# What is the link between the normative power output Wth/m² and the a0 coefficient (optical efficiency)?

The thermal power output (Wth/m2) listed on the front page of the technical data sheet is a normative value calculated from the η0 coefficient determined during the SOLAR KEYMARK certification ) and a reference insolation G = 1000 Wth/m2.

## DualSun SPRING 300 Black

SOLAR KEYMARK (n°16458 Rev.2)

 Thermal power output = a0|0m/s x 1000 = 629 Wth/m2  Windspeed  = 0 m/s : a0|0m/s = η0 + 3 * c6 = 0,488 + 3 * 0,047 = 0,629

(case of the insulated panel)

## DualSun SPRING 375 Black Shingle

SOLAR KEYMARK (n°16826 Rev.1)

 Thermal power output = a0|0m/s x 1000 = 660 W/m2 Windspeed  = 0 m/s : a0|0m/s = η0 + 3 * c6 = 0,543 + 3 * 0,039 = 0,660

(case of the insulated panel)

The a0 and a1 values on the backside of the SPRING panels datasheets are characteristics used in modelling software to simulate thermal performances of panels with a wind applied in order to best simulate real-life conditions:

a0 = η0 – c6 * ( windspeed -3)
a1 = c1 + c3 * (windspeed -3)

In the Spring datasheet, a windspeed of 1m/s is applied to determine a0 and a1 values in order to be consistent with module power values given on the first page of the SOLAR KEYMARK annex. For more information, please consult our article on the subject of PV/T thermal performance.