In order to determine the usual thermal efficiency parameters eta(0),(hem), a(1) and a(2) 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 FUL 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 (at) and absorber efficiency factor F, 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 eta(hem), during daytime operation for arbitrary G then is obtained from the Hottel Whillier Bliss equation using FUL from nightly heat loss measurements and the recalculated mean fluid temperatures. The new procedure allows the determination of eta(hem) with only a few hours nighttime measurements, provided the peak collector efficiency eta(0),(hem) and (at) is known. Otherwise, only one additional bright day is needed to perform a standard efficiency measurement at mean fluid temperature equal to ambient temperature. This effort has to be compared to typically one week of clear weather conditions necessary in today's standard steady state tests. The test procedure was successfully validated for two completely different flat plate collectors, a modified standard collector with backside aluminium-film insulation instead of mineral wool (FUL approximate to 4, 4 W/m(2)K at 70 K over ambient) and an ultra-flat prototype collector with only 40 mm height and no insulation on the backside (FUL approximate to 7, 0 W/m(2)K at 70 K over ambient). The results of nightly cooling measurements with subsequent conversion by the developed formula show full agreement to the results of eta(hem)-measurements according to ISO 9806:2013 within the common error bars. The new procedure yields an experimental error in daytime-efficiency of 3-4% and can help research institutes or R&D-departments of collector producers to shorten testing time to determine the thermal efficiencies of solar collectors. A broader application and test of the new method for further types of solar collectors by independent researchers is desirable. (C) 2014 Elsevier Ltd. All rights reserved.