.

Can anyone help?

Eellogofusciouhipoppokunu

Also, I'm paying a standing charge of £71 per year for 'surface water/highway drainage' which I believe I could get rid of by having the rainwater from the roof go into a tank and the overflow go into a stream that runs through our garden.

Presently, our rainwater goes into the sewer as the pipes are not separate. As part of the harvesting installation I would re-route the rainwater downpipes into a tank and then an overflow from ths into the stream.

EDIT: I've just phoned our water supplier and we are allowed to have the rainwater go into the stream, but it will only reduce the bill by £37 because the highway drainage part is compulsary.

SailorSam

This is an interesting page on rainwater harvesting. One of the links they give is to that 'centre for alternative technology' which does courses for all these eco friendly things. I'm sure that would be a good place to start.
http://www.self-build.co.uk/features/Essentials/article/?id=30

EricMears

FWIW, our houshold is 2 adults (and occasional visits from son) and a typical consumption for flushing WCs only is 100 litres /day (or 36.6 cu metres pa).

With 6 people (WC flushes same amount whether a toddler or OAP has used it) in your household I'd expect 300 litres per day (around 100 cu m per year).

If you can also use it for washing machine, amount per cycle ought to be listed in user's manual so have a stab at guessing number of times per year it's used and multiply up.

Showers would be a further 'bonus' (no idea how to calculate that !).

Costs of mains water vary considerably across country - your most recent bill will tell you yours. And of course your sewage charge is based on assumption that all the mains water (but only that) goes down the sewer so buying less water will also save on sewage charges.

However, there may be a snag. Total amount of rainwater you can collect is limited by size of roof and total annual rainfall. In our case that's 100 sq. metres x 1000mms/year or 100 cu m.

No problem if you have a very large roof and live in the Lake District; major setback for a small house in Norfolk.

Not paying for surface drainage can be another bonus. You might need a licence to discharge to a stream but wouldn't for a 'soakaway' (and with usage described, a very small soakaway would be fine).

Eellogofusciouhipoppokunu

Thanks for the replies. I put our water useage and location details through a tank size calculator and it came back with a recommendation that we go for a 4800 - 5000 litre tank. We have a large footprint on the house, have a large outbuilding that would also be linked in and live in an area of high rainfall, so filling the tank wouldn't be a problem.

I wouldn't use the water for anything other than flushing toilets and washing clothes because it's not potable and would cost more to purify it than it would cost to use mains water.

EricMears

Eellogofusciouhipoppokunu wrote: »

We have a large footprint on the house, have a large outbuilding that would also be linked in and live in an area of high rainfall, so filling the tank wouldn't be a problem.

If you have a large outbuilding and have space in it, you might consider buying a few free-standing one-tonne 'transitanks' rather than burying a 5 tonne tank under the lawn as most people seem to do. Tanks need to be kept out of sunlight to deter algae growth so inside a shed would be ideal. Mine are installed in the basement.

In my case, I was able to take some transitanks (which would otherwise have been scrapped) home from work so paid nothing for them but have bought another one more recently from eBay for only six pounds.

Try searching for "IBC" - but don't rush to buy the first one you see ! Some sellers are asking £100+ (& good luck to them) but others start at 99p and if you're the only bidder that's all you need to pay although it could take months to find that situation. There is currently one seller quoting a buy it now price of £30 and you certainly won't find and install a 5 tonne underground tank for anything like £150.

Have to say though that I didn't just install rainwater harvesting to save money (though welcome the spin-off). I really believe that it's a profligate waste of resources to purify water to drinking standard then flush it down the WC

EricMears

Eellogofusciouhipoppokunu wrote: »

I put our water useage and location details through a tank size calculator and it came back with a recommendation that we go for a 4800 - 5000 litre tank.

I used to have 2 transitanks and found that there were several weeks per year when we'd run out. Have never run out since upgrading to 4.

If 2 tonnes isn't enough for my 2 person household, I'd suggest 5 tonnes won't be enough for 6 people. Perhaps 10 tonnes would be more appropriate.

