1) I want to calculate the efficiency of each compressor at 300W heatload . How's this possible ? Let's say for the sc18cl .
Are we looking at cecomaf or the ashrae numbers ? For 300W it can make approx -40celsius . But how much power consumption will it have ? The "Power consumption" and "current consumption" don't match ... What am i missing here ???? The Power consumption VS the Cooling capacity is the effeciency and is usually stated as COP {co-efficient of performance} s 1.1, 1.23 etc. so a compressor with a cop of 1.2 and power consumption of 235 will cool a load at rated temperature to 283 watts.

2) Looking at the specs , i see that Is that up to the sc12cl the current consumption scales up pretty good . From the sc15cl the current consumption goes up very quick. Does this make the sc12cl the most economic compressor from the others when operating with the specific heat load ??? Yes and NO you have to compare the same temperature ranges at multople points.

3) Let's see this from the hardcore overclockers point of view . What are the benefits from choosing the sc21cl from sc18cl ? It's about 3 degrees cooler and it has less power consumption . I'm i right ? I would use a sc10. the number in danfoss is the displacment dont get too big it will not make enough differance for a single stage.
4) Can someone explain to me what the COP means ?? I understand that it is capacity/power consumption . Is this the efficiency ??? So , are we looking for the higher possible COP with our heat load ? See above yes awe are lookng tofor higher efficiencies. I have a 1/4 hp that has a LRA of 10 it will remove 163 watts art its rated evap temp of -20C with r123a With R402 it will remove 285 watts at -38C.
Thanks