Mael was the former owner of the Grace "Sunshine", which allowed him to increase his power as the sun goes up and decrease as the sun goes down. This is the same Grace which Escanor has now, and Escanor also has an ability called "The One", where for a minute at noon his power it is at its peak and Escanor in this form is said to be invincible.
Could Mael use The One like Escanor when he had the Grace Sunshine?
As far as we know, Mael does not possess "The One", but does not have a weaker form at night either. It must be noted that Ludociel refers to this Grace as Sun (「太たい陽よう Taiyō」), and not Sunshine, which may imply there is a difference between Escanor and Mael's use of this power.
A popular fan theory is that Escanor possesses an unnamed innate magical power which grants him power and strength based on the time of day ; and that this innate power interacts with the Sun Grace to create Sunshine. But this is only a theory.
