You're absolutely right. Just leave my pint alone.

I'm with Jeremy S: it drives me bonkers seeing things like "2 kWh per hour".

I think it's a lot easier just to work in watts. E.g., a 20 W bulb which is on 6 hours a day is equivalent to 5 W. Quick, work that out in kWh per year.

A good rule of thumb seen on this forum (sorry, can't remember who from): the cost of 1 W always on is £1/year.

Posted By: sydthebeatmaybe the 'per year' aspect of the unit is important??

I can't see why it should be.

I would think that new units, the kwy, pronounced, "kwiy" (kilowatt{hours}year) and the Akwy, pronounced akwiy (Average power in kilowatts{hours} over a year).
I think measuring property consumption in Mj per year, leaves one with a total lack of feeling about the electrical useage, is this figure high or low, and what are the likely bills going to be. It is a useful figure if you are considering heating the property by filling it with red hot cannon balls or you are thinking of dumping it into a calorimeter (like wot we mite have dun in physics).
Frank

Posted By: James Nortonthe American R value to european U value issue....

If you really want to scare yourself, look at how the Yanks calculate heat pump COPs.

Posted By: djhIf you really want to scare yourself, look at how the Yanks calculate heat pump COPs.

I'm not sure what's so scary here as the method used is simply COP = Power Out / Power In
Of course, if you want to get into EER, SEER and HSPF numbers then that's a whole different issue, but COP is the same the whole world over (just have to note at what conditions it is specified though as the ISO and ASHRAE definitions are slightly different)

Paul in Montreal.

Posted By: tom.harriganNow, using Ed's method calculate the cost per day for a 17W bulb on for 9.75hrs a day..

9.75 is a quite a bit less than 12. Decrease the 17 W by about the same proportion to, say, 14. Divide by 2 for a day and you get 7. Dead easy.

With a calculator: 6.90625 W. So 7 W is as accurate as either the 17 W or, probably, the 9.75 justifies.

I happen to remember that there are 8765 hours in a year [1] but it doesn't help much with getting an accurate enough answer in your head.

[1] Actually none of a common year, a leap year, a mean Julian year, a mean Gregorian year, a tropical year, an anomalistic year or a sidereal year are this length but they're pretty close and it's an easy number to remember.

Because were British

(and you can use them for whole house heat calculations)

xkcd is there before us, as usual... http://www.xkcd.com/526/

Posted By: djh
Well, the question is about units. In most places COP is a dimensionless ratio but in some American documents I've seen COP expressed in "BTU/kWh", which gives bigger numbers than the European/Japanese/etc way of doing it and so presumably makes American heat pumps "better" than European or Asian ones when trailertrash/chavs come to buy." alt="" src="http:///newforum/extensions/Vanillacons/smilies/standard/devil.gif" >

That would be the EER which is the standard US rating of air conditioners (actually, its SEER, the seasonal EER which is manadated to be a minimum of 13 now).

From http://www.icekubesystems.com/pdf/EER.htm:

EER: The Energy Efficiency Ratio is calculated by dividing the amount of cooling put out by an air conditioning system IN BTUH, divided by the amount of energy put in to it IN KW. If the air conditioning capacity of a heat pump is 48,000 BTUH, and the compressor, fan and pumps consume 3.43 kW (3,430 watts), the EER is:

48,000 / 3,430 = 14.0


To convert this to COPc (COP cooling), divide the EER by 3.412. The COPc is:

14.0 / 3.412 = 4.1

Not sure why you had to make a reference to Chavs. As a test for whether a statement is acceptable or not, try replacing "Chav" with "Jew" or "N*gg*r" and then seeing if you'd still make it.

Paul in Montreal.

Posted By: tom.harrigan... perhaps you would deign to enlighten me as to precisely how many kWh Ed Davis gets through each year? I may be bottom of the class, but I'm keen to learn!

I admit that I was a bit sloppy in that I should have made my units completely explicit. Nevertheless I would have expected a "scientist" (or anyone else with an interest for that matter) to understand that:

kW * hrs * days = kW * (hrs/day) * (days/year) = kWh/year

What sort of "science" do you do exactly?

Tom, hi.

I'd say you don't need enlightening, as your calculation looks fine to me. So, a 20W lightbulb running for 6hrs a day, using your method (and the unit I'm disputing) is 0.02kW * 6 * 365 = 43.8kWh/yr.

However, this is equivalent to:
20 J/s *( 365 * 6 * 60 * 60)s /(365 * 24 * 60 * 60)s
= 20 J/s * 6/24 [as the '365 * 60 * 60' in both the numerator and the denominator cancel out]
= 20J/s * 0.25
= 5J/s
= 5W;

that third from last step can be arrived at much more easily by saying "if it's on for 6 hrs in every 24 then that's a quarter of the time, so my average rate of energy consumption is a quarter of the continuous rate of energy consumption, i.e. of the rated rate of the appliance. Hmm... a quarter of 20, that'll be 5!" Likesay, that seems far simpler to me, all round - no need to bring years into it whatsoever - and I imagine that's what Ed's reasoning was.

