My latest energy savings toy -- a pressure cooker

Don't know what to get your Significant Other for the Holidays? Looking for a "green" present? Feel as if you haven't boosted the Chinese economy enough yet this season?

Let me recommend getting a pressure cooker.

I bought one a while back for canning, and have finally gotten around to using it for cooking. I'm impressed. It's now my favorite kitchen toy. Takes less energy, takes less time. Built like a tank. What's not to like?

I got the Presto Professional Pressure Cooker 8 Quart Stainless Steel. But as far as I can tell, they're all pretty good. Here's what I like about them.

1) Essentially, they're like diesel engines -- have to build them right in the first place, or they'll blow up. So it's a nicely-made cookpot.

2) If you're a complete and total dumb###, you can cause damage (by failing to turn down the heat once the pot starts steaming, which will blow out the safety plug.) I like consumer objects that have a built-in Darwin test.

3) Works as advertised. Soaked kidney beans, 3 minutes at pressure. Whole chicken, 20 minutes to falling off the bone. For the first time in my life, I can cook garbonzo beans and not have them end up crunchy.

4) It's just basic physics and chemistry. Higher pressure = hotter, hotter = faster chemical reactions.

5) Very retro, and yet so modern.

6) The "steam release" looks and sounds extremely cool. As in, if any of your other consumer goods did that, you'd get the h### out of the room in a hurry.

7) And if you actually want it made-in-America, there's a company that still makes big expensive heavy-duty ones here -- the All-American is made in Wisconsin.

 

Short answer: Not much. Near as I can tell, average US family spends about $100/year in the direct energy cost of cooking. But the average US family doesn't cook much.

Intermediate answer: No good data on the 'net that I could find. Lot of armchair calculations on based on BTU of a stove burner, ignoring the fact that you mostly simmer.

Long answer: Perform the experiment. Roughly 11 cents savings on a batch of hard beans, @15 cents/KWH for electricity.

Thought about using a Coleman stove and weighing the fuel bottle pre/post, but a hot plate and a Kill-a-Watt was a lot more convenient. So, 6 cups of water in my pressure cooker, and get the following energy use:

1) Bring to boil: 0.29 KWH
2) Simmer, lid on, no pressure: 0.50 KWH per hour
3) Bring to from boil to pressure: 0.09 KWH
4) Simmer at pressure: 0.36 KWH per hour.

Hypothetical pot of garbonzo beans:
Traditional: Bring to boil, simmer 2 hours.
Pressure cooker: Bring to pressure, cook 25 minutes, do not release pressure.

Estimated use and cost:
Traditional: 1.29 KWH
Pressure cooker: 0.53 KWH
Difference: 0.76 KWH
Cost difference @ $0.15/KWH: 11.4 cents.

Observations:
0) It does indeed take less energy to keep the pot simmering under pressure than it does merely covered. Lot of people say that, that seems to be my observation on the stove, and the data validate it.

1) The 0.09 KWH to bring up to pressure is partially recovered at the end. On this test, the cooker maintained pressure (hissing steam) 6 minutes after I turned off the heat. I accounted for that in the 25 minute time for the beans.

2) This does not account for HVAC effects, this is just the direct energy cost.

3) The cooker cost about $70 and has a 12 year warranty. Taking that as the lifetime, I'd have to cook about a pot of beans a week to pay for the cooker in energy savings.

4) At one pot of beans per week, the global warming effects are ambiguous, due to the tradeoff between lower C02 emissions and increased methane emissions.

 

Well, as my high-school physics teacher used to say, when in doubt, perform the experiment.

Experiment, continued: 6 cups water, covered pyrex container, microwave oven, bring to boiling: 0.40 KWH.

I wouldn't necessarily generalize that, but, for sure, my (elderly) microwave isn't as efficient at heating water as my closed-burner electric hot plate is.

Oddly, my results are well in line with US DOE averages. Resistance electric heat smooth cooktops are listed at 74% efficient, microwave ovens at 55%. So DOE data would project 35% more electricity use in the microwave, where as I found 38% more for my particular microwave and hot plate.

http://www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/cookgtsd.pdf

Everyone agrees that microwaves are more efficient than conventional electric ovens. (The document above lists a convention oven at 12% efficient). But for boiling water, the averages say no, and my particular experiment says no, the microwave isn't more efficient. Microwaves are not lossless. So it's a question of whether the microwave loses more than the particular resistance heater being used. For example, all sources seem to agree that a dedicated electric kettle is much more efficient at water heating than a microwave or a stovetop.

Not that I would boil beans in a microwave anyway.

On nutrients, its a time versus temperature tradeoff. Pressure cooker advocates claim better nutrient retention than open-pot cooking, though I have not seen the data to back that up. But as near as I can tell, the scientific literature on canning all boils down to this: Given the temperature-versus-time tradeoff, you preserve more nutrients with high-heat, short-time cooking. I've learned enough to know that the time/temperature tradeoff varies by particular nutrient, so the most I'd say is that I see nothing to suggest that a pressure cooking loses more nutrients than the equivalent open-pot cooking. Versus a microwave, it's completely plausible that the microwave does better due to the shorter cook time.

 

Huh. So I've been wasting energy by heating my tea in the microwave. I drink a lot of tea. Call it four cups a day, 2 min each, 1500W, x 365 = a truly impressive 73 KWH annually for tea drinking. Kettles are listed as being about 90% efficient, versus (say) 55% for my elderly microwave. At $0.15/KWH, I'd save about 28 KWH or $4.25 per year on my tea drinking alone, by switching to some type of electric immersion heater. That gives me less than a 4 year payback period on one of these:
http://www.amazon.com/dp/product/B000C3QSPQ?tag=priuschatcom-20 can't pass that up.

 

Got it -- my proposed solution is the "hot shot" device above. You fill your cup, pour it in, it heats that cupful, then you dispense it out the bottom of the reservoir. Turns out they've been selling those for decades (my mother-in-law had one when my wife was in college), so it must be a pretty decent design.