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Friday, July 4, 2008

Calculating Dishwasher operating cost

I was asked how I calculated and compared dishwasher operating cost. Aside from acquisition cost, there is the electrical consumption of the dishwasher itself, how much water it uses, the cost of heating the water, and the cost of detergent and additives.

Read on for the nitty-gritty and the tools that I used.

One of the variables in using a dishwasher is how often you use it. Typically our machine runs twice or three times a week. For us the Energuide rating, which is based on 215 cycles per year seems about right. This is fairly easy to adjust for your own use by dividing the Energuide rating by the number of cycles to get the per-cycle consumption, then multiplying by the number of times you use the machine annually.

For example, our Samsung DMR77LHS is rated at 315 KwH per year, which works out to 1.46 KwH per cycle. If we used the machine daily, that would work out to 534 KwH. So based on our KwH rate of $0.0827 (including tax) the annual electricity cost is $26.

Next is water consumption. This information is a little more difficult to make sure it is accurate. For example, the Energy Star Appliance Directory on NRCAN's site lists water consumption per "normal" cycle, but this is often a different number when you look at manufacturer data. For example, the Bosch SHX36L is rated at 10.9L of hot water consumption in the Energy Star directory, but the Bosch user manual says anywhere from 9-22L. Also if you don't use the normal cycle (we always used the Energy Saver cycle with the Kenmore) this number can be misleading. In my case it was worth it to compare water consumption on the "Quick" cycle (which was 20 L for the Samsung and 13L for the Bosch). This number is not so easy to find as some manufactures (like Blomberg) don't supply this data or don't have a Quick cycle.

Since our water is metered, it was fairly easy to calculate 20L (per cycle) x 215 (annual cycles) x $0.001 (avg. cost per litre) for $4.30 per year.

Now that we know our electricity cost and water consumption, we can calculate the hot water cost. I found this nifty calculator to figure out the annual hot water cost as follows:

I converted annual litres to U.S. gallons and divided by 365 to get "daily gallons" (4300L / 3.67 = 1172 gallons, then 1172 / 365 = 3.2 gallons per day). The calculator gives $21 per year hot water cost for our Samsung, based on an electric water heater at $0.0827 KwH.

Detergent can make a huge difference in annual operating costs. For example our Samsung takes only 15g of detergent for a normal load vs. 25g for the Bosch and 35g for the Asko. Based on a 3kg box of Electrasol powder at $10, (including tax, which works out to $0.003 per gram) we are looking at $9.65 a year for the Samsung vs. $16 for the Bosch and $22 for the Asko. And don't forget the rinse aid (we go through a 600ml bottle per year, for a cost of $5)

So to add all this up we have:

$26 (electricity) + $4.30 (water) + $21 (hot water) + $14.65 (soap and additive) for an annual cost of $65.95. The Bosch and Asko came in marginally lower, with their higher detergent consumption canceling out their slightly better energy and water consumption.

The big difference, and the justification comes from comparing operating cost of our 6 year old Kenmore:

30L water per cycle, 800 KwH annually and 60g of detergent means an annual cost of $157. Our $1000 Samsung dishwasher will basically pay for itself in energy savings in a bit less than 9 years (given 3-5% annual increase in consumables), which is a bit more than 1/2 of its expected life.

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