How Wasteful are Tankless Heaters?
Published at 12:49 on 23 January 2012
Let’s do a little math.
First, Wikipedia states that a navy shower can use “as little as” 3 gallons of water, while leaving the water running can use “as much as” 60 gallons. So, in the best case (this is important), a Navy shower uses 1/20th the water as leaving the water running.
Second, according to the US Green Building Council, a tankless heater can heat water as much as 34 percent (again, another best case) more efficiently than a conventional tanked one.
Now, let’s give tankless heaters the benefit of the doubt here: We’ll compare the two best cases, and then weight the comparison in favor of tankless heaters by ignoring how much more water one sends down the drain while adjusting the temperature.
In other words, we will assume that we’re heating 20 times the water, but doing it 34% more efficiently:
20 ÷ 1.34 = 14.9253
That’s right, a tankless heater is nearly fifteen times more wasteful than an old-fashioned tank-style heater if you’re already used to taking a navy shower (as I am; in fact, I prefer it). I’ll repeat that: fifteen times. Not fifteen percent more wasteful, fifteen times. An order of magnitude.
If you’re not already taking navy showers, you can therefore save far more energy just by changing your usage habits than you can by changing your water heater (and I’m not even including the energy cost of manufacturing and transporting the new water heater in this).
For other uses, the wastage isn’t so great. For rinsing off a few dishes, I figure I’m maybe using three times the water. Even then, it doesn’t look so good for a tankless heater:
3 ÷ 1.34 = 2.2388
Wow, just slightly over twice as wasteful. I’m so… impressed.
Given the choice, I’ll stick with with an old-fashioned tank, thank you very much.