A question about solar panels

[SamuraiProgrammer]

In a game that just completed, I had this transport design:

Bridge
Life Support
Crew Quarters
(5) Ion Engine I
(10) Cargo Bay I
(1) Solar Collector I

The engines use 10 supplies each, the Solar Collector supplies 50 supplies per star.

In a system with a star, why did the supplies run out? Does the activity of loading and unloading consume supplies?

Thanks in advance...