OddThinking

I overheard at a party recently that the total amount of water being stored by a rainwater tank is 4 cents worth. The implication was that a rainwater tank would never pay for itself.

This sounded a little pessimistic. I have never bought a rainwater tank, so I don’t have any real figures, so I did some searching. Can I work out the payback time for a rainwater tank?

How Big is a Tank?

Tanks vary in size (duh!) so the value of the water being stored varies.

400L is considered small. 100,000L is considered very large (Ref). Sydney Water offers rebates above 2,000L and recommends at least 5,000L if you plan to use it in the house and garden (Ref). I’ve lived in houses with rainwater tanks of around 20,000L (estimate based on childhood memories and pictures from vendors) so that’s about the size I think of when I think household rainwater tanks.

How Much Does The Water Cost?

The first question is, do you plan to drink it? I grew up in a region of Australia at a time when the mains water was notoriously “hard” and tasted poor; nonetheless, it was safe to drink. Rainwater from tanks was widely used as a more pleasant tasting alternative to mains water. Now, the Government advice is to not drink the rainwater, for safety reasons.

1,000L of drinking water costs either $1.264 or $1.634 depending on the amount you use per day. 1,000L of unfiltered water costs $0.801. (Ref)

I would assume that most purchasers of water tanks (who are on mains, and thus have a choice) have the highest marginal water rate, but I will use the (uncommon) unfiltered rate – if I can debunk the claim with the cheapest rate, there can be no complaints.

Here we go with the calculations:

• Tiny Tank (400L) holds 32c worth of water.
• Small Tank (5,000L) holds $4 worth of water.
• Typical Tank (20,000L) holds $16 worth of water.
• Very Large Tank (100,000L) holds $80 worth of water.

How Much Does A Tank Cost?

It is no surprise that it varies, based on size, type and installation costs.

The best finger-in-the-air estimate I found was “a minimum of $500 for a small 400 litre tank to around $8000 for a 100,000 litre tank” (Ref, dated post-1998).

Payback Time?

This is where I wuss out. Working out the payback time means you need to know how often your tank gets filled, which depends on rainfall patterns and the size of your catchment area (e.g. roof). You need to compare that with your usage patterns – you may not be emptying the tank completely between refills. I can’t answer this question – not even for a typical installation.

However, what I can tell you is that it would take at least 1500 refills of the 400L tank and 100 refills of the 10,000L tank for them to start paying their way compared to unfiltered mains water (assuming you are getting mains water connected anyway).

That sounds like a long payback time to me.

Caveats

I haven’t factor in lots of issues, like the unreliability of rain, the fixed service charges for mains, the Government rebates for water tanks and for pensioners, the availability of unfiltered water, the environmental impact. I was just trying to debunk a claim I overheard at a party.

A final thought, for the economists out there. What would be the economic and environmental impact of the Australian government(s) removing all existing water restrictions and tank rebates and instead taxing mains water at similar rates to petrol?