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May 142014
 
Which is more energy efficient to boil water?  This $10 plastic electric jug, or a $500 microwave oven?  The answer might surprise you!

Which is more energy-efficient to boil water? This $10 plastic electric jug, or a $500 microwave oven? The answer might surprise you!

We recommended either buying or making your own ‘Wonderbag’ type product and using it for an energy-efficient type of slow-cooking yesterday.  But what about cooking items when a slow-cook approach is not practical or possible?  For example, what is the best way of boiling water?

If you want to boil water, you probably have various choices – you can boil a kettle (or a pot) on a stove top element, you might have an electric jug, you could use an oven, or a microwave oven.

Now it goes without saying that using a regular oven would be a very slow and inappropriate process, but what about the difference between, eg, stove top, an electric jug, and the microwave?

We were able to exactly test the difference between an electric jug and a microwave oven, and we can empirically comment on the stove top as another alternative.

For our testing, we used a microwave oven that had a nameplate power rating of 1560 watts, and an electric jug with a nameplate power rating of 1500 watts.  We heated one liter of water in a glass container in the microwave, and one liter of water in the electric jug itself.  The electric jug did not have an immersion exposed element, but rather had a smooth base and the element directly below it.

We observed a rise of 34.9°C by the water in the jug, and 19.1°C by the water in the microwave during the two-minute period.  We also noted that the water in the jug was slowly continuing to rise at the end of the heating period – this was to be expected because the very hot electric element had some thermal inertia and was continuing to transfer energy after it was switched off.

So, a quick result is that there was almost twice as much net heating from the jug as from the microwave, even though the microwave was drawing slightly more power.  That would seem to argue conclusively in favor of using the jug rather than the microwave.

We were interested to know exactly how efficient each process was, so we did the calculation to compare the electrical energy consumed and the thermal energy created.

Two minutes of the jug at 1500 watts represents 50 watt hours of power.  Two minutes of the microwave at 1560 watts is 52 watt hours of power.

Increasing the temperature of 1L of water by 34.9 degrees requires 40.52 watt hours of energy.  So, for the jug, we got 40.52 watt hours of heat from 50 watts of electricity, which is an 81% efficiency rating.

For the microwave, the 19.1 degree temperature rise required 22.18 watt hours of energy, and we used 52 watt hours to create that.  This represents a 43% efficiency.

Clearly, the jug is much better than the microwave for heating water.

Where Did the Rest of the Energy Go?

You might be wondering what happened to the rest of the energy.  In the case of the jug, the balance of the energy was probably radiated away from the jug – heat from the sides of the jug, and more heat from its spout at the top.  An 81% efficiency rating is actually a reasonably good result.

The microwave’s much greater energy loss requires a bit more explanation.  First, we have the efficiency (or perhaps we should say, the inefficiency) of converting electricity to microwave energy.  This is generally thought to involve about a 40% loss of energy.  So, of the 52 watt hours that went into the microwave unit, 20.8 of them got ‘lost’ in the electronics.  More power was spent to spin the turntable, to illuminate the light, and to operate the fan (although these three things are all moderately low power drains).

Not all the microwave energy inside the cavity (and of the 52 watts, probably less than 30 watts actually ended up as microwaves) was absorbed by the water.  In addition, just like the heat that was lost out of the top of the electric jug, the open beaker we had the water contained within definitely was allowing heat to escape from the top.  If we had some sort of lid to put on the beaker, that would have probably made a measurable improvement.

So, the observed efficiencies are in line with the theoretical estimates of energy losses.

The Best Electric Jug?

If you don’t yet have an electric jug, we’d suggest you consider a plastic one, because the plastic will give you better insulation and have less heat loss through the jug sides than is the case with a pretty nice looking stainless steel one.

Our favorite jug (which is not the one we tested with) is this Proctor unit.  It is the one pictured at the top of the article.

It is plastic, it has a small minimum fill requirement, it has a fully exposed element for best heat transfer, and – wow – it is only $9 at Amazon.  What a deal that is.

Hidden Microwave Advantages

On the face of it, you’d think there’s never a reason to use a microwave oven instead of a jug when you want to boil water, right?

Well, actually, wrong.  If you are boiling a jug, you need to put a minimum amount of water in it, no matter how much water you need to heat up.  Indeed, our test jug suggests a 1.3L minimum fill (but note the Proctor unit is happy with only 300 mls).

With the microwave, you only need to put a single cup of water in it, if you are only needing to heat a single cup of water (a cup of coffee requires maybe 400 mls, depending on how large a cup you want).  In such cases, this may compensate for the microwave’s lower efficiency.

