Just add all the power suppliers outputs together and if that number exceeds the 32 EU/t it will blow up a bat box.
So 33 Solar panels connected to bat box (assuming no cable losses occur) will blow up bat box when they start producing. (32 solars are fine)
I was running a generator to one face of a batbox and 5 solar panels to the top. The batbox never exploded but some of my machines did.