Energy Down the Drain, Literally

In a previous post, I patted myself on the back for replacing my shower head and saving 34% on my water usage during showers.  As an aside, I said that it was a "bonus" that I was also saving energy too, because I burned less natural gas to heat less water.

Well, I've been thinking about this - Is the energy savings just a nice little bonus, or is it actually a huge benefit?  Here are the numbers, if you're interested:

I don't know the exact temperature of the water being put into the water heater, or the temperature of my shower, but I've estimated 60 F and 105 F, respectively.  That 45 F degree difference is exactly a 25 Kelvin difference.  To raise the temperature of 1 gallon of water by 25 K takes about 400,000 Joules (heat capacity of water near room temperature: 4.2 J/(g-K)).

That means taking a shower uses energy at a rate of 13,000 joules/second, or 13,000 watts.  That is huge!  My shower is consuming energy at a faster rate than my whole-house air conditioning unit (see previous post)!

Granted, it's only for 10 minutes, but still.  Let's put it in terms of total energy.  I'm probably using about 6 kwh/day in electricity at this time of the year.  A ten minute shower uses 2 kwh, meaning my shower is 1/4 of all of my energy use in my house.  jeeez.  So in that respect, using 34% less water is HUGE, a savings of over 8% on my total energy use in my house, just by using a more efficient shower head.  awesome.

Note 1:  This is probably a lower limit on the amount of energy used because the water loses some energy as it sits in the tank, waiting for me to use it.  Thus, it needs some reheating.

Note 2:  All of these units of energy are interesting, but the energy to carbon dioxide conversion is not the same for electrical generation (via burning coal) and heating (via natural gas).  But that's for another post...