In the spirit of this question about paper towels vs electric dryers, which types of faucets are most sustainable?

The marketing reasons for using motion-activated faucets are that they reduce water usage, and since they don't need to be touched, require less time and chemicals for cleaning.

However, they do require electricity and motors, and when (as often happens) the motion activation part isn't working well, people touch them anyway (or hit them, grab them, squeeze them, etc, as they grow more and more frustrated (not that I would know anything about that...)).

It seems like something with a bumper (like many drinking fountains) or a foot pedal would offer the best of both worlds.

  • I bought a used kohler commercial grade touchles faucet for 60 dollars, It claims to run on a single 9 volt battery with 4 years of average commercial use. so far so good! – Richie Jun 13 '18 at 5:48
  • @Richie that's a good point... I dug around on the Kohler website a bit, it looks they have faucets that use batteries, or AC adapters. I couldn't find any information on how long they last though. It'd be interesting to know how much energy they require per liter of water, or per activation. – LShaver Jun 13 '18 at 15:41

I don't think there is a clear answer to this question.

In a public environment where there is the risk of someone leaving the water running, then almost certainly yes. In a household where people can be taught and trusted to use water respectfully, especially hot water, then probably no, because the negligible resources saved is likely to be offset by the need for batteries or mains power. There is also the issue of complex equipment like this being more likely to fail than a standard tap, and less likely to be repairable. I'm a strong believer in KISS when it comes to issues of sustainability.

Motion activated taps do have other advantages though, in terms of hygiene and ease of use for some with certain disabilities.


The units themselves aren't terribly complex. The valves use the pressure of the water to open and shut. There is a small solenoid with a mechanism similar to a retractable ball point pen. One surge of power, clicks open, second surge, clicks shut. The rest is a simple motion detector. For a sink this is just a photocell that measures the light reflected off the sink bowl. When the light drops the circuit runs the solenoid.

There are variations in design. May run for X seconds. May run for Y seconds after the light returns to normal. May have a max of Z seconds.

Anyway: Simple design + large quantities + low cost = low resource usage (first approximation)

So the next question: Do they save water? A faucet with an aerator screen (makes the volume seem greater) runs about 2 liters a minute -- a cup every 5-8 seconds. Most bathroom hand washers are really hand rinsers, and only run the tap for a few seconds. Probably a large fraction use under 1/2 cup about 1/8 of a liter.

Payback: Another post mentions buying used ones for 60 bucks. Used plumbing at our local habitat for humanity Re-store tends to run 1/3 to 1/2 of new costs. Suppose a new one was $300. If water is scarce you may be paying as much as $5/cubic meter, or .5 c/liter (For comparison, my cost is about 10 c/cubic meter) Suppose that motion activated cuts water usage in half, to 1/16 of a liter. 32 handwashes per liter saved. 32,000 hand washes per cubic meter, 192,000 hand washes to pay for the faucet. If they have a 5 year life span, that's under 40,000 per year, about 5 per hour in a 24 hour facility, like an airport.

If water is cheaper (likely) the payoff is longer.

Compare this to the 1 gallon (4 liters) used to flush a toilet, or the 1 liter to flush a urinal, this is small usage. Add to that some large fraction of people don't wash their hands.

From a sustainability standpoint this is NOT low hanging fruit. Automated urinals that were smart enough to flush only every 3-4th usage if urine was close to clear would save more water. Or a system that used a mist to rinse down the sides of the urinal. Or a downdraft air current that whisked away the stink using no water.

Motion activated faucets have other merits: Conventionally, people with soiled hands are turning the faucet on, washing their hands, then turning the faucet off, recontaminating their fingers. More significantly, they are doing so with not just their germs, but with strangers germs. It may actually be safer to NOT wash your hands -- you are likely used to your own germs, and contacting people with only one set instead of 50 sets may be safer for them too.

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