Originally Published: January 5, 2018 5:59 a.m.
Editor’s Note — Part 1 of this series outlined the general guidelines and components of a rainwater harvesting system, now we will look at the calculations needed to establish the right size system and cost.
First action is to calculate the runoff from your catchment (roof) system:
Take the dimensions of the footprint of your roof and convert them to inches.
Multiply the roof dimensions by the number of square inches of rainfall.
Divide by 231 to get the number of gallons
Roof pitch is given as the number of inches in height change over the distance of one foot. For a pitch angle of 4, divide by 12 and get 1/3. Square the result 1/9. Add 1, then take the square root. Adding 1 gives 1 + 1/9 = 10/9, then taking the square root 1.0541.
Now multiply by the square footage of a single floor of your house. For a home with a 2000 sq ft single floor area (20001.0541=2108 sq ft roof area). Now convert the sq ft to sq inch 2108144=302552 sq inch roof area. Now multiply by the monthly average rain fall in Prescott and convert to gallons (Fig 1.).
We now have to establish how much water is required for landscaping. There are a number of ways of doing this; for existing homes use the water bills over a 12-month period. Or you can run the landscaping water system for 1 hour measuring the water meter before and at the end, and multiple by the number of times a month you run the system. For a new home list all the transmitters used and their flow rate per hour and add them together. The example below uses a typical water bill analysis. The water used (gallons) is the actual bill, the landscaping is the bill minus the in-home basic 2,425 gallons of water used and described by the town as sewer (recyclable) home use water (Fig 2).
As can be seen the ideal tank size would be 12,550 gallons, but even at that size the system will still need an additional 9,600 gallons from the city water utility per year. If you select a 10,000-gallon tank just substitute the January 10,234 with 10,000 and the February 12,550 with 10,000 and note the following months changes. Note: you can plug in any size tank at the appropriate time (5000 gal in September). It will just change the amount of city water needed for back-up.
What about the economics? Does any of this make financial sense.
An installed active rainwater system (i.e., with pumps and associated components are more expensive than passive systems) will typically cost between $2 and $6 per gallon of system size, depending on system specifics (above or below ground, amount of piping required, number of pumps, DIY or professional installation etc.), not including gutters. As an example, the 12,550-gallon inground active system above could cost between $15,000 and $20,000.
So, the final assessment (Fig 4) compares the municipal water costs versus the rainwater harvesting system investment. The way the water bills are calculated include a large upfront service charge and then a tired water usage charge. As an example, my Prescott bill shows a cost of $66.15 for my base usage of 2,425 gallons. From the model above the annual cost of municipal landscaping water is $164.80.
The rainwater system model uses a 12,550 gallon inground tank based on an installation cost of between $15,000 and $20,000 without maintenance and municipal backup water costs of 9,620 gallons per year. If we use the annual cost of municipal water at $164.80 per year the payback for the rainwater system would be at a cost of $15,000 91 years and at $20,000 121 years. Most reasonable investment projects should payback between 5 and 10 years.
As regards incentives, Arizona State provides an Individual Water Conservation Systems Tax Credit, of up to $1,000, and Prescott provides incentives for irrigation systems, rainwater catchment, and indoor fixture replacement to a Maximum allowance per property of $2,500.
Small rain barrel solutions targeted at specific gardening projects cost a lot less, but may still not be financially sound; however, in this case, it is a hobby project where cost is less of an issue. However, for many of these programs there has to be a financial incentive, and in this case, it is not obvious.
Contact Paul Scrivens at greenhomeenergyadvisors.com.