Solar Heat Gain Coefficient

So I was looking at windows yesterday and noticed on a lot of them this number, the solar heat gain coefficient.
We asked the guy at the store, and looked it up on wikipedia, but I can't figure out from either of these sources what number is "better" for a passive solar house.

The coefficient is between 0-1, and it refers to how much solar energy is transmitted through the window. Maybe I'm being a little dense, but I don't know if we want a higher or lower number. The windows we saw ranged from 0.3's to 0.5's.

Any help clearing this up would be greatly appreciated.

Thanks,
-WY

For the South facing glazing on passive solar designs you want high SHGC. Generally for the East and West elevations, a lower SHGC is preferable. For North facing, SHGC doesnt really matter. For all windows, the lower the Ufactor the better.

Most passive solar designers shoot for a SHGC of .5 and above on South sides. A .45 may be acceptable with a really good Ufactor to go with it. A SHGC of .3 is much too low for the south sides of most Passive solar designs. If the South facing glazing is more than the usual recommendation of 12% to floor area then it can make sense to choose lower SHGC numbers but such designs tend to overheat late in the day and lose too much heat through the windows overnight.

Thank you very much for clearing that up. The 2 large windows that we purchased will be south facing and have a SHGC of 0.55. I'm not sure what the Ufactor is, but maybe it says it on the tag somewhere.

-WY