thought there is the standard window 1.20 x 1.48 and then there is a Uw value on it and he confirms that he installed the windows according to RAL installation.
You thought correctly. According to the KfW guidelines, the required Uw value can be proven in two different ways:
It is allowed to rely on measurement values from the window manufacturer (not the installer). In this case, the Uw value for a window measuring 1.23 * 1.48 m is MEASURED at the manufacturer, according to a procedure described in DIN EN ISO 12567. The testing laboratory then issues a certificate / confirmation stating:
... bla bla ... measured according to DIN EN ISO 12567 with profile setup XY glass ZZ triple glazed 12-4-12-4-12 mm ... etc. window with 1.23 * 1.48 m achieves Uw value of 0.7 W/m²K
Stamp, logo, signature.
This "standard window" then basically serves as a sample for your house, which meets the requirements. Certificate to your energy consultant and that’s it. Available from every major window manufacturer for their respective profiles and designs.
Second option:
The window installer / profile assembler calculates according to DIN ISO 10077-1 the Uw value for all windows from the individual parts, considering the respective proportions of frame and glass surfaces - on average, the required Uw value must result. Usually, the carpenter’s software outputs this already. Then just submit the complete offer to the consultant, showing this.