Here is a screen snip showing the 20 optimizer limit. The newest guidelines would allow 25 optimzers per string 15,000 / 300 / 2 = 25. The data sheet indicates a max of 30 optimizers per string. It can be found here. http://www.solaredge.us/files/pdfs/products/power-optimizers/se-P5-series-commercial-add-on-power-optimizer-datasheet-na.pdf