I had this posted in another topic and decided creating this topic makes more sense as the question really is about which SMA unit I should use.

I am looking at an array of 12 Siemens SP150s (they are 24v for a total nominal voltage of 288v).

Granted it's only 1800 watts; this array will be on the roof & space is really at a premium...

I think I am in the "sweet spot" in terms of the SB2500 voltage range with a single series string, but am wondering if maybe I should look at a SB1800 (I see that the 1800 is out now at www.sma-america.com).

I see from the SMA string sizing application (a very cool web app!) that I could use the SB1800 and create 2 strings of 6 each. So the only downside I see at this point is that there is not really much system expansion possible if I go this route (which is probably not an issue - there is an outside chance I might build a patio cover that could be home for 12 more SP150s).

Would being at the higher end of the SB1800 inverter capacity be better (or worse) in any way as it relates to performance?

Or is the possibility that I could add another array a reason to go for the SB2500?

Anyone have other thoughts? Thanks!

IMHO, I would spend the extra $300-$400 for the SB2500.
- You can add up to 9 more 150s (2 strings of 11, according to the string spreadsheet).
- It is better to operate at partial capacity than full capacity -- extends the life of the electronics.
- On a really hot day, the Vinput may approach the lower threshold on the 1800, causing MPPT cut-out.
- The 2500 gives you 120VAC on both L1 and L2, while the 1800 just delivers 1 120VAC ckt.

I hope this helps. Either choice is solid.

Roger:

You could really go either way.
The efficiency will be about the same with a slight (0.5 to 1%) advantage to the 2500.
Expansion is not that much of an issue since your roof space is limited. If you expand onto a patio with a different orientation them we would suggest a seperate inverter for the different orientaion anyway. We should have the 1100 and the 700 Watt units out at the end of 2002.