U value is the reciprocal of R- Value so a window with a U value of .3 is going to have an approximate r-value of 3.333 and a window with a U of .15 is going to have an r value of 6.666 so it is going to be a much better insulating window as far as being worth 50% more that is very subjective as to your desire, in the Northeast where I am from the the higher U-value can be very beneficial, however where my mother is in Florida, not so much.
Talking Solar Heat Gain again it can be very subjective. the farther south you go the less heat you want to absorb through your windows so the better the solar heat gain the better off you are, however up here int he Northeast solar heat gain will cut more in heating costs (8 months of the year) than it will cost you in cooling costs (maybe 2 weeks up here in NH for A/C use) Solar Heat Gain is a good thing up here but not in the south.

Visible light or VLT the .40 is only 40% of visible light that would go through a completely open hole is what will come through the hole when the window is in place so .5 is 50% therefore you lose 10% of the potential visible light potential with a .4 over a .5 window. This can be a good thing on the south or west side but on the North side you may be trying to get as much light in as possible.

In short it is all a balancing act as to what you are looking for in a window and what is more important to you as far as function.

Great answer, thank you. I am also in the northeast (slightly northwest of Manhattan).

I read in a diff. thread here, a post by oberon, that SHGC only benefits you when the sun is shining directly on the window, but "that solar heat gain is going back out again as soon as the temperature of the glass is below the temperature of the room air". So I would imagine it benefits you the most to have a high SHGC in a south-facing window (which the 2 windows I'm replacing with a bow and picture both are).

Nevertheless, to get the energy tax credit, the SHGC must be .3 or less.

And to get a low U-factor, the SHGC tends to be low as well from what I've read/learned. One window I'm looking at is U=.16, SHGC=.24, Vis.Lt=.43. Obviously, I would love the SHGC to be .30 so it's a little higher, but still qualify for the tax credit. And I would also imagine that the amount of $$ you save with the tax credit would probably offset any benefit you get from going with a higher SHGC that doesn't qualify for the 30% credit, especially since a higher SHGC usually comes with a higher U-factor as well.

So I wonder if it would ever make sense to sacrifice a low U/SHGC/medium Visible Light with the tax credit, with a higher U/SHGC/high Visible light without the tax credit. My guess is no. And my guess may also be no even if the credit wasn't in the equation. Sound about right?

I'm currently running projections using ResFen (do a google search, awesome tool), that should provide me with some answers, I'll post my findings here shortly to see if you folks concur with the software's cost-benefit analysis of changing the U & SGHC factors up/down.

.15? Are you absolutely sure the figure you have is the overall U value of the entire unit? Many dealers will toss out a "Center of the glass" figure which is missleading. Per law, a window label must include the U value of the complete unit.

this figure is what the tax credit requires to be .30 or better (better being lower). The second figure is the Solar heat gain. The feds want this to be .30 also. Hence the 30-30 reference to windows that qualify for tax credit.

In the south where the heat from the sun is the biggest issue, better SHC (solar heat gain) is import. it is not so important in northern climates. However, the Fed still requires the 30-30. If you go much below .30 SHC, then you are blocking out sunlight that can add some additional heat to your house. The more SHC, the darker the windows will appear. This is reflected in another measurement for Visibility (not a factor for tax purposes)

SHC is also a factor of UV rays. .30 is sufficient to protect your carpets, furniture, drapes etc from fading or getting sun rot.

UV coatings can also have a small affect on the U value. Getting the whole unit down to .30 can be tough for some manufactures and every little bit helps. Some window manufacturers have to use more UV coating to get their U performance down to the .30

Because it takes a great deal of enginering, design and glass options to reach the .30, I am very skeptical that a window could have a .15 U rating. This would be like a care manufacturer boasting 50 MPG for a full size SUV with a regular, gasoline engine. Car manufacturers do all kinds of tweaking to increase MPG a tenth. The same is true with window manufacurers.

Just because you asked, here is the link to that u-factor

Here is a link that might be useful: tristate