How much total surface of the moon is illuminated by the sun when it is at quarter phase?

50% of the moon is always illuminated, however during it's quarter phase means that we only see a quarter of what's really lit up. So it LOOKS like the moon is only 25% lit and 75% dark, it's truly 50/50. We only see that 25% since we can see it from one angle.

Amplify and switch electrical power and electronic signals