Stovetop Cooking Considerations

Okay, so that sort of explains the relativity of microwave ovens to electric jugs.

But what about boiling water on the stove top?  That is a bit harder to establish without special test equipment and digging in to the stove’s wiring or gas pipes to accurately measure energy consumption, and it also varies from case to case depending on the efficiency of the heat transfer from the heat source to the heat recipient (such things as the size and shape of the pot bottom, the size and shape of the element/burner, etc), the pot material (glass, aluminium, copper, steel, etc) and so on.  Two different scenarios could give you two massively different results, with one twice as good/bad as the other.

However, there have been some studies done which have clear and interesting results, and if we assume reasonably optimized setups, we can make some generalizations.

The least efficient form of heating is invariably gas.  You are lucky to get about a 35% – 40% efficiency from a gas burner on a stove – that is, for every three units of gas energy, you get one unit of heat transferred into your pot.

Regular smooth flat electric elements are rated as about 70% – 75% efficient, and induction cookers are about 80% – 85% efficient.

Another source claims 55% efficiency for gas, 65% efficiency for regular electric, and 90% efficiency for induction cooking.  As we said, a lot depends on the specific setup you’re using.  While the numbers are different, the relativity is the same.  Gas is the least efficient, regular electric in the middle, and induction way in the front.

In particular, if you have gas, make sure the flames do not spill over the sides of the pot.  That’s totally wasted heat.  Any time you see the water boiling first around the side of your pot, you know you are wasting gas heat and should turn down the gas.

For electric cooking, make sure the pot bottom sits flat on the element surface, and is clean.  Dirt acts as an insulation barrier, and if there are air gaps, then you are heating the air rather than the pot.

Induction Elements

Normally, when electricity is abundant and relatively inexpensive, no-one cares about the greater energy efficiency of an induction cooktop, and you have to be more specific about the types of pots you use with an induction cooktop, too.  Many of us also prefer the greater control of gas compared to traditional electric elements, and although gas is less efficient, it is also usually cheaper, per unit of energy, to use gas rather than electricity, so the efficiency issue is sort of cancelled out by the cost saving.

But WTSHTF and all energy becomes scarce and costly, it becomes very beneficial to consider an induction cooker.  There are other benefits to induction cooking, too – it is a bit like gas because it too can instantly increase or decrease the energy being applied to your pot, with no ‘thermal lag’ as is the case with regular electricity.  It can also do clever things like detect if your pot has boiled dry or not.

The good news is you don’t need to go out and buy a whole new stove top right now.  You can simply buy a single free-standing induction cooker.  Amazon has them for about $60 – $100, they are available elsewhere too of course.

We see some model induction cooktops are rated at 1300 watts and others at 1800 watts.  While you might instinctively go for the 1800 watt unit, there’s a potential small problem there.  1800 watts on 120 volts requires 15 amps of current.  So make sure you run it off a 20A rated circuit, and make sure you don’t share the circuit with anything else that consumes much power, or else you’ll trip the circuit breaker.

Needless to say, practice with the induction cooker, so you know its quirks and how to get best (and most energy-efficient) use from it.  And make sure you have the appropriate pots to go with it too – ideally pots the same diameter as the induction heating circuit.

Oven Cooking

An oven can be either an efficient or an inefficient means of cooking.  It is efficient if you are cooking large amounts of food for a long time; it is inefficient if you are heating up leftovers the next day.

You can sense this without needing to measure, just by doing a thought experiment.  Say you turn your oven on and heat it up to 350 degrees, then when it is hot, you put something in and cook it for 45 minutes.

How long does it take to heat the oven to 350°?  Probably about 15 minutes, maybe longer.  So there is 15 minutes with the oven elements on full, all the time.  Your oven probably has 3 kW – 5kW of heating elements; let’s average and say it has a 4 kW heater inside.  You’ve used 1 kWh of energy just to heat up the oven prior to cooking in it.  If you have a daily energy budget of 10 kWh, you’ve used 10% of it just to heat up your oven.  Ouch!

If you then have it cooking for a while, the oven is probably only cycling the heating elements on for 25% of the time or thereabouts, so for 45 minutes of heating, you might use another 0.75 kWh of energy.  So 45 minutes of cooking uses 1.75kWh of energy total, but if you were cooking something for more than twice as long, eg, two hours, you’d use much less than twice as much energy (ie 3 kWh for two hours of cooking).  The oven becomes more efficient, the longer it is cooking something.

The other issue to do with oven efficiency is how much food you have in it.  Most of the energy in an oven goes to keeping the air in the oven hot, and the heat transfer to the food is slow and inefficient.  It costs little more to cook ten pounds of meat or whatever in your oven than it does to cook 10 ounces of meat or whatever.

