Battle Coffee Pot vs. Microwave: An Energy Cost Analysis

For Christmas this year I got a new coffee pot. Among the wonders it offered over and above our old coffee pot (in addition to working) was a fabulous keep-warm feature that holds a pot of coffee at steaming hot perfection for four hours! Amazing, what will they think of next?

While I love the convenience of on-demand hot coffee, Erica raised an interesting question: is this the most energy efficient way to keep our coffee hot? Is it, she posited, more energy efficient to make a pot and re-heat extra cups in the microwave when we need a refill?

"In this corner, they call him 'The Energy Maker'!

Battle Coffee Pot

My efforts to calculate the answer got sidetracked with estimates of how closely a half-full pot of coffee approximates a black body and if the radiation emitted follows Planck’s Law. Internet research and back-of-the-envelope math told me only one thing: the winner of Battle Coffee Pot vs. Microwave was going to be too close to call without empirical research.

To get the hard facts I needed, I walked down to the local library and checked out a Kill-A-Watt, a handy plug-in power meter that the local utility company makes available through the library system. (Read more about how we used a Kill-A-Watt to assess the energy cost of our garage freezers.)

Ready now to collect some data, I tested three scenarios:

  1. Using the coffee pot’s keep-warm feature
  2. Reheating coffee in the microwave
  3. Brewing two smaller batches

In all three cases I started with the same temperature water (90 degrees Fahrenheit) and kept the temperature of the kitchen the same (66 degrees Fahrenheit).

For the keep-warm and reheat approaches I made a four-serving batch and poured two servings of coffee right away, a third one after an hour, and the fourth and final one after a second hour.

When reheating the coffee in the microwave, I took the third and forth servings to 168 degrees Fahrenheit, the temperature at which a refill of the keep-warm brew ended up after being poured.

For the two-batch method I simply made a half-size two serving batch, waited two hours, and made another half-size.

I ran each method twice and averaged the results. I didn’t get into wildly different approaches (French press, espresso, Italian style moka pot, etc.).

"And in this corner...don't mess with, 'The Little Nuker.'

Results

Coffee Pot Keep-Warm

  • Initial brew: 150 Watt-hours
  • Hold for 1 hour: 60 Watt-Hours
  • Hold for 1 hour: 60 Watt-Hours
  • Total: 270 Watt-hours

Microwave Re-heat

  • Initial brew: 150 Watt-hours
  • Reheat 1 cup: 30 Watt-hours
  • Reheat 1 cup: 45 Watt-hours
  • Total: 225 Watt-hours

Two Smaller Batches

  • Initial Brew: 120 Watt-hours
  • Second Brew: 110 Watt-hours
  • Total: 230 Watt-hours

In the end, the Microwave Re-heat approach won the day, by a scarce 5 Watt-hours over the Two Smaller Batches approach and 45 Watt-hours over the Keep-Warm function.

Now I’m not saying that this is inconsequential, but in the great scheme of things, it isn’t very much. Compared to other energy uses, even the larger 45 Watt-hours energy cost equals:

  • Leaving a single 60 Watt light bulb on for ¾ of an hour.
  • Leaving all my kitchen lights turned on full for just under four minutes.
  • Running an electric oven for thirty seconds.
  • About 0.014% of the average American home’s daily energy consumption.

So am I advising those of you with coffee pot keep-warm features to shut them off and head to the microwave instead? Honestly, not necessarily.

The margin of improvement is small enough that you may want to think about where you spend your conservation attention – and if that attention might be better directed elsewhere. Spend a minute warming your coffee and leave the oven on during that time…and you’ve undone all the good that your carefully planned coffee strategy could have done you. So would forgetting a cup in the microwave and having to come back an hour later to re-re-heat it (and I’ve done that more often that I’d like to admit!).

As conveniences go, 45 Watt-hours for is pretty cheap in my book, a reasonable price to pay for the simple joy of hot coffee waiting for me when I come in from wheelbarrowing woodchips around the garden or waking up early to get on a transatlantic conference call.

But if conservation is part of your very being, a continuous effort to pick at every detail of waste and inefficiency in your world, then those 45 Watt-hours may well make the difference between satisfaction and frustration.

So the ultimate decision is one of values and tradeoffs – pick yours, and enjoy a cup of coffee however you choose to make it!

The Fine Print

As fun as kitchen geek stuff is, the results of these same experiments done in your kitchen will vary according to the appliances you are using. Here’s what I used.

