Water-Efficient Technology Opportunity: Rainwater Harvesting Systems

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Graphic shows a schematic of a rainwater harvesting system next to the exterior of a house.
Rainwater harvesting system schematic

The Federal Energy Management Program (FEMP) identified rainwater harvesting systems as an alternative water technology that is relevant to the federal sector, is commercially available, and offers a significant opportunity to offset freshwater use.

This overview provides agencies with key information to deploy innovative products and systems that may otherwise be overlooked. It also helps agencies identify water-efficient technologies for consideration when entering into energy savings performance contracts and utility energy service contracts.  

FEMP considered the following when selecting this technology.

  • Underutilized in the federal sector
  • Broad applicability across the federal sector
  • Lack of information available to agencies on alternative water systems
  • Offset of freshwater
  • Market availability
  • Produced by multiple manufacturers

Technology Description

Rainwater harvesting can provide a key source of alternative water to federal facilities. Alternative water offsets the use of freshwater from surface and groundwater sources and helps agencies improve their water security. Rainwater harvesting captures, diverts, and stores rainwater from rooftops for later use. Uses for rainwater include landscape irrigation, ornamental pond and fountain filling, cooling tower makeup, and toilet and urinal flushing. Typical rainwater harvesting system components are outlined in this illustration.

Technology Considerations

The following are important considerations when planning for rainwater harvesting.

  • Site location: Consider the amount of rainfall an area or region receives. For more information, see the Rainwater Availability Map.
  • Size of catchment area (roof size): A larger roof area can capture significant precipitation even in areas of low rainfall availability.
  • Rainwater storage capacity: Areas with lower available precipitation may require larger tanks to provide more storage capacity. An increased tank size will increase equipment cost.
  • Roof pitch and type: Roof material and pitch influence the amount of water that can be harvested. Lower-pitched roofs tend to lose less water than steeply pitched roofs. Smoother roof textures will facilitate runoff better than textured roofs.
  • Water rates: Areas with more expensive water rates will make rainwater harvesting projects more economically viable.
  • Permits: Rainwater harvesting permits may be required. Check with your local or state government. For more information, see the Rainwater Harvesting Regulations Map.
  • Operation and maintenance: Rainwater harvesting systems require regular operation and maintenance:
    • Monitoring tank levels
    • Cleaning system parts, including gutters and first-flush diverter, to prevent the system from becoming clogged with debris that can reduce the amount of rain collected
    • Monitoring for leaks
    • Maintaining treatment systems, including filter replacement and disinfection equipment maintenance
    • Testing water quality.

Harvesting Potential

Capturing rainwater on-site can reduce costs by helping to offset potable water use. The cost of potable water is increasing across the country. The American Water Works Association evaluated the water rate increases of utilities across the U.S. and determined that the annualized increase in water rates from 2004 to 2014 was 5.5%, compared to the consumer price index annual increase of 2.4% over this same period (source: Annualized Rate Increases from 2004 to 2014).

Aging infrastructure, utility debt, water scarcity, and supply risks all drive water rate increases (source: America’s Water: An Exploratory Analysis of Municipal Water Survey Data. Growing Blue). The potable water savings potential of harvesting rainfall depends on the amount of rainfall an area receives and the size of the rooftop where the rain is being collected.

Use the Rainwater Harvesting Tool to estimate the amount of rainfall that can be collected from a rooftop or other hard surfaces.

The monthly rainwater available for harvest can be calculated using the following formula.

Monthly Rainfall Collected (gal)

= Catchment Area (roof size)(square feet) x Monthly Rainfall (inches)

x Conversion Factor x Collection Factor


  • Catchment area (roof size) is the size of the roof where rainfall will be collected.
  • Monthly rainfall is the number of inches of rainfall for the month.
  • Collection factor is a factor applied to the total monthly harvesting potential to account for losses in the system. The Texas Manual on Rainwater Harvesting recommends using between 75% and 90% depending on how efficient the rainwater harvesting system is at collecting rainfall.
  • Conversion factor is a factor of 0.62 used to convert inches of rain that fall onto the roof area to total monthly gallons of harvesting potential.

The above formula can be used to size a tank. Properly sizing the tank is important because the tank generally drives the cost of the project. Several factors need to be considered when estimating water storage:

  • Application water demand: Compare the amount of rainfall that can be collected to the water demand of the application (e.g., irrigation) over a specific timeframe.
  • Analysis timeframe: Collect precipitation data over a short timeframe if possible (e.g., weekly); this will provide the specific amount of water that can be collected, stored, and used to help optimize the size of the tank.
  • Variation in rainfall: If there are large variations in rainfall throughout the year, a larger tank may be necessary to store rainwater during wet months to be used during drier months.

Additional Resources

FEMP created a series of alternative water maps to help federal agencies strategically plan where to implement alternative water projects. Three of these maps cover rainwater harvesting:

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