A new, low-flow toilet may pay for itself in a year

July 15, 1946. That's the date stamped inside the tank of my guest room toilet. That explains a lot.

My house was built in 1946, so it's an original toilet. You can hear the humming of pipes for several minutes after someone flushes. It takes all that time to fill back up. I am not imagining this.

The Environmental Protection Agency says toilets manufactured before 1980 use an average of five gallons per flush, sometimes more.

The EPA also says toilets use more water than anything else in our homes (30 percent). Amazing! That's why I wanted to know whether switching to low-flow toilets was worth it, how fast those toilets would pay for themselves, and how much money I - and you - could potentially save. So I plugged various scenarios into the EPA's WaterSense Calculator.

The calculator asks how many people live in your house and the age of your toilets. If my whole house was full of 1946 toilets, like the one in my guest room, and my mother-in-law were visiting, which would make us a family of four, our savings would be extra impressive. By switching to high-efficiency WaterSense toilets, which use just 1.28 gallons per flush, we would save 27,000 gallons of water and $230 per year.

Next I peeked into the tank of the toilet in my master bathroom. The mint-green color and Space Age shape hint at the time period, and, sure enough, this one was stamped Feb. 6, 1990. Happy 28th birthday, baby! Toilets made between 1980 and 1994 averaged 3.5 gallons per flush.

So imagine if my entire house were outfitted with these, and that my mother-in-law still hadn't left. In this case, by switching to WaterSense toilets, I would save 16,000 gallons of water and $140 per year.

Unsure when your own toilets were manufactured and how much water they use? Your toilet's "date of birth" should be stamped inside the tank, as mine were. Some manufacturers also list the number of gallons per flush. This could also be inside the tank or in the small space between the seat hinge and the tank. It will say something like 3.5 gpf, which stands for gallons per flush.

So why and how have manufacturers managed to create toilets that use 1.28 gallons of water to flush, rather than five or more? In 1995, the National Energy Policy Act began requiring plumbing manufacturers to make - and homeowners who were remodeling to buy - toilets that used 1.6 gallons of water per flush or less.

Some states then adopted - or exceeded - that national standard. During the worst of its drought, California stipulated that only toilets that use 1.28 gallons per flush could be sold in the state. These are called "high-efficiency" or "WaterSense" toilets. Colorado and Texas adopted similar standards. Georgia requires you to install WaterSense toilets if you're building or renovating. Connecticut, Nevada, New York and Washington state have emulated the 1.6 gallon federal standard.

Unfortunately, the first generation of low-flow toilets did not work well and turned many homeowners off. They often required multiple flushes to do the job and clogged easily. The EPA is so defensive about these early failures that it created a "Flush Fact vs. Flush Fiction" pamphlet. The pamphlet explains that toilets bearing the WaterSense label "must meet strict criteria for water efficiency and flushing performance" and must "meet or exceed national plumbing performance standards."

Independent testing by Consumer Reports confirms these claims. The magazine awarded its top five toilets scores ranging from 74 to 78. All five use between 1.28 and 1.6 gallons of water per flush, "resist drain clogs," and are rated "excellent" or "very good" at solid waste removal.

What did not rate well in Consumer Reports' testing were dual-flush toilets, which use less water to flush urine. Here the scores ranged from 42 to 58. Testers said some of the models did a poor job of bowl cleaning and only a fair job of solid waste removal. Perhaps it's no loss, because the average amount of water used by a dual-flush toilet, in its two modes, is similar to what a WaterSense toilet uses.

When I searched for high-efficiency toilets on an EPA portal, I had 91 pages of choices. Clearly, there's plenty of competition to make and sell these ultraefficient 1.28-gallon models. And with competition come cost cuts! Three of Consumer Reports' top picks cost just $240 to $275.

I can save $230 a year by switching my house over from pre-1980s toilets to WaterSense toilets. So in a single year, I save enough to cover the cost of one toilet.

I can also save $140 per year by switching my house from 1980-1994 toilets to WaterSense toilets. In that case, it'll take two years for the savings to cover the cost of a toilet.

So peer into your toilet tanks and start doing your own math.