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How much energy to heat water (in Watts/Wh)

  1. Aug 20, 2015 #1
    I'm heating a liter of water from 25C to 60C. I'm under the impression that a Calorie (kcal) heats one liter of water by one degree, and so heating one liter by 35 degrees requires 35 Calories.
    I want to convert this to Watthours but I'm not sure how....does bringing time into the equation determine how much energy I need to expend? For instance, is there a difference if I need to increase the temperature to 60C in 5 seconds versus a minute?
    I could spend a few hours figuring this out for myself but I figured someone else could illuminate me with much less effort
  2. jcsd
  3. Aug 20, 2015 #2
    The watt is a unit of power=change in energy/change in time. So the watthour is actually a unit of energy.

    Watt is Joules/second. So a watthour is essentially joules/3600 (there are 3600 seconds in an hour).
  4. Aug 20, 2015 #3
    To answer your question, yes the time required would matter if you were concerned about power. But since the problem asks for watthours, a unit of energy not power, the time required is of no relevance.
  5. Aug 20, 2015 #4
    Can I convert kcals to watts in this instance? ...as in 4.184 calories/second, so .004184 watts would heat one cc of water by one degree? ...and in an ideal world (not accounting for conduction/convection/niggling details) a watthour would heat .004184 x 3600 cc's of water by one degree?
  6. Aug 20, 2015 #5
    Damn, I screwed up my math and my units...I'm probably going to have to consider this deeper until my internal inconsistencies diminish enough to ask questions that actually make sense. Thanks for answering!
  7. Aug 20, 2015 #6
    I think what you would do is convert kcal to joules and then joules to watthours.
  8. Aug 20, 2015 #7
    I think I figured it out...your answer prompted me to look up energy vs. power and I realized I originally did the math [somewhat] correctly but forgot to account for time factors cancelling themselves out which led to an answer that didn't make sense and subsequent searches consisting of the wrong terms.
    1 kcal = 1.162 watt hours so 35 kcal = 40.667 watt hours
  9. Aug 20, 2015 #8
    Yup thats correct.
  10. Aug 20, 2015 #9


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    If you want the answer in watt hours you don't really need calories at all...

    Energy (in joules) = specific heat capacity of water(joule/kg °C) * mass of water(kg) * change in temperature(°C)

    = 4186 * 1 * (60-25)
    = 146510 Joules

    1 joule = 1 watt for 1 second

    so to convert to watt hours divide by 3600

    146510/3600 = 40.7 Watt hours.

  11. Aug 20, 2015 #10
    Thanks, I was going with what I knew offhand and was almost too tired to put two and two together
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