The amount recovered from this ability varies with the summoner's summoning magic skill (in addition to any gear that adds to the skill), weather of the area, and the day of the week. Optimal results seem to be obtained with capped summoning skill and matching the summoned spirit with the elemental day and/or weather present in zone.
Using a spirit that matches the weather when it is double weather gives more MP than when it is single weather.
(Summoning Skill - 50) x (Bonus%) = MP restored.
Minimum base return of Elemental Siphon is 50 MP. Minimum Summoning skill needed to improve upon this is 101. Weather bonuses and penalties still apply. The maximum base return prior to the level cap increase was 300 MP. This limit was removed with the first level cap increase and will likely continue to rise until the final level cap.
Weather and day multiply the amount of MP restored as shown below. These figures are without the Elemental Siphon Bonus granted by the Tatsumaki Sitagoromo:
0.65 for opposing day and double weather,
0.75 for opposing double weather,
0.80 for opposing day and weather,
0.85 for opposing double weather and matching day,
0.90 for opposing day or weather,
1.00 for matching day and opposing weather or matching weather and opposing day or neutral day/weather,
1.10 for matching day or weather,
1.15 for opposing day and matching double weather,
1.20 for matching day and weather,
1.25 for matching double weather, and
1.35 for matching day and double weather.
Example: Thunder spirit on Lightningday with thunder weather and 310 summoning skill: (310 - 50) x (1.2) = 312MP.
The Scholar Storm spellset affects the bonus or penalty obtained by the same multiple as single weather. If the same weather is already present the bonus is applied as if in double weather and so on. However it cannot exceed a maximum bonus multiple of 1.35.
Using a spirit that is weak to the day/weather will incur a penalty to the amount of MP obtained equal to the inverse of the bonus for matching. (i.e. Using a Thunder Spirit on Earthsday will get you .90 your base MP returns. Using a Thunder Spirit on Lightningday with single earth weather will cancel out and you will receive the base.)