EricMears

OffGridLiving wrote: »

I've seen negative comments (on other forums, happily not on this one) about the economic feasibility of installing a rainwater harvesting system, which is something I'm looking to do in the next 12 months. While my situation lends itself to a cheaper installation of RWH (because I'm already doing some remedial work to drains, etc.) it still probably won't pay in purely economic terms, but it does have a rapid payback in sustainability terms, and in the end that's surely what really matters?

Welcome to the MSE pages. Hopefully we'll be able to discuss rainwater harvesting in this thread without the intrusion of irrelevant thoughts from some third rate journalist getting in the way.

It's a personal crusade of mine that treating mains water to drinking water standard then flushing a third of it directly to waste is a profligate waste of resources. That was the primary reason for ensuring that our WCs are flushed with rainwater.

But as it happens I do save money by doing it.

Our last couple of half-yearly metered water bills have been around £60 and using the assumption that WC Flushing accounts for a third of water consumption (which I've found pretty accurate in our case) I'd therefore say we're saving around £60 per year.

Working backwards from a 10% ROI, that would justify spending £600 on a RHW system and because I made extensive use of recycled components in our system I'd estimate I actually spent around £200 (giving 30% ROI). But if you're happy with a smaller %ROI spending even £2000 on a RHW system to save just £50 per year would still be better than most current ISA rates.

But as OffGridLiving has pointed out, feeling the warm glow of 'sustainability' has got to be worth a few bob to anyone.

Also, RHW works best if you go back to the 'old fashioned' idea of having a header tank in the loft from which your WCs can be filled. Anyone with a 'modern' system who has had to rush around flushing their WC with a bucket of water from the local stream after the water mains were interrupted will join me in cursing whichever bright spark decided the header tank was redundant.

Cardew

The difficulty in estimating financial savings is the huge spread of metered water/sewerage charges across UK; they range from well under £2 a cubic metre(for both) to well over £5 in the South West.

EricMears

Cardew wrote: »

The difficulty in estimating financial savings is the huge spread of metered water/sewerage charges across UK; they range from well under £2 a cubic metre(for both) to well over £5 in the South West.

Good point.

FWIW, Severn Trent were charging me 152.p pence per cu m for metered water plus a £12.97/half year 'standing charge' in November last year (and apparently that utility is VAT-free).

No idea what they charge for sewage or rainwater disposal; they don't offer that service here. People who it does affect should note that 'metered sewage' is usually estimated from metered water purchase rather than actually measured so RWH will save money on sewage costs as well.

I guess that makes this one of the cheaper areas for water supply. In which case, if I can justify collecting rainwater on economic grounds practically everyone else in the country ought to be able to do that too.


Whilst I had the STW webpage open, I also checked quantity of water supplied on my last bill. I was charged for 30 cu m for the May-Nov half year. From my (very rough) pumping estimates, I'd say that I use around 100 litres/day or 18 cu m per half year which works out that WC usage is approx 37% of our total water consumption. The 'standard estimate' of one third for WC usage is hence confirmed as reasonable.

OffGridLiving

Does anyone know if water companies will make a fuss if they find out you are rainwater harvesting and you're connected to a mains sewer? Our sewage water bill is calculated on the amount of incoming water we use, so if we use 60 litres over the billing period, we're charged for the disposal of 60 litres of water into the sewers.

If our incoming water goes down to 30 litres because the other 30 litres is now coming from stored rainwater, our outgoing water will still be 60 litres but we will only be billed for 30 litres. I just can't see the water companies allowing that to continue, especially if RWH becomes more mainstream?

Just had another thought. As the OP has mentioned, you get a rebate if your rainwater goes to a soakaway or other water source rather than to a sewer. If you divert this into a rainwater tank and use it to flush the loo, then it's going back into the sewer system. Only the overflow from a full tank will now go into a stream. Again, I can't see the water companies allowing people to keep this rebate when most of their rainwater is going into the sewer.