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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthortony
    • CommentTime5 days ago
     
    I like kWh/m2/y
  1.  
    When I was designing my house I ran the PHPP with different internal temperatures to see what the effect was. These figures are obviously very specific and are measured in kWh/m2/a. They are for space heating followed by total energy use.
    20C 12 51
    21C 15 54
    22C 17 57
    23C 20 60
    24C 23 64
    • CommentAuthortony
    • CommentTime4 days ago
     
    Is that 20C 1251? Or 12.51?
    • CommentAuthorEd Davies
    • CommentTime4 days ago
     
    Posted By: PeterStarckThey are for space heating followed by total energy use.
    Posted By: tonyIs that 20C 1251? Or 12.51?
    So that's 12 kWh/m²/a for space heating and 51 kWh/m²/s for total energy use (so 39 kWh/m²/a for non-space-heating energy use).

    The non-space-heating energy use increases slightly from 39, 39, 40, to 41 kWh/m²/a as the temperature is increased. Is that just a matter of rounding or is there some other reason? Pump consumption not counted as space heating or something? Of course, as Tony would point out, pretty much all energy use is space heating, deliberately or otherwise and wanted or not.
    • CommentAuthorEd Davies
    • CommentTime4 days ago
     
    Posted By: djhSorry, but kW per hour doesn't make much sense as a unit. It makes my brain hurt even thinking about it.
    I've seen it used properly.

    Somebody on another forum used it for the rate of installation of his PV panels. He's a professor of electrical engineering and knew exactly what he was doing - namely, taking the piss. :wink:
    • CommentAuthortony
    • CommentTime4 days ago
     
    Thanks chaps,
    • CommentAuthorJeff B
    • CommentTime4 days ago
     
    djh wrote: Sorry, but kW per hour doesn't make much sense as a unit. It makes my brain hurt even thinking about it.

    Fair enough!
    :wink:

    1 kW per hour = 1 kWh doesn't it? Somehow I just can't get my head around the heat output from the burning of wood pellets. Examples of electrical appliances are easy: 1 kWh is equivalent to running a 100W light bulb for 10 hours, a 10kW shower for six minutes or a 2kW kettle for half an hour. So if a wood pellet boiler (or any kind of boiler for that matter) is rated at 20 kW what does that mean in reality? If burning 1 kg of wood pellets produces approx 5kWh of energy, do I take it then that a 20kW boiler is capable of burning a maximum of 4 kg of pellets per hour when going flat out?
  2.  
    Posted By: Jeff BSo if a wood pellet boiler (or any kind of boiler for that matter) is rated at 20 kW what does that mean in reality? If burning 1 kg of wood pellets produces approx 5kWh of energy, do I take it then that a 20kW boiler is capable of burning a maximum of 4 kg of pellets per hour when going flat out?

    That would be my understanding. A wood pellet boiler would have a (much) shorter time to reach its max. output than a log burner(which could be anything up to an hour + based on my experience) and I would expect a gas (or oil) boiler to be fairly instant in its reaching max output.
    • CommentAuthorEd Davies
    • CommentTime3 days ago edited
     
    Posted By: Jeff BExamples of electrical appliances are easy: 1 kWh is equivalent to running a 100W light bulb for 10 hours, a 10kW shower for six minutes or a 2kW kettle for half an hour.
    Yes. You multiply the power by the time to get the energy.

    Posted By: Jeff B1 kW per hour = 1 kWh doesn't it?
    NO!

    A watt is like a knot [¹] in that it's already a rate. Talking about 1 watt (or kilowatt) per hour doesn't make any more sense than talking about 1 knot per hour (other than odd cases like a rate of change of power or rate of change of speed of a supertanker respectively).

    To put it another way “1 kW per hour” is doing division, “1 kWh” is doing multiplication.

    1 kWh = 1000 W x 1 hour = 1000 J/s x 3600 seconds = 3'600'000 J = 3.6 MJ.

    1 kW per hour, if anybody was to use it properly, would be: 1 kW/h = 1000 J/s / 3600 s = 0.2777… J/s². Sort of like the acceleration due to gravity on the Earth's surface being roughly 9.81 m/s².

    [¹] 1 knot = 1 nautical mile per hour. 1 watt = 1 joule per second.
  3.  
    I worked with someone who thought it would be purer to state all physical energy measurements in electron-volts, apparently that meant his heating bill as well. There was also something about energy being interchangeable with mass, which is defined with reference to time, but I'd lost him by that point.

    I gave up on being a units pedant after that, it was no fun with folk like that around.
  4.  
    Posted By: Ed Davies
    Posted By: Jeff BExamples of electrical appliances are easy: 1 kWh is equivalent to running a 100W light bulb for 10 hours, a 10kW shower for six minutes or a 2kW kettle for half an hour.
    Yes. You multiply the power by the time to get the energy.

    Posted By: Jeff B1 kW per hour = 1 kWh doesn't it?
    NO!

    A watt is like a knot [¹] in that it's already a rate. Talking about 1 watt (or kilowatt) per hour doesn't make any more sense than talking about 1 knot per hour (other than odd cases like a rate of change of power or rate of change of speed of a supertanker respectively).

    To put it another way “1 kW per hour” is doing division, “1 kWh” is doing multiplication.

    1 kWh = 1000 W x 1 hour = 1000 J/s x 3600 seconds = 3'600'000 J = 3.6 MJ.

    1 kW per hour, if anybody was to use it properly, would be: 1 kW/h = 1000 J/s / 3600 s = 0.2777… J/s². Sort of like the acceleration due to gravity on the Earth's surface being roughly 9.81 m/s².

    [¹] 1 knot = 1 nautical mile per hour. 1 watt = 1 joule per second.

    So is 1 kWh 1 kW in 1 hour whereas
    1 kW / hr is something else
    • CommentAuthorEd Davies
    • CommentTime3 days ago
     
    Posted By: WillInAberdeenI gave up on being a units pedant after that, it was no fun with folk like that around.
    It's not just pedantry, there's real confusion on significant matters. See further up this thread.

    Posted By: Peter_in_HungarySo is 1 kWh 1 kW in 1 hour whereas
    1 kW / hr is something else
    Yes, it's something else.

    Here's a junior-school-level illustration I thought of this afternoon:

    Sean [¹] and his friends install 6 kW of PV panels in 5 hours. The rate of installation is 6 kW / 5 h = 1.2 kW/h. Division.

    The next day the sun shines on the panels for the equivalent of 5 hours of full sunshine. The amount of electricity generated is 6 kW × 5 h = 30 kWh. Multiplication.

    [¹] The professor who wilfully used kW/h to wind people up.
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