heating water- microwave, stove, water heater?

BrockBrock Solar Expert Posts: 633 ✭✭✭✭
The other day we were chatting about the best way to warm up a gallon of water for a floor scrubber which recommend the water be 150F+.

Strictly using electricity I thought the best way to warm up a gallon of water was in the microwave in a glass bowl. The other options were on an electric stove and turning an electric water heater way up or using one of those 4 quart "hot shot" water heaters for making tea and stuff.

I would think the electric water heater would be most efficient, but not if you only need a gallon or two, and using an electric stove would loose a lot of heat to the surrounding area. So I suggested using the microwave.

This got me to thinking, is a microwave really any more efficient at heating water than a submerged electrical resistant heating?

Thoughts?
3kw solar PV, 8 L16's, xw 5548, Honda eu2000i, iota DLS-54-13, Leaf EV, 4 ton horizontal geothermal, grid tied - Green Bay, WI

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,625 admin
    Re: heating water- microwave, stove, water heater?

    A microwave is probably only 60-70% efficient...

    An immersion heater is close to 100% efficient.

    Water heaters have losses through the insulation (natural gas much worse than a full electric--because of the flue?). You also have the gallon +/- of water that you you have to use to pre-heat the pipes.

    And you have the main water heater issue of over heating too much water for a small usage.

    I would vote (if electric is the only choice) for a electric kettle or small point of use water heater (turned off as water is drawn off).

    We have a small 2 cup plastic electric travel kettle... It boils the water far faster than the full sized microwave on a similar amount of power. In fact, it is much faster than the full sized natural gas burner (10,000+ btu or ~2.9+kW electrical equivalent) heating the same amount of water on our kitchen stove.

    The stove is probably between the electric kettle and the water heater.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,804 ✭✭✭✭✭
    Re: heating water- microwave, stove, water heater?

    I'm a fan of the induction hotplate, it's about the same as an electric tea kettle, very little wasted power. Electric hot coil on a stove is only about 40% efficient.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • phred01phred01 Solar Expert Posts: 35
    Re: heating water- microwave, stove, water heater?

    One has to look @ the efficiency of generating microwave energy which is about 80% taking in to account the running a microwave oven. Say if u want to warm a cup of water this takes 1min to raise the water to around 70 C. Now a kettle one generally heats up more than one cup so the energy can be more than the microwave oven. The most efficient way is to use heat water a vessel with an element inside and insulated on the outside. Here is an example in heating water with gas. A gas heater with a slow continuous heater heating 210l uses less gas than a flash point boiler the difference being 7 to 1, so the break even point is 30l.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: heating water- microwave, stove, water heater?

    hmmm, this is a very good and debatable question. it would boil down to what gets the heat into the water best without losing said heat. although resistance heating will in theory work best as bb said one would wonder about that as the element has to heat up then transfer the heat to the container and then into the water. i should note here that in all cases if the water or any other material is exposed to cooler air that it will lose its heat so all heating methods should minimize that. induction will heat the pans and then the water, but a microwave oven, even though more inefficient due to creating the waves, will heat the water nearly direct. these waves don't need to heat other items that are exposed to the air and the fact that you need to heat anything else at all increases the need for the number of btus to reach the water. loss will occur if said waves aren't allowed to fully be absorbed by the water as in allowed to escape without redirecting waves back to the water. i won't get into this in detail, but think of it as the difference of a flashlight being able to direct light to a target and an open bulb of the same used in the flashlight not illuminating the target as well.
    as such there will be surrounding air losses for all other methods and extra materials to be heated, i'll vote for the microwave oven. this could be provable under controlled conditions as the electricity will still yield about 3.14 btus/w/hr no matter the heating method, but what gets to the water is also provable by the water temperature be it timed or not and the results can also vary by circumstance.
  • zeuspaulzeuspaul Solar Expert Posts: 59 ✭✭✭✭
    Re: heating water- microwave, stove, water heater?

    I ran some tests about two years ago using a kill-a-watt meter.

    It took .06 kWh to boil two cups of water in an Aroma electric tea kettle. It took .1 kWh to boil two cups of water in my microwave.

    Starting water temp was 73 F in both cases. The kill-a-watt meter was blinking during the microwave test indicating an overload so I am not sure the microwave test was accurate.

    My conclusion was the electric tea kettle was the most efficient and I now use it for tea and for pre heating water for other cooking purposes.

    Zeuspaul
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: heating water- microwave, stove, water heater?

    Heat pump !!!

