Sorry for the slow response

ER for those that are interested is short for Energy Rating. For more info see:

http://oee.nrcan.gc.ca/publications/infosource/pub/renovate/windowsanddoors/section6.cfm?text=N&printview=N

The ER approach (ER= gains-losses) is a heating climate approach. First used in Canada, the UK rating system is based on a similar approach, see:

http://www.tangram.co.uk/TI-Glazing-Window_Energy_Rating.html

Recently the ER was changed.

In the original equation an ER of 0 means the window loses as much energy as it gains over a heating season.. A positive ER means the window gains more energy than it loses over the heating season. A negative ER means……you get the picture.

The ER was never meant to predict actual energy use/contribution of a window in your house. Rather like the mileage ratings for cars it’s meant to serve as a comparison between different windows. 
 

So in general,

ER =  solar gain -  conduction losses – air leakage losses

Or more specifically,

oldER = 72.2*SHGC  - 21.9*U – 0.54*L75/A

72.2 is the avg solar gain on a vertical surface for Canada in W/m^2 (it’s an avg of NSEW for the major cities per hour over the 211 day heating season)

SHGC is the whole window SHGC for a stnd sized window

21.9 is the average Canadian (major cities) indoor outdoor temperature difference over the 211 day heating season in degrees Kelvin (Celsius)

U is the whole window insulating value in W/m^2K (Uimperial * 5.678 = Umetric)

0.54 is the Pressure Factor

L75/A is the measured air leakage at 75 Pa for a standard sized window in m^3/h

For those that are interested ER has units. They are W/m^2.

If  I was doing my own ER calculations from manufacturer’s data I'd ignore the air leakage term – on an one example leaving out this term over estimates the ER by only 0.2 W/m^2

Recently the ER equation was changed

1. the stnd or reference sizes were increased
2. the SHGC term was multiplied by 0.8 to notionally account for shading.
3. a fourth term (+40) was added to end of the equation to avoid manufacturers the embarrassment of having to report negative numbers

I still like the numerical elegance of the older version, that’s why I started with that version

If you compare the Canadian ER to the UK version, you can see they’ve accounted for their cloudier and warmer climate.

There is also yet another variation called ERs or Energy Rating Specific.  It uses city specific indoor outdoor temperature differences and orientation specific solar data. This allows orientation by orientation comparisons.

As an alternative to all this you may want to try LBL's Resfen program. It's free. 

http://windows.lbl.gov/software/resfen/resfen.html



And that’s probably enough for now

Todd:

It sounds like you are interested in something more accurate than the averaging inherent in ER. Better than the ER is the ERS. I mentioned it in my previous post:

“ There is also yet another variation called ERS or Energy Rating Specific.  It uses city specific indoor outdoor temperature differences and orientation specific solar data. This allows orientation by orientation comparisons. “

Before explaining about ERS, I will again remind everyone that:

ER (and ERS) were never meant to predict actual energy use/contribution of a window in your house. Rather like the mileage ratings for cars it’s meant to serve as a comparison between different windows. --- in a heating climate.

If you are interested in predicting energy use, you need a computer simulation tool like; Energy-10, HOT2000 or the like.

However, ERS is still useful. It’s more accurate than ER - because it requires that the 3 co-efficients for sun, temperature and wind be changed for local conditions.

Todd, you’re in Cleveland so I’m going to give you data for Windsor, ON

A)    Coefficient for Sun

In the ER equation the average solar gain for a vertical surface is 72.2 W/m^2 (averaged the 211 day or 5064 hr heating season for and average CDN city)

For Windsor the average solar gain by orientation is as follows:

S             121.9

SE/SW    102.2

E/W         67.1

NE/NW   42.0

N             36.6

So Windsor is pretty average, for Canada, when it comes to solar gain

B)     Coefficient for Temperature

In the ER equation the average indoor outdoor temperature difference is 21.9 over the 211 day (5064 hour) heating season in degrees Kelvin (Celsius)

For Windsor, ON the average temperature difference is:

19.0

So Windsor is warmer than average CDN city

C)    Coefficient for Wind

In the ER equation the Pressure Factor is 0.54

For Windsor, ON the average temperature difference is:

0.344

So Windsor is less windy than average CDN city

(Again, this term has very little meaning for a new window – even with a relatively drafty new window its hard for this term to affect the ER by more than 0.5 W/m^2K)

And that’s probably enough for now

21.9 is the average Canadian (major cities) indoor outdoor temperature difference over the 211 day heating season in degrees Kelvin (Celsius)

Good morning Stephen, 

Okay.  Had shower. Had breakfast.

Now I understand what you meant.  Makes perfect sense...

When I read the calculations the first time thru, all I could think of was that 22K is somewhere about -250C and I just wasn't able to wrap my head around the Kelvin (Celsius) part of the answer.

Metric easy? 

What can one expect from a system invented in France (with due apologies to all our Francophile friends)(J/K BTW!). 

Thanks for the clarification and have a great day.