Sizing a System

Determining Your Water Use

First step in sizing a system is to calculate your water usage.

If you're already connected to municipal water, you should study your monthly water bills to get your average household usage. It should tell you how many units that you used, the time frame, as well as how many gallons of water are in a unit. In metro Detroit one unit equals 748 gallons.

With water-conserving plumbing fixtures and little or no outside watering, per-person usage can range from 50 to 100 gallons per day.

For residential home system this includes toilet flushing, bathing, clothes washing, dish washing and outdoor watering.

The following can help you estimate your daily water needs:

• Toilet = 6 flushes/day per person X flush volume of the fixture.

• Faucet = 5 minutes of use/day per person, X the actual flow of the faucet.

• Shower = 15 minutes/day per person, X output of the shower head.

• Bath = Actual volume of the tub X number of baths.

• Clothes washer = 25 gallons per load X number of loads a week.

• Dishwasher = 1 use per day at the rated gallons per load.

• Irrigation = Based on the system and months of year used.

• Outdoor faucets = Based on its gallon-per-minute

These calculations will give you a good approximation of the amount of water your family uses.

Next, determine rainwater availability in your area of the country.

It should be noted that sizing a system in areas with less than 24 inches of rainfall per year, building a large enough collection area and cistern for a adequate water supply may not be feasible.

Finally you must determine the size the cistern or tank you need to provide enough water storage for reasonably expected dry periods. What is a reasonable dry period in Maine is going to be different that in Arizona. So you will have to decide that.

The Texas Guide to Rainwater Harvesting includes detailed information on carrying out all of these calculations, including an advanced method that factors in storage capacity. It is a good source of information if you are considering a rainwater harvesting system.

The American Rainwater Catchment Systems Association web site is loaded with contact information about every aspect rainwater harvesting.



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