    With the right ambient temp you should get a 2:1 or higher transfer.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: heating water- microwave, stove, water heater?

    good point ken as heat pumps will exceed the normalized 3.14btu/w that electricity has through resistance heating. getting a heat pump that goes to heat water to 150 degrees is another problem as most heat pumps aren't designed to go that high.
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: heating water- microwave, stove, water heater?

    Ones for DHW heaters could, this would be their upper limits.
  • BrockBrock Solar Expert Posts: 633 ✭✭✭✭
    Re: heating water- microwave, stove, water heater?

    Yes, Ken I like the heat pump one and that is how our pre-heat tank is heated, but to get it the rest of the way from 110F-120F to 150+ was what I had in mind :)

    I was thinking along the same lines as niel, that even though heating water with a micro wouldn't be as efficient, it would all be contained. I suppose this is why, as zeuspaul noted, that an electric tea kettle works well, all the heat is contained, basically a miniature water heater :)

    Good stuff!
    3kw solar PV, 8 L16's, xw 5548, Honda eu2000i, iota DLS-54-13, Leaf EV, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: heating water- microwave, stove, water heater?

    Brock,
    I know of the perfect solution, you need to move south. So the heat pump will work better.
  • BrockBrock Solar Expert Posts: 633 ✭✭✭✭
    Re: heating water- microwave, stove, water heater?

    It's ground source, works pretty well, but I don't want to keep my de-superheater at 150F :)
    3kw solar PV, 8 L16's, xw 5548, Honda eu2000i, iota DLS-54-13, Leaf EV, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • System2System2 Posts: 6,290 admin
    Re: heating water- microwave, stove, water heater?

    I have a similar question. My wife likes to heat about 7 cups of tea water on the electric stove with a standard 8 inch coil burner, and leave it on warm all morning. If I insisted that she do each cup in the microwave, that would be approximately 60 watts per mug. I don't know how much the stove uses on warm. I have a kills a watt somewhare, but I'm not sure it works on 220, and the outlet would be difficult to access. Can anyone give me a good approximation of the current usage on the stove? I think it's 1500 watts on full power, but I have no idea on warm.
    We need space heat for half of the year, but we heat with wood. from the previous discussion, it sounds like a good electric teapot would be somewhat more efficient than the microwave.
  • BB.BB. Super Moderators, Administrators Posts: 30,625 admin
    Re: heating water- microwave, stove, water heater?

    If you have propane or other fuel for cooking, you may be better off with stove top coffee maker like this one from Coleman (Icarus and a couple others have used them here--If I recall correctly). Assuming your choice of drink is coffee.

    For heating otherwise, an electric tea pot is probably more efficient than a microwave--But I would suggest placing the hot beverage in a thermos as the best "low energy" solution (as opposed to reheating)... I don't drink coffee/tea, so I cannot attest to the flavor of thermos coffee/tea.

    As to measuring/estimating current, you could go with a whole house monitor (like a T.E.D.), measuring current with a current clamp meter, etc...

    Not sure if you are on grid or off grid--But if your family like stew/soups/etc., a popular method of cooking is to use a large open top thermos.

    Heat the food/liquids (brown meat/vetables) on the stove, then pop into the thermos and let set for a few hours (for stews, I will reheat once and let cook for a couple more hours). You may be able to find these cooking thermoses in Asian markets.

    We have a couple of these at our home (no longer available--other models should be). Many times, I will just brown in the cooking pot, add liquids, bring to a boil for a couple minutes (heat up interior of chunks of food), then put in thermos. They are real easy to use and save a bunch of fuel.

    You can even find websites that talk about thermos cooking too...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zeuspaulzeuspaul Solar Expert Posts: 59 ✭✭✭✭
    Re: heating water- microwave, stove, water heater?

    I agree heating in an electric kettle and then pouring into a thermos would be the more energy efficient way to go.

    I keep tea hot all day long in insulated Durotherm cookware http://www.kuhnrikon.com/products/duro/group.php3?id=3
    on a thermostically controlled Cadco cooktop http://www.amazon.com/Cadco-PCR-1S-Professional-Range-Stainless/dp/B00028X2Y6

    Considering the cost of the Durotherm you may never realize net savings. However you have an investment that will last your lifetime and that of your heirs. I also find other good uses for the combo.

    A quick search came up with this insulated electric tea kettle. Perhaps there are others. http://www.foodnetworkstore.com/ProductDetail.aspx?R=194535&ccaid=FNFROOG194535

    Zeuspaul
  • BB.BB. Super Moderators, Administrators Posts: 30,625 admin
    Re: heating water- microwave, stove, water heater?

    Went through this a year ago looking for an electric hot water kettle for some relatives overseas... It is darn near impossible to find a reliable electric kettle that does not have plastic somewhere in the water path (for those trying to avoid plastics).

