U-value of windows - differences

Even though I myself do not arrive at your 2160 Wh after 10 minutes of calculations, the 39€ additional gas costs per year for 0.9 instead of 0.7 windows sound realistic. Basically, insulation always costs as much as you can save during the usual lifespan phase.

I still need better windows to reach Kfw55. Currently, my heat calculation is based on 0.9 windows.

Does anyone know what I have to calculate as an extra cost from 0.9 to 0.8 (per sqm)?

Many greetings
Oliver

Would suspect that this is extremely individual.... on evofenster for example for a window 900x1230 with Uf of 0.87 W/m²K and a Ug of 1.1 W/m²K ~ 1.0 in total and once with Uf of 0.87 and Ug of 0.7 ~ 0.8 in total - price difference ~5%.

Do you happen to have a € value as well? Unfortunately, I don’t know the price of windows at all.