Compact fluorescent bulbs, that look like:
are driven at the wattage the ballast is designed to drive them to. Some ballasts will handle more than one wattage, but those require changing how they are connected to the different wattage bulbs. I have put higher wattage bulbs in a light and the bulbs worked, but didn't last very long before the ends blackened. But, I also have changed the wiring, per the manufacturers directions, and used the same ballast successfully.

are driven at the wattage the ballast is designed to drive them to. Some ballasts will handle more than one wattage, but those require changing how they are connected to the different wattage bulbs. I have put higher wattage bulbs in a light and the bulbs worked, but didn't last very long before the ends blackened. But, I also have changed the wiring, per the manufacturers directions, and used the same ballast successfully.