Robert, there are losses associated with any system, distance from inverter to panels, heat losses through power generated and as others have said the efficiency of the panels and inverter. I have a 4.5kw system and mid summer 90deg and no cloud plus temp below 35deg C, I get around 4kw, so the numbers you are getting sound good. I have noticed that as the temp gets above 35deg C, the efficiency does drop a few percent. My next door neighbor has a 7kw system and he is getting identical output percentages to me. The guy that sold me the system said I would be doing well if I averaged 80% of the rated output. I have checked this carefully since it was installed and when cloud is not around and the sun is at zenith my panels are delivering approx 85% in mid summer, this obviously drops off as the sun migrates to the northern lattitudes so to speak, and that is expected. My best day since I have had the system installed in July was 34kw generated by my panels, obviously that was on a coolish clear day mid summer, now I am getting around the 28-30kw mark on clear cool days, this will obviously continue to drop off now as the days get shorter. As I said in another thread, I now no longer have to pay monster power bills of over $500 / quarter, and I get money back from the power company for the excess power I export back into the grid, which in the last quarter was around 1600kw. Since that reading on 30 Dec I have exported 840kw back to the grid. I think that corresponds to around 20kw per day exported, so I am happy with that.
Granted I still use power from the grid when the PV's are not operating, but that is only about 40% of what I used to use.