    I have a small 2 cup 120 volt travel sized kettle (black plastic with is supposed to be the "worst" for those plastic-phobic) and it heats a cup or two amazingly fast (and not that much Watt*Hours). Got to be more efficient than a microwave.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarvicsolarvic Solar Expert Posts: 1,053 ✭✭✭✭
    Re: heating water- microwave, stove, water heater?
    BB. wrote: »
    If you have propane or other fuel for cooking, you may be better off with stove top coffee maker like this one from Coleman (Icarus and a couple others have used them here--If I recall correctly). Assuming your choice of drink is coffee.

    For heating otherwise, an electric tea pot is probably more efficient than a microwave--But I would suggest placing the hot beverage in a thermos as the best "low energy" solution (as opposed to reheating)... I don't drink coffee/tea, so I cannot attest to the flavor of thermos coffee/tea.

    As to measuring/estimating current, you could go with a whole house monitor (like a T.E.D.), measuring current with a current clamp meter, etc...

    Not sure if you are on grid or off grid--But if your family like stew/soups/etc., a popular method of cooking is to use a large open top thermos.

    Heat the food/liquids (brown meat/vetables) on the stove, then pop into the thermos and let set for a few hours (for stews, I will reheat once and let cook for a couple more hours). You may be able to find these cooking thermoses in Asian markets.

    We have a couple of these at our home (no longer available--other models should be). Many times, I will just brown in the cooking pot, add liquids, bring to a boil for a couple minutes (heat up interior of chunks of food), then put in thermos. They are real easy to use and save a bunch of fuel.

    You can even find websites that talk about thermos cooking too...

    -Bill
    I have the coleman coffe pot and I recomend an insulated coffee carafe. Got mine from Amazon. The carafee is "Bodum chamford thermal double wall vacume carafe 1.01 34 oz. It will hold all the coffee from the coffee maker except for 2 cups which I drink when I brew it. If anyone wants one of those don,t make the same mistake I did. First I got the coffee press which I don,t like. Second time I got the carafe which is very good and will keep the coffee hot all the day. :Dsolarvic:D
  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: heating water- microwave, stove, water heater?

    Coleman "Mr Coffee", insulated carafe (pre heated with hot water) and it keep coffee hot all day. (I even use a "cozy" made of a wool hat to keep heat from escaping through the top.

    The trick to using little energy is to get it into something, and then keep it there,, ie insulation.

    Tony

    Ps at the very least, a camp stove, burning refillable propane from a 20 #tank is way cheaper than using electricity.
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: heating water- microwave, stove, water heater?

    Kettle on the wood stove using junk firewood. Heat going into the water ends up in the room anyway so there is virtually no cost to energy diverted from the stove. Using a high quality fuel source like electricity or gas is always less desirable for heating low temp purposes than making use of low quality fuel. Of course, not a real good option in the summer.
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: heating water- microwave, stove, water heater?

    I use a Bunn Coffee maker for hot water 24/7. It has a 30 watt heating blanket on the tank. If that watt drain is not acceptable to you on a 24 hr basis then won't be interested in the rest.

    It has a tank that keeps about 3/4 of a gallon water hot. It has a 700 watt catch up element in the tank that is on a thermo disk that would come on if you dumped in a whole pot of water, it won't with small amounts. I " disconnect " that element by pulling off a spade terminal and put some heat shrink on it. It's really not necessary unless your in a hurry with a large amount of water. When you use a whole pot it takes about a hour without it to heat back up and I find that acceptable without the element.

    It is a direct displacement, you pour a cup in you get a cup back out.

    The pot warmer element uses 130 watt.

    I am a coffer drinker so it's a small price to pay.
  • zeuspaulzeuspaul Solar Expert Posts: 59 ✭✭✭✭
    Re: heating water- microwave, stove, water heater?

    Sometimes it's useful to look at the numbers.

    Using the Nebraska water heating cost calculator and my current energy cost of 10 cents per kwh for electricity and $2.83 per gallon for propane. http://www.nppd.com/My_Home/Services/Additional_Files/waterheater_calculator.asp

    Based on 100 gallons per day usage which should help factor out standing losses and beter estimate actual water heating costs my cost is

    2.2 cents to heat a gallon of water with electricity and 3.8 cents to heat a gallon of water with propane. So in my case using electricity to heat water is about half the cost of propane.

    I made tea yesterday with an electric tea kettle and placed it in a half gallon thermos for a day trip and the tea was still hot at the end of the day. My total cost to heat and keep hot one half gallon of tea was about one penny.

    Zeuspaul
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