So, an electric oven is good for large quantities of food cooked for a long time, but it is bad – very bad – for a small quantity of food cooked for a short time.

Let’s come back to the ‘heating leftovers’ example.  Maybe you heat your oven to 350° then heat the item for 30 minutes.  That’s 1.5 kWh of energy.  Compare that to perhaps 6 minutes in the microwave, which would be 0.15 kWh – ten times less energy.  That makes for an obvious choice, doesn’t it!

Summary

There are several things we can conclude from all of this.

1.  If heating the same amount of water, an electric jug with immersion water heater is the most efficient way to do this.

2.  If heating less than half the minimum amount of water you’d need to heat in a jug, use a microwave oven.

3.  If cooking on the stove-top, induction elements are the best, and gas elements are by far the worst.

4.  Traditional ovens work best when cooking large amounts of food for a long time.  For small amounts of food, that only need a short time in the oven, it is usually better to use a microwave oven instead.

As preppers, we suggest you ensure you have three cooking appliances as part of your kit.  An electric jug, an induction cooktop, and a microwave oven.

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David Spero[suffusion-the-author display='description']

  6 Responses to “What is the Most Efficient Form of Cooking?”

Comments (5) Pingbacks (1)
  1. A very interesting and informative posting! (And not the first one from you, David). I hadda toss that in.
    I have no induction cook top, (And I won’t be requiring one).
    I have a two burner propane campstove. It is all stainless. I have been using it for around 8-9 years, and it still looks like new. Cost me 39 bucks on sale at Wallyworld. These things cost a whole bunch more, these days……..
    I have one of those tiny folding backpacker things, that burns solid fuel. (Or- I can use one of those beercan alcohol burner things, that I learned how to make online, )
    I have a propane single-burner, uses those 1 lb. Propane bottles, (which I can scavenge outta the dumpster- and re-fill from a 20 lb. tank). My camping propane heater and lantern also run off these things.
    And I have one of those homemade ‘rocket stoves’, (I can go to the park across the street, and collect enough fallen twigs to run that). That one cost nothing- I fished the metal cans out of the dumpster.
    Our hydro went down this past winter, for 24 hours. My apartment was quite comfortable for 16 of those, but it dropped rapidly after that- so I retired to bed to wait,
    For making soup or coffee, I used the one-burner, (With a window cracked open).
    After the power came back on, everybody jumps online to discuss this. One guy sez: “What if the power does not come back on for a week?”
    And he’s right- that has happened elsewhere, over wide areas! So yeah- that’s worth preparing for.
    Then- somebody else posts a question online: ” What if the power never comes back on?” (And you got family members and people that you care about, who are gonna depend on you……..Yaknow what I’m saying!

    • Hi, David

      Thanks for your interesting comments.

      Propane cookers are good, but as you point out, you need to vent the CO2 (and carbon monoxide) that comes from running them. If you don’t crack your window open enough, the consequences are, ahem, rather severe! And maybe the heat or cold that you are letting in/out of your apartment becomes an issue, too.

      I’ve always hesitated to recommend them for that reason, and although I have one of the same sorts of things you do, too, I only use mine outside. If the budget allows, getting a small generator/inverter that will power a single burner induction heater (ie 2 kW or less) might be a better approach. We discuss those here :

      http://codegreenprep.com/2014/05/small-electricity-generator-short-term-level-1-power-outages/

      It is a shame that people don’t really truly learn from such experiences, and after a day or two of wondering about the implications of the power outage, they then return back to their complacent lives full of assumptions about the ever-present nature of power once more.

    • To David, above: I have both the 15-20 lb and the 1-lb type propane stoves…I didn’t know you could re-fill the small bottles from the large one, that is a great idea! How do you do that – is there a kit or adapter that is needed, or instructions available somewhere?

      • Yup- Mr Heater has these propane tank refill adapters. (I have another one by a different mfg, but that is out in the shed). Cost: about 15 bucks. (These adapters are banned in Canada, but you can still get them online). Make sure the 1 lb bottle is in good shape. Cool it in the refrigerator. Screw the adapter on the large tank, (valve closed). Screw the 1 lb bottle onto it. Turn the large tank upside down, open the valve for one minute, You want the liquid propane- not the gas. If you want to, you can weigh the small bottle before and after, to make sure you have a pound of fuel in it. Do this outside.

      • I forgot to mention: Any time you are working with gas connections, fill a small spray bottle with water, add about a teaspoon of liquid soap to it- shake it up real good, then spray all the connections you have done with this. (It’s called a ‘bubble check’- all gas fitters do this). Stay safe!

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