The Smiths live in Greenville, South Carolina. The Johnsons live right next door to them. Both the Smiths and the Johnsons are families of four, and both families use 94,900 gallons of water every year. If the Smiths have a gray water recycling system and conserve 35% of their water, how much smarter are the Smiths than the Johnsons?

The answer: this much (you can’t see me, but my arms are spread really wide).

All kidding aside, these sorts of word problems drove me crazy in school. But sometimes you have to bite the bullet and do a little number crunching. Today, I’m looking at the local price of water and calculating an average payback period for a RGW-250.

Look out–here come the numbers.

If each member of the Smith family uses 65 gallons of water a day, the family as a whole will consume 260 gallons per day (65 gallons x 4 people). That means the Smiths will rack up a whopping 94,900 gallons of water per year (260 gallons x 365 days per year).

According to the Greenville Water System, the Smiths will owe $25.32 as a base fee per quarter. On top of that, they will owe $2.03 per thousand gallons consumed, which equals $192.65 for the year. When these figures are combined, the Smiths are spending $293.93 annually on fresh water alone, which isn’t very much when you think about it.

Now let’s look at where the real savings start: wastewater treatment. How much does it cost for ReWa to treat the Smiths’ water and make it all good again? Well, the base fees for wastewater are similar to fresh water: $25.50 per quarter.

But the cost of treating the water is more than double the actual cost of the water: $4.45 per thousand gallons. That means the Smiths will spend $422.31 for the city to treat the water that goes down their drains. When the base fees are added to the wastewater treatment fees, the Smiths are looking at $524.31 per year.

Confused? Hang in there.

Breaking It Down and Bringing It All Together.

Below is the same calculation as above, but without the step-by-step explanations.

Fresh water base fees per year: $101.28 ($25.32 x 4 quarters)
Wastewater base fees per year: $102.00 ($25.50 x 4 quarters)

Fresh water consumption per year: $192.65 ($2.03 x 94.9 thousand gallons)
Wastewater treatment per year: $422.31 ($4.45 x 94.9 thousand gallons)

Total fresh water costs: $293.93
Total wastewater costs: $524.31

Grand total: $818.24

Payback Periods: Like Them or Love Them?

So what does this grand total mean for the Smiths? Well, with a Brac gray water recycling system installed in the home, the Smiths can expect to save anywhere from 35 to 40% on their annual water consumption. The retail cost of the RGW-250 is $2,390, so a simple division of the amount of money saved yields the payback periods. Check it out:

Conserving 35%: 8.3 years ($2,390 / $286.38)
Conserving 40%: 7.3 years ($2,390 / $327.30)

Before you discount these payback periods as too long, let me emphasize something: these periods are estimates at best. Payback varies depending on location, flush and flow rates, and other variables. The Smiths enjoy very inexpensive water in Greenville, so if you are located in a big city or a water scarce region, chances are your payback will be even quicker.

A Final Word or Two About These Numbers.

For these calculations, I was working under the assumption that the average American uses 65 gallons of water per day, which is conservative. Many experts say we use as much 100 gallons per day. Neither number includes water needed for irrigation.

Also, while crunching away, I was looking at the rates of a standard ¾” meter on a home located outside of the Greenville city limits. These calculations do not include sales tax and shipping, nor does it include any fees or surcharges associated with your water bill.

Whew. That was tough. If only my fifth grade math teacher could see me now.