Power Consumption

A few weeks ago, somebody posted a question about Joule power consumption. I just completed an exercise that might provide some information.

I recently purchased a device (Kill-A-Watt or KAW) that measures power consumption of individual appliances. You plug the appliance into the KAW. You then plug the KAW into an electrical outlet. To obtain the most accurate readings, leave the appliance connected for several hours. The device reports power consumption in terms of kilowatt hours (KWH). You can input the rate your utility charges per KWH. The device will then estimate the cost of the power consumed by your appliance per hour, day, month and year. My utility charges $0.13 per KWH. I tried the device on several appliances (freezer, refrigerator, etc.) and the results seemed plausible.

I then tested my beloved Joule. The test involved preparing a 2” thick 12 oz eye of round steak at 129* for 24 hours. The water bath sat in a Rubbermaid 8L container filled to 7.5L. Before connecting the Joule to the KAW, I preheated the water bath to 129*. The amount of power consumed during preheating would vary by the amount of water, type of container and the starting temperature of the water. I didn’t want to deal with these variables.

After 24 hours, I recorded the results. Power consumed was just shy of 3 KWH. The cost of the power consumed at $0.13 per KWH was $0.38. Seems like a bargain to me.

The eye of round was between rare and medium rare. The texture reminded me more of sirloin than round. The flavor was wonderful. I’ve tried to prepare round at this level of doneness with conventional cooking methods. You pretty much need a jackhammer to cut the thing. Since round is quite a bit cheaper than sirloin, the money one can save using Joule with the cheaper cut far outweighs the cost of power consumption. The savings should eventually pay the cost of the Joule.

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Comments

wolfiegirl

Cool! Thanks for sharing.....it really is a bargain to cook with Joule! Low electrical usage and fabulous results with cheaper cuts of meat! WIN-WIN!

fisher23

Thank you.

Clinton_Ingram_416399

Thanks for sharing that. I've wondered about it myself but hadn't bothered to do a real test. People on here have come up with some pretty clever insulation solutions for long cooks as well. Would be cool to see what kind of difference that makes.

Steve_372295

I didn't insulate my container. Therefore the surrounding air (68*) would tend to cool the bath making Joule work a bit harder. Also, there was evaporation. I added a cup or so of water about three times during the 24 hours. Again, Joule needed to work a bit harder for a couple of minutes to bring the bath back up to 129*. I don't believe the added power consumption from both of these causes would be more than one or two percent.

Clinton_Ingram_416399

I think you'd be surprised how much of a difference even adding a plastic-wrap cover would have. In addition to the heat required to get the added water up to your target temp, there's the latent heat of vaporization to consider for the water that was lost.

Here's a quick bit of math to estimate that, assuming you lost 3 cups to evaporation over 24 hours:

3 cups of water weighs roughly .71 kg

The latent heat of vaporization for water is 2257 kJ/kg

1 kWh is equal to 3600 kJ

So that's .71 * 2257 / 3600 = .45 kWh lost to evaporation

Of course those are all physics numbers, so distilled water at sea level, etc etc... but you get the idea... it's a decent percentage of your total energy used.

Either way, 3 kWh for 24 hours of cooking is only 125W average usage, which is pretty darn good under those conditions. If you do another test with a cover or some insulation, I'd love to see those results.

Cheers!

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