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Energy Efficiency Blog

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Posts Tagged ‘energy’

Sep 09

The next appliance to get the wireless power meter treatment is my 3 month old dishwasher.

It’s a Fisher & Paykel DW60CDX2. It is actually the thing that inspired me to make the sokitt appliance comparison tool because it was inordinately difficult to work out which dishwasher under $1000 gave me the best water and energy efficiency.

Anyway, here’s the 24hr energy profile of the beast:

DW60CDX2 Dishwasher

DW60CDX2 Dishwasher

As you can see, we only used it once, and it spiked up to over 2000W (2kW) on 2 occasions. The first spike must be the initial water heating and the second spike the heater for drying the freshly washed dishes. It’s interesting to see that apart from the heating actions, the dishwasher uses very little power for the rest of its cycle.

Zooming in:

DW60CDX2 Dishwasher wash cycle detail

DW60CDX2 Dishwasher wash cycle detail

This wash cycle was the eco-cycle (of course!). The cycle lasts about 90 minutes and if there was no heating, then the dishwasher would use about 130W for the 90 minutes, that would be a power use of 0.130kW x 1.5hours = 0.195kWh which in cash terms = 0.195 x $0.15 = 3 cents.

However because the dishwasher is connected to cold water only, inefficient resistive heating has to take place causing these spikes, this brings the power use for one wash up to 0.714kWh (as calculated for me by the power meter’s internal accumulator). That means that each cycle costs me 11 cents in electricity. In terms of annual electricity use, the dishwasher uses 365 x 0.714 = 260kWh per year.

[ The energy rating by the Aussie government benchmarks this dishwasher as using 233kWh per year (3.5 stars), so my DIY calculations are pretty close to their test lab's results :-) . ]

One other thing from the charts above – this dishwasher is pretty good in standby. It only sucks 0.7W when not washing, that’s about the same as a mobile phone charger and costs me about 70 cents a year in electricity. I can live with that.

So what’s the scope for reducing the energy use here. I have 3 options.

1) use the dishwasher less
2) buy a more efficient dishwasher
3) heat the water more efficienctly

1′s a no no because we generate too many dirty dishes.

As for 2? Lets use the sokitt advanced comparison tool and see what my options are.

If I go to the advanced page then choose my dishwasher, sokitt shows me all the dishwashers that i can buy that use less energy, and calculates the payback. Let’s give it a go:

Step 1 go to www.sokitt.com.
Step 2 click on the advanced link under the main selection box.

Advanced Appliance Energy Comparison

Advanced Appliance Energy Comparison

Step 3. Choose Dishwasher, click Go

Step 4 – Choose your dishwasher make and model

should you upgrade your dishwasher?

should you upgrade your dishwasher?

Step 5 – look at the results here

This screen is showing me all the dishwashers I can get in Australia which have a better energy rating than mine, and details of how much energy they use, plus how much they cost, and how long it would take me for any savings in electricity to pay for the new appliance.

Interestingly all the appliances that have a better energy rating actually use more electricity (look at the kwh/yr column). They get a better energy rating because they have a bigger capacity (i.e number of place settings), and the energy rating is based on electricity use per setting.

This means that the payback for each of these is “never” because they will cost more to run than my current dishwasher. At least that puts my conscience at ease!

So that leaves option 3. Heat the water with a more efficient source, e.g gas or solar. This means either putting hot water into the cold inlet on my washing machine, or finding a new washing machine that has a dual connection – e.g hot and cold water. Putting hot-water in the cold connection is a bad idea because then you are using hot water for everything, including the parts of the cycle that only need cold water. That would waste more energy than we save.

Dishwashers in Australia with 2 water (hot and cold) connections are rarer than rocking horse poo. I can only find 2 for sale, both Bosch. The SGS55E42AUW and the SGU55E55AUSS.

(on the to do list – add a filter to show dual connection dishwashers on sokitt)

If I got one of these and piped solar heated water into the hot water connection, my use could potentially drop from 260kwh per year to (0.195kWh x 365) 71kWh per year – saving 190kWh per year. However, the reality is that because dishwashers use so little water these days, it would draw mostly the cold water in the pipe and very little hot water, and have to heat it anyway. So I would need very short pipes between the hot water tank and the dishwasher.

Or I could get a device like the Chilli Pepper Hot Water System for about $200. This nifty little thing recirculates the cold water in your pipes until hot water comes thru. Neat eh? This would guarantee that your dishwasher filled with hot water and didn’t have to use the dishwasher’s crappy resistive heater.

So cost for saving energy with my dishwasher:

New dishwasher with dual water connection $999
Chilli pepper hot water thingy $200

Possible savings (assuming water is heated by non electric means) 190kwh/year = $28.50 per year.

Hmmmm…