Coffee Machine: 12 cup Cuisinart

Microwave: 1300 Watt Panasonic

What Seattleites consider a “cup” of coffee, and therefore the quantity of all servings used for these experiments: 16 oz.

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Comments

  1. I think that the best solution to this one might be a thermos. Which has the added advantage that you can take along your fair-trade organic coffee on the road to the tune of (I think I figured) about $0.30 – 0.50 per cup. On account of, now you’ve got a spiffy thermos that you’ll want to use all the time. :)

  2. Nice analysis.

  3. Love it. We go back and forth with the French press, which feels more “off grid” but requires me to do work before my morning coffee. :)

    Either way it gets dumped into a large thermos and then stays warm for a good part of the day.

    • Just Nick says:

      French presses are great – I use one at work a lot because the company supplied coffee is indigestible! I have this vision of doing a second round of this analysis and looking at all of the “alternative” coffee technologies: presses with various ways of heating the water, instant (aack!), various types of espresso machines. Now if I could just get Williams Sonoma to sponsor that post by donating a half dozen different types of coffee technology…

  4. Great analysis! We usually only pour one giant mug in the morning so we turn it off right after we pour. Sometimes Tom will have one in the evening but he rebrews it.

    • Just Nick says:

      Thanks! Unfortunately, Erica and I are both “just keep it flowing” types when it comes to coffee. We’ll change over to decaf sometime between 4-6pm, but otherwise it is a more-or-less continuous stream of Java. Homebrew and coffee…the twin fuels of the NW Edible Life blog!

      • elizabeth says:

        We have a Gevalia that we use and the coffee is brewed right into
        a thermos-carafe. It stays hot for quite awhile. The maker is years
        old now, maybe 10 or more. It seals tight also so if we make at home
        can take along for the day.

  5. My last 2 coffee pots were exactly like yours. First one lasted for several years, but the last one only worked for only 8 months before dying. In December we bought a new Cuisinart coffee pot with a thermos style carafe. The coffee stays hot for a couple of hours but would last longer if the lid had a better design, I believe. The current design allows you to pour without doing anything to the lid, so heat escapes over time. After only a few months we’re finding that the lid has opening and closing issues that can leave a huge mess when pouring. With a little tweaking, the new Cuisinart could be a real energy saver.

    I found the biggest cost savings when it comes to making coffee is weighing out the coffee instead of just scooping. The old Cuisinart is very large for one or two people. It’s very easy to over fill those scoops and your coffee pot takes more coffee than the regular size pots. I found that by weighing the coffee we saved many, many dollars on coffee in a very short time.

  6. Love the analysis. And the constant stream of Java explains why Erica has all the energy I perceive through the blog ;D (I save coffee for special occasions and therefore am mostly kinda confused during the day)

    • A Finn – actually, my energy levels are much more consistent and even (and not, surprisingly, particularly low) when I cut out all caffeine. I actually think I perform better without it. But that presupposes a decent night’s sleep, which has been in short supply around here for awhile. ;) When I’m sleep deprived, which for me is anything under 8 solid hours a night, I turn to the java beast to fill in the energy gaps…all….day….long. :)

  7. I’ve been cold-brewing my coffee lately. You can make a big batch of concentrated coffee at once and then mix it with hot water or milk as you drink it. The energy it takes to heat the water or milk needs to be accounted for, of course, but once iced coffee season hits I’m golden!

  8. Arrowleaf says:

    We make a Chemex of coffee in the morning and leave it on a trivet on the woodstove all day. Summer is a one pot season, or is supplemented by cold coffee drinks made from coffee cubes of frozen java…Either way, I relish my morning coffee and it is surely one of the greatest joys in my life!

  9. Just Nick says:

    Wow, it has been an absolute joy reading some of the different ways that people prepare and enjoy their coffees! Not just from an energy standpoint, but thinking about how we all do it – one big batch, many little methodical batches, wood stoves, etc. Great variations on the ritual of the coffee!

  10. Lady Banksia says:

    Here’s my scenario – (from which I star by saying I concur with the thermos/insulated carafe platform). I scored big at a local thrift shop by finding a perfectly good, yet needing a cleaning, high-end 10-cup thermal carafe coffeemaker for $5.00. (I add that it originally retailed for over $150.00. I don’t the previous owners knew what they had…it makes fabulous coffee.)

    Anyway, it doesn’t have a warming plate underneath that stays on. It warms to heat up the carafe while it is brewing and filling, but it doesn’t stay on. Yay, because that way, the coffee isn’t cooked on the warming plate, as is common for glass-carafe units. I sometimes pre-warm the carafe by rinsing it with warm water (just a little…), but beyond that, I’m set for several hours. If it does start to cool down a touch before I’m finished, I just warm it in the M/W for a few seconds… good as new.

