H2OmeCalc background and assumptions
H2OmeCalc recognises there are a wide range of water sources available to the householder. The challenge is to find the optimum mix of sources.
H2OmeCalc is supported by a simple financial model with a number of assumptions about the future price of water, the cost of rainwater tanks and greywater systems, and the installation cost. These costs (particularly the installation cost) depend on your individual circumstances. H2OmeCalc provides an indicative cost based on data users have entered to help with the decision process. Please note: these estimates are only a guide; a quotation for your specific house and garden is required to confirm all costs.
For the purpose of H2OmeCalc average water volumes are used to estimate the flows of greywater from the appliances in the home. These volumes were taken from two key references:
- DEUS (2007) NSW Guidelines for Greywater Reuse in Sewered, Single Household Residential Premises. Department of Energy, Utilities and Sustainability. NSW Government, Sydney, NSW, Australia.
- Roberts P (2005) Yarra Valley Water - 2004 Residential End Use Measurement Survey.
There are a number of assumptions made in H2OmeCalc which allow for quick and simple calculation of water savings and supply and demand graphs. The starting volume in the rain-tank is assumed to be full. Any significant impact that this has on the long term monthly water supply is negated by running the rainfall model for five years.
| Case | Greywater diversion systems (permanent) | Greywater treatment systems (permanent) | Rainwater tank and internal plumbing (permanent) | Rainwater tank (permanent) | Temporary |
| System (Tank or greywater system) | 700 | 7,000 | 1,207 | 1,207 | 100 |
| Installation cost | 0 to 1,500B | 1,500 | 550 | 550 | 0 |
| Pump cost | 200 | 650 | 650 | 650 | 0 |
| Internal plumbing cost | 0 | 730 | 730 | 0 | |
| Irrigation system cost (supply and installation) | 500A | 50 | 100 | 100 | 0 |
| Operating & Maintenance cost annual | 50 | 500 | 20 | 20 | 0 |
| Totals | 1,450 | 10,430 | 3,257 | 2,527 | 100 |
A Irrigation area calculated as area irrigated with dripper (m2) × $5/ m2 (i.e. example above is irrigation of 100 m2 of garden or lawn.
B Greywater cost from discussions with plumbers in Melbourne. Costs in other cities and town could vary significantly. Rainwater costs estimated from MJA (2007) The cost-effectiveness of rainwater tanks in urban Australia. Canberra: Marsden Jacob Associates for the National Water Commission. Intial plumbing costs for permanent greywater divsersion systems were dependent on the type of connection: Greywater ready houses $250; Laundry only $500; New connection to existing house $1500.
Discounted payback calculations assume:
- A new pump (where relevant) is required after 10 years.
- Increase in prices of water annually was 7.5% annually
- The initial water cost in 2010 was $2.50/KL
- Operational costs increase by 2.5% annually
- Net present value was discounted at 5.5%
Rainfall tank costs were estimated from available prices from several retail locations in Melbourne and a line of best fit derived for calculating tank cost
Tank cost ($) = [549.9 x Ln(Volume in KL)] + 322.4
Note: Pump operating and cost may not be required for rainwater use, gravity may be an option. Estimates give indicative cost, however, consult your specific supplier for onsite costs.