Saving energy through windows?

I am looking for windows that could help me save on heating costs, but unfortunately I am a layman on this subject. How thick do they need to be, how strong should the glazing be? They should be thick, inexpensive windows, I don't care about the design.

Hello Baulurch,
You order good windows best online today, if you trust yourself to install them :-) But you have to make sure that you don’t get Polish windows, that can come back to bite you badly ;-)
I bought my windows online.
System with 0.7 glazing + warm edge, is top quality. German profiles from Aluplast in Karlsruhe, fittings from Winkhaus and the manufacturing is also in Germany. The price is more than attractive and I am really satisfied with the workmanship. They even have installation partners all over Germany, if you don’t trust yourself with measuring and installation ;-) But they also have YouTube videos and technical support for everything, basically nothing can go wrong. Just have a look if it suits you.