How does a 90 watt fan consume power from a 500 watt solar panel?
A PV panel generates voltage like a generator by separating positive and negative charges. It will do so in the moment when it is exposed to light. When the circuit between the different potentials is open-circuited, no current can flow, the PV panel will produce voltage but no power. Its just unused. So e.g. your panel produces 100V, then 100V*0A=0W.
You may use any fraction of energy you want from an electrical source. This is always the case if you do not consume all the power which can be generated. But as soon as a consumer demands the provided power generated, it will be used.
This is why you can use the same batteries in an energy saving remote control and a high power torch. Any consumer &quot;tress&quot; to sink the current needed, and if the source can deliver it it's OK. If a consumer does not need all the power a source can deliver, it will just work and leave some energy for other consumers.

PS: In an AC power grid with rotating generators, if not all power is needed, the generators will spin faster and the frequency will rise. The plant will then reduce power generated by the turbines, so the generator reduces rotation speed again. The same happens if too many consumers demand power at once: frequency drops, and the power plant will increase power generated by the turbines so the generator rotates faster again.