Toilet Flushing with Harvested Rainwater in Hawaii

Using harvested rainwater for toilet flushing in Hawaii is a practical way to reduce your water bill and conserve municipal supplies. With 63.7 inches of average annual rainfall, a 1,500-square-foot asphalt shingle roof in Hawaii can collect approximately 45,229 gal per year. toilet flushing requires roughly 20 gallons per day per person, year-round. With this setup, rainwater can meet approximately 100.0% of your toilet flushing needs, saving an estimated $219.00 per year.

Annual Harvest 45,229 gal 1,500 sq ft roof
Annual Demand 14,600 gal toilet flushing (2 people)
Demand Met 100.0% of toilet flushing
Rec. Tank 5,000 gal storage capacity

Monthly Supply vs. Demand for Toilet Flushing in Hawaii

The table below shows how your monthly rainwater harvest compares to toilet flushing demand throughout the year. Green values indicate months where your harvest exceeds demand (surplus water for tank storage), while red values show months where you will draw down your stored reserves. Toilet Flushing is a year-round use, so demand remains constant regardless of season. Tank sizing should bridge the gap during your driest months.

Month Harvest (gal) Demand (gal) Balance
January 3,738 1,240 +2,498
February 3,140 1,120 +2,020
March 4,486 1,240 +3,246
April 4,112 1,200 +2,912
May 2,841 1,240 +1,601
June 2,392 1,200 +1,192
July 3,364 1,240 +2,124
August 3,588 1,240 +2,348
September 3,738 1,200 +2,538
October 4,336 1,240 +3,096
November 4,859 1,200 +3,659
December 4,635 1,240 +3,395

Harvest by Roof Size for Toilet Flushing

A larger roof collection area directly increases your harvest and the percentage of toilet flushing demand you can meet with rainwater. The table below shows how different roof sizes perform in Hawaii for toilet flushing (calculated for 2 people).

Roof Area Annual Harvest Demand Met Rec. Tank Savings/Year
1,000 sq ft 30,153 gal 100.0% 2,500 gal $219.00
1,500 sq ft 45,229 gal 100.0% 5,000 gal $219.00
2,000 sq ft 60,307 gal 100.0% 10,000 gal $219.00
2,500 sq ft 75,383 gal 100.0% 10,000 gal $219.00

Setting Up Rainwater Harvesting for Toilet Flushing in Hawaii

Connecting rainwater to your toilets in Hawaii requires a dedicated non-potable plumbing line (typically purple-colored pipe to distinguish it from potable supply). This is best planned during new construction or major renovation, though retrofit kits are available. The system includes a storage tank, submersible pump, pressure tank, sediment filter, and a backup connection to municipal water with an air gap to prevent cross-contamination. Most jurisdictions in Hawaii require a plumbing permit for this type of installation. The payoff is substantial — toilet flushing accounts for roughly 30% of indoor water use, and switching to rainwater for this purpose provides year-round savings regardless of season. The recommended tank size for toilet flushing should bridge at least your driest month, ensuring continuous supply.

Metal Roof Upgrade: Toilet Flushing in Hawaii

Upgrading to a metal roof increases your collection efficiency from 80% (asphalt shingle) to 95%, boosting your annual harvest to 53,712 gal from a 1,500-square-foot roof. This increases your toilet flushing demand coverage to 100.0% and raises annual savings to $219.00. Metal roofs also last 40-70 years compared to 20-30 years for asphalt shingle, and their smooth surface sheds debris more effectively, reducing maintenance on your collection system and improving water quality.

Other Uses for Rainwater in Hawaii

Explore how harvested rainwater can serve other needs in Hawaii:

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