John Berner (president of SMA America) when asked this question recommends quantity (18 -24) 12 volt (nominal) panels for a SB2500U.

My really world experience in Florida (and actual testing put the minimum at 20 panels. It gets warm down here and the mpp point (maximum power point) is in the 12-13 volt range this time of year.

The SB2500U has a minimum voltage requirement of 240 Volts DC , so if the string under heat has a mppt below this the unit will track down to 240 VDC and power production will be lost.

My KC120's are spec'ed at 0.08 volt drop per C above the standard rating of 25 C (77F). So nice dark blue panels in hot Florida sun = big temperature rise. At 13 volts mpp this mean about a 50 C rise above standard , which I have verified with a thermal probe

Doing the math , 18 panel should work down to 13.3 volts mpp , but in my experience this is cutting it to close

If you wondering ... it's not that bad on the overall array output because in this hot weather we have lots of edge-of-cloud effect.

Yesterday I pegged the isonolation meter which stops at 1250 Watts/meter squared. The KC120's are rated at 7.1 amps and I have seen reading almost hitting 9 amps ... it's for a short burst but it really help make up for the voltage drop !!
[img]images/smiles/icon_wink.gif[/img]

Sweet, got to ove those cloud edge effects. Its been rainy and cloudy out our way -- little sun, little kwh.

By the way, there is a string sizing spreadsheet available from John Berdner or Kent Sheldon at SMA. It allows you to input the manufacturer, type and quantity of PV panels and model their performance with a Sunny Boy under different ambient conditions. Its free and in Excel format.

On of my goals in the next few weeks is to get stuff like this onto this server. Pete Prossen is being a kind sole and working to get the web-page inplace. Once that up I intend to gather such goodies and have them here.

Initially we will have a links/documents page for each manufacturer ...Xantrex and SMA

(I can't seem to delete this message - anyway, I moved my question to a more appropriate topic: Which Sunny Boy is right for me now that I see the SB1800s are an option). -R

[ September 15, 2002: Message edited by: Roger ]

This is the most common question we get asked. We developed a web app that can help you what if different array configurations.

For now it is only up on our site www.sma-america.com. The scripts are a little slow so please be patient after you click the buttons.

Feedback is welcome and appreciated.

Thanks John, for your reply here & on the "Which Sunny Boy" topic. I think that the string sizing application on the site is great... I like "what if" tools.

However I noticed in your post, the address includes a "." after ".com" and this takes us to the "www.sma-america.com." site hosted for SMA Regelsysteme GmbH instead of your site with the string sizing application.

I use www.sma-america.com and it works great.

-R