We report on the investigation of terahertz (THz) emission from gold coated nanogratings (500 nm grating constant) upon femtosecond laser irradiation (785 nm, 150 fs, 1 kHz, ≤1 mJ/pulse). Unlike common assumptions, THz emission - both in the direction of laser propagation and reverse - is not only observed in case of rear side irradiation (through substrate) of the nanograting, but also in case of front side excitation (through air). Based on that we suggest a new approach to describe the newly observed phenomenon. Using a highly sensitive and fast superconducting transition edge sensor (TES) as calorimeter it was possible to directly measure the absolute energy of the emitted THz pulses in a defined spectral and spatial range, enabling for the first time a quantitative analysis of the THz emission process.