Advice 1: UW value

It is an important parameter informing about the heat transfer of the window. Uw value always refer to a window of specific dimensions, what allows comparing different window models. Windows from the same model, but different dimensions usually have different values.

Uw is the amount of heat that escapes 1m2 of the window per hour when the difference on both sides (interior/ exterior) equals 1K (1°C). Many factors contribute to that value, but still it is the most important information defining a window.

Example:
Window model 1. has Uw = 1,3W / m²K,
The window model 2. has Uw = 1,0 W / m²K,
Which parameter will be better?
Of course Uw = 1,0W / m²K!
 

Lower Uw value limits the heat loss. It is most apparent in case of passive windows - these reach Uw value of  0,8 W/m²K. However, if you are not building a low-energy house, but just look for some windows to your flat, you can consider windows with Uw of around 1,0 - 1,3 W/m²K.