Thank you for the reply Paul.
I did not understand you this part:
"you can see the pump power for time steps that Q useful > 0."
What is the "> 0." ?
The "Q useful" you mentioned is the "Q collector to Tank (kW), Hourly" from the SAM 2013 screenshot below:
www.dropbox.com/home/stuff?preview=shw_SAM.jpg
?
(I am using the 2013 version because this was the last one which supported my operating system: windows XP).
How does SAM calculate the hourly Pump power (W)?
For example in the upper screenshot, there are hours when the hourly Pump power (W) is 40 (equal to "Circulation Pump power), but also 26.66, 6.66 and so on.