Saving Pennies or Dollars is a new semi-regular series on The Simple Dollar, inspired by a great discussion on The Simple Dollar’s Facebook page concerning frugal tactics that might not really save that much money. I’m going to take some of the scenarios described by the readers there and try to break down the numbers to see if the savings is really worth the time invested.

Kathy said, My husband takes his lunch almost every day to work. If my son and I go out we’ll take our lunch as well. This equates to big meal cost savings but now that we’re using TONS of tupperware to cart our bag lunches I have to run the dishwasher constantly. With water, electricity, soap, etc. Are we still coming out ahead? Is this saving us big dollars or just cents?

Let’s do the math, shall we?

To test out Kathy’s question, I took out a bunch of our own reusable containers that we use for our lunches and filled up the dishwasher with them. I wanted to see exactly how many it would take to fill up our rather typically sized dishwasher.

What I found is that I was easily able to get eighteen containers with lids into our dishwasher pretty easily. Some of these were pretty large containers, while others were pretty small, so I think eighteen is a pretty good average.

Energy Your mileage may vary somewhat, but this data indicates that the typical dishwasher uses 2 to 5 kilowatt hours per load (including heating the water). We’ll use a 3.5 kWh average. A kWh of energy costs roughly $0.11, so the energy cost is about $0.38.

Soap You can find soap or detergent at a lot of different prices. I was able to quickly find dish detergents and soaps that varied from $0.10 per load to $0.35 per load. Let’s figure $0.20 per load.

Water The average dishwasher load uses about 15 gallons of water. Water is usually sold by the acre-foot – 325,851 gallons for $120 (or so). You’ll end up spending a cent or two on the water depending on how your municipality taxes it.

Your cost per load of dishes is about sixty cents, all told. This means that your cost per container for running them through the dishwasher is just a hair over three cents.

Remember, each time you use a container, you’re eating inexpensive leftovers for a meal instead of eating out. You should easily be saving multiple dollars each time you use a container.

What about the cost of the containers? Unless you’re drastically overpaying for such containers, you shouldn’t have spent more than a few dollars per container. It only takes a leftover meal or two to recover the initial cost of the container.

Thus, the math pretty clearly shows that washing your own containers and using them for leftovers saves dollars, not pennies.

Of course, there is a small time investment of loading and unloading the dishwasher, but the savings for that small amount of time makes it quite worthwhile.