Why this discrepancy?

[TomW]

I am in my first month of producing electricity with my second grid tie system, a SMA SB3000 inverter with 2100 watts of yingling panels. My first system is a couple years old, an Outback GTFX with a small battery array and 1600watts of panels attached.

I am a little surprised at the outputs of the two systems. For instance, right now I am looking at 1760 watts coming out of the SMA and 960 watts coming out of the Outback (Panel voltage = 80v, current = 12A).

Since the panels on the Outback system are rated about 3/4 the size of the ones on the SMA system, I would expect the power to be about 1300 watts on the outback, not 960. At first i suspected maybe i had a bad cable connection, but everything checks out ok. and the short-circuit amperage is just what its rating says it should be.

Does anyone have any theory about why there is this discrepancy?
Thanks.

[Mike90250]

Maybe the MPPT system is less robust ?

[solarc]

Dear TomW,

could you provide some more details of you systems, such as # of strings and # of panels per string. Is there a inverter being used in your Outback system or does it exclusively charge batteries.

In this case the maximum power is not determined by the power output of the panels but by the maximum power the charge controller can handle and much more by the charging state of the batteries. In case the batteries are full allready, the charge controller will limit the power completely.

Only with almost empty batteries you will be able to compare a battery system with a grid tied system which practically can be considered as a battery of infite capacity.

Solarc

[Sunking]

Tom I suspect it is because your batteries are getting near full capacity charge. If the batteries are fully charged, and you have no load, then your power input from the panels will be ZERO if you have no dump load.

[TomW]

Thanks for your responses. The outback GTFX has a 48v battery array, but it sells 120vac back to the grid when the batteries are fully charged. Therefore, even when the batteries are full, there should still be the potential to sell as much power as the panels are capable of producing.

There are some differences in the two systems that might explain some of power reduction. Panel angle of the outback system is 60 degrees. Angle of the SMA system is about 40 degrees. Maybe this winter things will even out a bit as the steeper angle of the outback system will become more advantageous. The second item is possible I2R loss. The outback is designed for 80v at 20 amps max transmitting power through about 135 feet of awg #6 cable to to the inverter. The SMA system produces 240 volts at 9 amps max. but the distance to the inverter is less than 10 feet.

It would be really interesting to see side-by-side comparisons of the various systems by disinterested third parties.

[Sunking]

Tom panel orientation will make quite a bit of difference.