Why is solar-only power hard for a lunar base?

Answer: The long night needs huge storage

The long night needs huge storage ✓ — Correct! A lunar night lasts about two Earth weeks, so a solar-only base must carry enough batteries or other storage to bridge many days without sunlight. That makes mass, reliability, and thermal survival much harder than a simple daytime solar setup.

Moonlight weakens solar cells — Not quite. Moonlight is just reflected sunlight and far too weak to be the main problem. The real challenge is not dim moonlight, but surviving long stretches with no direct solar input at all.

Panels fail in vacuum — Not quite. Solar panels can operate in vacuum, and spacecraft use them routinely. The problem on the Moon is that panels stop generating when the Sun is down, while a base still needs life support, communications, heating, and machinery.