Guide · 6 min read
Reading a water bill for irrigation savings
An audit produces gallons; a client pays a bill. Turning one into the other honestly requires reading the bill the way the utility computes it, and most bills hide three traps: unfamiliar units, tiered pricing, and sewer charges that may or may not follow irrigation use.
Getting this right is what separates a defensible savings estimate from a number a client can later wave back at you.
Units: kgal, CCF, and cubic metres
Utilities bill in thousands of gallons (kgal), in hundreds of cubic feet (CCF, also written HCF), or in cubic metres. One CCF is 748 gallons; one cubic metre is roughly 264 gallons. Convert the audit's gallons into the bill's unit before applying any price, and quote the client in the unit their bill uses.
The audit arithmetic itself is fixed: one inch of water over 1,000 square feet is 623 gallons. That constant already includes the depth conversion, so area in square feet times depth in inches times 0.623 gives gallons directly.
Tiered rates and which tier savings come from
Many utilities price water in tiers that rise with monthly use. Irrigation typically sits in the top tiers because it stacks on household base use. That cuts in the auditor's favor: every gallon saved comes off the most expensive tier first, so pricing savings at the customer's top marginal rate is usually the honest choice, not an inflation.
Use the marginal rate from an actual recent bill rather than a published average. Averages blend cheap base tiers into the figure and understate what outdoor water really costs the client.
Sewer charges and dedicated irrigation meters
Some utilities bill sewer on metered water use. Where irrigation runs through the domestic meter, saved irrigation water can also reduce the sewer line item; where the property has a dedicated irrigation meter, sewer typically does not apply to it at all. Check which situation the client is in before adding sewer savings to an estimate.
Winter-average sewer billing is a third pattern: sewer is set by winter use, when irrigation is off, so irrigation savings do not move it.
Why the estimate is not a guarantee
The audit's savings figure compares the current schedule with a proposed one under the measured precipitation rate, an assumed irrigation season, and an assumed rate. Actual bills also move with weather, rate changes, plant growth, and how faithfully the schedule is kept. Present the number as an engineering estimate with its assumptions stated, which is exactly how the Irrigaudit report words it.
Frequently asked questions
How many gallons is one inch of irrigation over 1,000 square feet?
623 gallons. The constant 0.623 gallons per square foot per inch of depth comes from 7.48 gallons per cubic foot divided by 12 inches; multiply area in square feet by depth in inches by 0.623.
What is a CCF on a water bill?
One hundred cubic feet, sometimes written HCF, which is 748 gallons. Divide the audit's gallons by 748 to express savings in the bill's CCF unit.
Which rate tier should irrigation savings be priced at?
The customer's top marginal tier from a recent bill, because saved gallons come off the most expensive tier first. Do not use a published average rate, which blends in cheap base tiers.
Do irrigation savings reduce the sewer charge too?
Only sometimes. If sewer is billed on the same meter's use, yes. With a dedicated irrigation meter or winter-average sewer billing, usually not. Check the client's bill structure before adding sewer savings.