Hi Steve,
I don't have MATLAB, so could not test your code directly. However, when I replicated it as an LK script in the SSCdev tool that comes in the SAM SDK package, I think I get different results than you describe:
POA before simulation = 1000
Tcell before simulation = 25
POA after simulation = 965.85
Tcell after simulation = 47.2424
Here is the LK script I used for the test -- you can copy and paste it into the SSCdev Script Editor to run it yourself:
Do you get a different result?
[BEGIN SCRIPT]
clear();
var('year', 2012);
var('month', 6);
var('day', 21);
var('hour', 12);
var('minute', 0);
var('lat', 39.742);
var('lon', -105.18);
var('tz', -6);
var('beam', 970);
var('diffuse', 86);
var('tamb', 25.4);
var('wspd', 3.6);
var('snow', 0);
var('time_step', 1/60);
var('system_size', 100);
var('derate', 0.95);
var('track_mode', 0);
var('azimuth', 180);
var('tilt', 40);
var('tcell', 25);
var('poa', 1000);
outln("before poa="+var('poa'));
outln("before tcell="+var('tcell'));
run('pvwattsfunc');
outln("after poa="+var('poa'));
outln("after tcell="+var('tcell'));
outln("after dc="+var('dc'));
outln("after ac="+var('ac'));
[END SCRIPT]
Thanks,
Paul.