Performance of solar collector is often presented as a graph, or set of three performance variables. Values may be provided based on gross area, aperture area or absorber area. It doesn't really matter which values is used, as long as you use the correct value. ie. Don't use absorber area when using performance values based on gross area.

The three performance variables for the AUCKLAT collector are provided by the SPF testing Insutitue in Switzerland (SPF report C762.) They are as follows:

Conversion Factor: h0 = 0.623
Loss Coefficient: a1 = 1.52 W/(m2K)
Loss Coefficient: a2 = 0.0146 W/(m2K2)

The equation for calcaulating basic effieiency is:
( Tm=average manifold temperatue,Ta=ambient temperature, insolation level (G) in Watts/m )

Below is a basic efficiency garph generated by SPF.

You may note the curve is very steady.
One factor which is not considered in the straight performance calculations outlined above, is the affect of transversal IAM values (Incidence Angle Modifier) on solar collector output throughout the day. Please read next section to learn more about IAM.