    One of the best $5.00 investments I”ve ever made.

    Oh, and a tip for iced coffee lovers – freeze your remaining morning coffee in a dedicated ice-cube tray, then into a freezer bag. Voila! Coffee cubes ready for your blender, however you like it fixed – milk, sweetener of whatever sort, etc.

  11. I use to use a Thermal Carafe for coffee. But it got old (the carafe, not the coffee) So I put waway my cuisinart computerised coffee pot and ot a black and decker Brew and Go. Makes on cup at a time, and if you opt not to use the cheap plastic travel mug and use a better mug (I use my 1980 Stainless steel starbucks wide body mug) it stays warm enough to drink for over an hour! I have a friend who swears by cold brewing her coffee in the reefer, then popping it in the microwave, cup by cup. But I am happy with my little brewer!

    • PS- This is not one of those “pod” coffee makers- You still gotta scoop a scoop of coffee in to the basket. And it doesn’t have a warming plate either.

  12. We use a Senseo (with refillable pods, so we can use whatever coffee we like) and make one cup as needed. I’m not sure what its energy usage is, but considering how much “leftover” coffee we were pouring out with a more conventional coffee maker, I’m sure we’re coming out ahead. Would be a different story if we were actually buying those disposable pods for it (I’m not sure they even make them anymore.)

    Anyway, I love this kitchen geek stuff, but I can’t help but wonder how much energy was used getting to the library to check out the energy meter. ;)

  13. Good for you! :)

  14. I brew in a french press (because the flavor is better) then pour it directly into a thermal carafe. It stays warm until the next day, and you can taste all the subtle nuances. I haven’t used a coffee maker in about 20 years.

  15. I agree with your analysis for coffee. However microwaves do destroy the enzymes in food so the healthier choice would be one of the two other options. I’m just not up to date on coffee enzymes. Does it even have them?

    I’m not one to talk though. I gave up coffee for tea, especially chai. Making chai involves heating individual cups or brewing a big batch on the stove and reheating. Either way I am microwaving and milk does have enzymes. I’d love to be an enzyme purist but who has time anymore.

    Thanks for the energy info. Good information for me to store away for the future.

    • Just Nick says:

      I doubt coffee has any enzymatic activity anyway…the heat of roasting the beans would certainly destroy any enzymes they’d contain (and if they didn’t I suspect the brewing water temperature is high enough to do so as well). In brewing the rule of thumb is that if something gets much about 160 degrees then you can count on enzymes shutting down. That’s usually what gets you with microwaves, too, localized hot spots take the temperatures up enough to denature the more delicate bits.

  16. The thing is that energy consumption is just one factor. From a flavor standpoint, the worst thing to do is brew one pot & let it sit on the warmer all day. This cooks down the coffee – makes it more concentrated, and also burns it. I know several big coffee drinkers who insist that microwaving a cold or lukewarm cup of coffee causes no appreciable difference in taste. So I would say the other two methods are comparable, as each time you’re getting a good cup for about the same energy usage. The brew two batches option would be the better flavor alternative, but you’re doing the same work twice and isn’t always an option. At my work there’s a pre-measured coffee packet – 1 per carafe. I suppose I could calculate the coffee and water carafe by volume & divide each by half, but the amount and speed with which the pot is finished varies from day to day. Is it hot outside? Is there a meeting? Have too many people been burning the midnight oil?

  17. We use a cold-brew system from http://www.hourglasscoffee.com/

    The “brewing” uses no electricity – and we reheat it in the microwave. It makes a good tasting cup of coffee and it supposedly has a lower acid level than conventionally brewed coffee. You can also “brew” the same grounds twice.

  18. This is awesome! Would you be able to do the same thing with the dishwasher versus washing by hand? My DH insists that it’s more energy-efficient to use the dishwasher, but I like having the kids do them by hand. (I call it “Life Skills Class” and count it as homeschooling! Ha ha.) Is there any way to calculate this?

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  23. Great read Thanks!

    I am not a NWerner, but enjoy my coffee and prefer to leave the burner on so that I do not have to stand around in the kitchen for 75 seconds or so twiddling my thumbs. It does not last long enough for it to get a “burnt coffee taste (that is like 8+ hours which means turning the coffee maker back on 4-5 times)

    16oz cups here a considered normal on the far east coast of New Jersey. No “soup cups” for me though, coffee gets cold before I finish it, and I do not like iced coffee

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