(FYI I mostly do marine physics, with occasional little bits of chemistry)

sorry to have got your goat, but I was trying to use your previous comment as an example of the unnecessary complexity introduced by the 'kWh/year', and hence the ease with which folk can come unstuck when trying to apply it.

best wishes, Jeremy
------------------------------
Syd - hello.

the point about Watts is they are already a 'rate' - the rate of energy (J) used per second; so, W/m2 is equivalent to J/s/m2, or energy per unit of time per unit of area.

With regards 'W/m3', that would be right if Passivhaus was expressed as rate of energy use per unit volume, but it appears to be per unit of floor area (presumably the ground-level footprint of the building - can anyone confirm/deny this?). I imagine that the 'PHPP software that calculates the energy use of a building for comparison with e.g. Passivhaus, does indeed have to take account of contained volume, as well as a whole load of other things (like thermal resistance and heat capacity of all of the building components, and ventilation and so on), but in the end, the rate of energy use for the whole building required to maintain a specified temperature throughout some period of seasonally varying outside temperature and passive solar gain is averaged over the floor area.

This last bit brings me to a spin-off question: the annual outside temperature (presumably taking account of wind chill, as that's going to vary as well) and the annual sunshine hours are going to vary fairly substantially from year to year, and not always in some directly correlated way, so I'd be surprised if it was valid to just take a, say, ten-year average of each, and then plug that in to get your building rating - anybody know how it's done? anybody used 'PHPP'?

best wishes, Jeremy

ref: http://www.passiv.de/07_eng/phpp/PHPP2007.htm
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Paul/djh, hi.

now I've really got to laugh - SEER is in BTU/h/W? (N.B. I don't think Paul's source is reliable, it's not BTU.H, but BTU/h, try: http://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio)

So that's: heat required to raise 1 lb of water by 1degF per hour/ J/s - who in hell came up with that? That's mixing imperial units with S.I. - does this mean that whoever's come up with this is happy enough to accept that the input energy consumption of the device is rated in S.I. units, but, come hell-or-high-water, is determined to cling to the Imperial measure of cooling that they've 'always' used (or at least for the last 50 years or so!).
Importantly, this is not a dimensionless ratio, i.e. it must be specified with a unit, because the numerator and the denominator have different units (i.e. apples per orange), and hence does not give a direct measure of efficiency; if it did, a 100% efficient system would have an 'energy out/energy in' ratio of 1 (which is simple to understand); with the SEER a 100% efficient system scores 3.415 (which isn't!). The required standard is a minimum of 13 BTU/Wh or, dividing by 3.415 to get a dimensionless ratio (which can be considered as W/W, or BTU/BTU, or oranges per orange), equates to 3.8; so you have put 1 whatever of energy in to shift 3.8 of them out.

Oh... hold on... it's a US government standard... Classic! Those jokers, eh?

Sorry if that appears to be going off-topic, but it still highlights the sense in thinking carefully about the units used, how they can/will be applied and, most crucially, how they can be usefully interpreted.

cheers - Jeremy

P.S. I use the phenomenally useful 'CONVERT' tool from Josh Madison - it's free! http://joshmadison.com/article/convert-for-windows

Jeremy, It's pretty safe to assume the majority know what a Watt and a Joule are, and are entirely up to speed on the difference between energy and power. My initial issue was with what I perceive as an unhelpful way to calculate the annual running cost for a particular electrical item. It's artificial ease of use being a result of a temporary and accidental scale factor.

As I'm sure you are aware, being a scientist, that units are often chosen for convenience. To me the Watt is the most natural unit when discussing instantaneous power. I prefer it for heating (in kW please) and lighting (in incandescent bulb W equivalent), but find it less useful when averaging is involved, then kWh/(day,quarter,year) makes life easier. I do however draw the line at kWh/h, even though meaning Unit/hr is clear.

When economics impacts on our attempts to be green, Instantaneous power is far less useful than a conveniently expressed energy/time. I read on this forum with intense envy of people's 50kW log boilers, but how much wood do they use a year? The fact is that we are invariably billed in energy, the bills come at regular intervals, and they vary with the seasons on an annual basis.

Excellent dog!

Does it consume 2 dog biscuits/day or 2.3 x10(-5) dog biscuits/second?

Note : 1 WTF = 1 dog biscuit/second

Sorry for that. Still trying to learn how to put pics on forum. Took laptop to apple shop and forced a technician to show me how to do it - he didn't know - but somehow we managed to get a shot of the dog on the forum. . . . . . will try and make the next pic much more relevant to the thread.

Posted By: SimonHIf the real unit of energy is a joule why not sell and refer to everything in joules?

Because it's inconvenient and most energy sources are not sold by their energy content but by their actual volume (though natural gas was billed in Therms). Only electricity was metered in kWh.

As for the odd definition of SEER/EER in the US, that's because air conditioners are rated in "tons" and electricity is metered in kWh so it made sense to come up with a comparison figure-of-merit that was based on actual figures users were familiar with - the higher the SEER (or HPSF for heat pumps etc.) the better and so enabled an apples-to-apples comparison to be made between brands.

And as for "tons" of air conditioner capacity, 1 ton = 12000BTUh which is about 3.5kWh and comes from the old days when ice was used to provide air conditioning - 120000 BTUh being the rate required to melt 1 ton of ice in 24 hours.

Paul in Montreal