conecting 12v to 24v panel in paralell

renaultscenic2 said:

The vmp for the 250w is 31.14v while the imp is 8.03A,while for the 2,100w panels I must find time to check cos they are mounted on the roof.The Cc is tristar ts60 mppt

Your 250 watt panel is a 60 cell panel.  Your 100 watt panels are probably 36 cell panels.  They are not a good match in series or parallel.

--vtMaps

You could wire the two 12v modules in series to get a 24v string, then parallel that 24v string with the 24v module. It wouldn't be a perfect match, but it would work.

You'd lose some efficiency because the MPPT would probably end up picking an operating voltage somewhere between the two (split the difference) so neither the 24v string nor the 24v module would be working at their maximum power point. In other words, the array as a whole would be operating at the maximum power point of the array, which will be something like 3v higher than the MPP of the single module, and 3v lower than the MPP of the series string (assuming your 100w modules are 36 cell / 18vmp).

If your 100w modules were 32 cell / 16vmp, then you'd be so close that it wouldn't matter enough to bother even worrying about.

Actually...I suppose it would work the same way and have about the same efficiency handicap if you just wired all three in series to make a 67v series string.