Magicka Regeneration is slowed during combat, outside of which it naturally regenerates at 3% of the Dragonborn's maximum magicka per second.
Thus, having 100 maximum magicka yields 3 magicka per second. This means that an increase to magicka is usually preferable to an increase to magicka regeneration, as the former naturally comes with a certain amount of the latter. For instance, if a character with 100 magicka gains an extra 100% magicka regeneration, that character will regenerate their 100 magicka at 6% per second, or 6 points. If that same character gains an extra 100 magicka instead, they will regenerate their 200 magicka at 3% per second, or 6 points. While the magicka bar will appear to refill more slowly, this is simply because there is more total magicka to regenerate.
However, magicka regeneration items will be preferable if the Dragonborn has already been investing a lot in base magicka as they level up. For example, a character with 300 base magicka and 100% magicka regen bonus will regenerate 18 points per second (300 * 6%). If that same character gains an extra 100 magicka instead, the regeneration rate will be 12 points per second (400 * 3%).
Use a mix of regenerate and fortify magicka effects for optimal results. Starting with 300 base magicka, plus 50 from Highborn and 50 from Savos Aren's Amulet, results in 400 magicka and 12 points regenerated per second. Adding 300% regeneration (from Apprentice Stone, Morokei Mask and the Archmage's Robes), gives recovery of 12% of the Dragonborn's magicka or 48 points per second, which is sufficient to cast powerful spells without worry of being vulnerable for long periods of time.
Potions with Regenerate Magicka can be made at an alchemy lab. When consumed, they temporarily increase the rate at which magicka regenerates. The strength of the effect is determined by the Dragonborn's alchemy skill and any fortify alchemy enchanted gear equipped.
The following ingredients contain this effect:
Chest pieces, headwear, and rings can be enchanted with Fortify Magicka Regen at an arcane enchanter. Apparel enchanted with this effect increase the rate at which magicka regenerates while equipped. In particular, robes often possess powerful Fortify Magicka Regen effects. The Amulet of Akatosh is notable in that it is the only amulet to have this effect.
- This is the only enchantment in Skyrim that doesn't match the found items. Even though rings and helmets found or purchased can have a Magicka regeneration rate of up to 100% and found or purchased robes can have a Magicka regeneration rate of up to 150%, the maximum enchanting rate using only skill level and perks is only 60%.