How much does Irisalis's ultimate cost?

You need two full energy bars before Floral Descent will cast at all, so one bar on its own gets you nothing. The bar may be counted in charges, segments, points or energy, but it is the same requirement every time: two.

That price is why timing matters. Casting while an Iris Shadow clone from her second skill is already active puts two clones on the field instead of one, so the same two charges buy more damage. Recasting just to keep a clone around is the poor trade: the ultimate's clone is reset and its 30-second duration refreshed, so you pay two charges again for a clone you already had.

Avoid firing it under a ceiling. Floral Descent often bugs out there, dealing no damage or failing to spawn its clone, and that wastes the two charges you spent building the bar. The linked article on clones, cost and bugs covers the rest of how the ultimate behaves.