Rainwater tanks are not just for rural environments, they also have a place in cities where they can provide an excellent secondary source of water supply. Under the current building code they cannot however be the sole source of supply if a mains supply source is available. Clearly the incentive for homeowners to have a rainwater tank is greater if they pay on a volumetric basis for water but that payment approach is likely to become more common in the future so it is as well to be prepared. Use water collected from tanks for non-potable uses inside and outside the home. Given that water used for potable purposes and direct contact eg showers is less than half the household water use, the current approach is to get their use established for non-potable uses before addressing the potable uses.

If rainwater is for garden use only there is likely to be less installation cost. If the rainwater is to be used within the house, there normally needs to be a pump and separate pipes and taps for the tank supply. Your local Council will have certain requirements regarding the connection.

Rainwater tanks of about 4500 litres are optimal for many urban areas but rainwater calculators are available. Generally basic rainwater tanks cost about $2500 plus installed but those most suited for urban use can cost more due to lack of manufacture in New Zealand and the need to import them form Australia, in itself not that sustainable. With increased interest in urban rainwater tanks the number of tanks required will increase and the cost of manufacture come down. Some councils are considering rainwater tanks as a mandatory requirement for new homes or an acceptable solution under the Building Code to address water shortages or stormwater detention. They need to be fitted with a backflow valve preventor to avoid rainwater getting into the mains supply. The configuration of an efficient system is shown on www.bluescopewater.com

Despite the up-front cost, installing a rainwater tank makes good financial sense over the long run. An average size house with a 150m2 roof using a 4500 litre tank would save about 100,000 liters a year. At current water prices a tank and installation will have about a 16 year payback period. However if water prices rise the payback period will come down. In addition, some councils have a rebate, generally of $500, for the purchase of a rainwater tank which again shortens the payback period on your investment. The Waitakere City Council website is a good source of information and has a calculator to help select the size of tank. Using-Rainwater-Brochure.pdf and
raintank_flyer.pdf
However for an accurate assessment of rainwater tank size for either water use or stormwater detention a model such as PURRS should be used. The following website has very detailed information on rainwater tanks and which includes a pathway to the use of PURRS. http://www.eng.newcastle.edu.au/~cegak/Coombes/

The yellow pages have a list of water tank manufacturers. Products and suppliers also on www.ecobob.co.nz

Tanks that are specifically designed for small urban sections can fit against a wall, under a deck or under a concrete pad. Selecting the best size tank and design is important to obtain maximum benefit and make it aesthetically pleasing.