Solar panel and 120mm Computer fan
The fan requires 12V, 150mA (0.15A). About 1.8W as listed. That is for standard power. You can provide less volts, for less powerful blowing, at a relatively lower current draw.

As such you need a solar panel that can provide 12V and 0.15A, again 1.8W, in regular light. Unless you have 100% unobstructed direct sunlight, no clouds or anything, you will need a solar panel that can provide alot more than just 1.8W. So you need to over size it.