Several elements need to be taken into consideration to analyze the benefits and the cost of rainwater harvesting systems. To come up with estimations, different scenarios will have to be created. For simplicity we will assume the following:
Household size: 4
Rooftop size: 2500 square ft
Rainfall data: annual average of Harris County-48.8 inches. Non-uniform throughout the year (highest rainfall during the summer).
End-use of rainwater: for Non-potable purposes (irrigation).
Maximum average family water consumption: 400 gallons per family per day (Based on household size). Assume water consumption is the same throughout the year.
Cost Estimation Rainwater harvesting systems do not usually have a standardized overall cost since they can be customized depending on the person’s needs. Other important factors that need to be considered are rainfall data, demand, and end-use of the water collected. Consequently, different cases can be set up to approximate an overall cost of rainwater harvesting systems.
For the assumptions made, water does not necessarily need to be treated, which means systems for irrigation purposes tend to be cheaper. As an example, the cost of a simple system (non-potable water) can be estimated by looking at different components and by adding each cost. The following table accounts for only 3 components of a rainwater harvesting system.
Table data was based on cost ranges provided in the Cost Overview
The cost range for a simple system tends to be less than $1000. For this example, an overall total cost of $828 was estimated. Note that other expenses such as labor are not taken into consideration.
Demand
The demand of water for irrigation purposes tends to increase from an average of 30% to almost 60% of the total water use (gallons per day per person) during the summer. Of course, this also depends on the size of lawn. With the assumptions stated above, the amount of water used for irrigation can be calculated in a per year basis by multiplying 400 gallons per family per day times 30 days times 12 months. This gives a total of 144,000 gallons of water used in a year. For this case, it can be assumed a 30% use for irrigation, which means 43,200 gallons are used anually for irrigation.
Water Bill Estimation Water rate ($/gallons) data can be used to calculate the cost of water consumption in one year. The water rate increases significantly if the amount of water consumed surpasses 12,000 gallons. The water rate for 6000 gallons is $32.42 ($5.4/1000 gallons). The rate changes to $5/1000 gallons until 12,000 gallons are met. After 12,000 gallons, the rate increases to $8.23/1000 gallons. For this case, the amount of water consumed in a month is 12,000 gallons, which would give a cost of $62.42. For a year, it would be $749. Note that only water rates are needed for this particular case analysis. There are usually other costs in a water bill. [55]
Does Supply Meet Demand?
For this case, calculations will be required to come up with the maximum rainwater harvesting capacity. The calculations follow a similar procedure as the one made in Harvesting Potential. The following table explains how much we can harvest based on the assumptions made. Runoff coefficient refers to the efficiency of the water running on the rooftop and down the gutters. Safety factor accounts mainly for spillage.
Note: Average rainfall data can be found in the environmental data scenario 1. Other data was based on the assumptions above and the rainwater harvesting system petitioner manual. [36]
Even if it was possible to collect and use the water in a efficient way, the assumption of a regular daily rainfall is unrealisic. Rainfall changes along the year with some days raining the same amount it rains in a whole month. For this study case, it was assumed the rainwater system tank had a capacity of 500 gallons. However, the water can be used during sunny days, and this one could be refill with more rainwater during the year (assume 10,000 gallons in total refill in a year, 20 cycles). This would still not meet the demand of water needed for irrigation (43,200 gallons). More storage capacity would be needed (bigger tank), thus a more expensive rainwater harvesting system.
Any Savings? Yes. Even if the demand does not meet supply for this particular case, there are still some savings made. The overall water usage in a year went from 144,000 gallons to 134,000 gallons. In one month average, the consumption was reduced from 12,000 gallons to 11,166 gallons. Converting that to dollars, the cost went from $62.42 to $58.25. This means $4.17 were saved in a monthly bill and $50 in a year. However, these are not real savings yet! $828 were originally invested to build the rainwater harvesting system. This means that real savings will start appearing after 17 years.It is important to take into consideration that with a bigger tank, more rainwater can be collected, thus more savings can be made. Also, the bigger the tank, the cheaper the cost per gallon. [1]
Remember that this whole analysis was based on assumptions that might not work for every homeowner.