In order to determine the usual thermal efficiency parameters g0,hem, a1 and a2 of solar collectors according to the current international
standard ISO 9806:2013, a new and simple short-term test procedure has been developed. It bases upon the experimental determination
of the fluid-temperature dependent loss coefficient F0UL without insolation using stationary heat loss measurements during night. To
account for the fact neglected so far, that without insolation the mean absorber temperature is lower than the mean fluid temperature,
a conversion to daytime conditions has been evolved: depending on hemispherical irradiance G, transmittance–absorptance product (as)
and absorber efficiency factor F0, the new conversion formula calculates the (lower) mean fluid temperature during daytime operation, for
which the absorber has the same temperature as during the nightly heat loss measurements. The thermal efficiency ghem during daytime
operation for arbitrary G then is obtained from the Hottel–Whillier–Bliss equation using F0UL from nightly heat loss measurements and
the recalculated mean fluid temperatures. The new procedure allows the determination of ghem with only a few hours nighttime measurements,
provided the peak collector efficiency g0,hem and (as) is known. Otherwise, only one additional bright day is needed to perform a
standard efficiency measurement at mean fluid temperature equal to ambient temperature