How large of a Magicka pool do you need to effectively cast the biggest nuke spells and the biggest on touch damage spells? This is assuming your Destruction skill is at 100.

200+ Maybe 300 to 400 can give you a very comfortable spell casting pool. However there is one spell, the highest level Fingers of the Mountain, requires ~700 mana to cast with 100 destruction. Not worth it though at the expense. There are far more effective destruction spells you can make yourself, using the spellmaking alter, that cost far less mana than that.

the big ones are around 600-800 per spell.

You can cast any spell that you can buy in the game with under 200 magicka.

You can of course always make spells that you can't cast. For all practical purposes there is not largest spell.

Thanks for the info. I have a character whose main weapon (source of damage) is the bow. I'm debating on whether or not I should use Blade weapons as my secondary damage source, or if I should go with Destruction spells.

I'm not raising my Endurance, and I won't be wearing any extra Health gear, so I won't be able to take a beating in close combat. I was thinking I could stick and move, possibly using a On Touch Destruction Nuke as opposed to a Blade weapon.

My character will max out Intelligence, but will probably only get Willpower to 50. I didn't intend to add any Magicka gear either, so I wouldn't have a very large Magicka pool. I'm doing this on purpose, because I don't want to be an especially powerful caster.

I'm just trying to decide between Destruction or Blade as a close combat, hit and run, type of option.

Or consider Restoration. The Absorb Health effect is cheaper to cast than Destruction+Healing.

My no melee mystic archer uses illusion (mostly mind control spells) as a potent support for her bow. When the long range sneaky poison thing doesn't work, she immobilizes or commands her foes and opens fire.

When in a pinch (like underwater), she blocks with her bow and uses a touch absorb health spell.

She only uses destruction for drain speed and weakness to poison. She doesn't care about fire/frost/shock because dealing direct damage is what her bow is for.

Her endurance and strength are both permanently at 30.

She dresses only for fashion, not armor rating. If she needs some temporary armor rating, she pops a couple homemade shield potions.

She's quite the glass cannon with a couple of exceptions: She has 50% magic resist that she inherited from her Breton father (she's a Bosmer like her mother). She also was born under the mage so she doesn't have the magic weakness that an apprentice birthsign could provide.

Absorb Health is also much better than any Destruction spell since it drains its target and gives it to you, Des and Res in one spell. However it may not be as dramatic as a heavy-duty lightning bolt.

But if you want that extra Magicka before a big fight, nothing is as useful as your own home-made potions which also can give you a whopping Willpower boost to regain it really quick. It also helps having the right Greater Powers.

Much to ponder here. Nobody thinks I should skip the damage spells, use a blade for close quarters, and save my Magicka for Heals and Illusion spells?

I haven't started this character yet, so I'm open to ideas. The thing I wish to avoid more than anything is being overpowered. I was planning on taking the Lord Birthsign, skipping Restoration altogether, and only occasionally using potions to help out in the healing department.

While I want to do most of my damage with the bow, it would be nice to have an alternate form of damage that would somehow work out better than the bow in rare situations. This would be for the sake of variety more than anything else.

Heal Legendary Wounds - 466
Movement Mastery - 495
Legendary Soul Trap - 810
Enemies Explode - 842
Blizzard - 836

A good spell to go with a bowman is an area attack of either: paralyze, demoralize or command. this way if you get overrun you just cast it and they all fall down/run away/stop attacking.

Don't forget that casting cost decreases a lot with higher mastery levels - Enemies Explode, for example, costs 168 for a Master of Destruction to cast. A mere 20% of its base cost, which is what the Wiki lists (I assume that's where you got the figure of 842).

If you mix elemental weakness and damage you can double the damage for little cost. If you use an enchanted weapon you can just use weakness to element 100% for some duration to double the damage from the enchantment. With a weapon enchanted with shock from a level 17+ sigil stone you do 50 in shock damage for each hit if you cast weakness to shock 100% for 10 seconds or something on the enemy.

if you add weakness to magic you become real lethal, my pure mage used shock damage 50 + weakness to shock 100% for 3 seconds + weakness to magic 100% for 3 second as her favourite spell. very nice for killing bosses at high level.

I was going to mention this. Yes.

This is where Destruction becomes slightly... broken. If you start adding Weakness to X effects to your spells, you can seriously start to beat the stuffing out of things. Weakness to Magicka is the king of these effects, since it will amplify... everything, including itself, provided it's not delivered by a weapon.

With that knowledge in hand, the best Destruction spell is no longer a 400 point monster. It's a 100-or-so point thingy that probably contains 100 Drain Health, but also contains several seconds of 100% Weakness to Magicka plus some Weakness to whatever other forms of elemental damage the spell dishes out.

The point is that every time you cast this spell at something, before the timers in the previous casting expire, its damage snowballs. 100% becomes 200%, 200% becomes 400%... and so on, and so forth.

The "best" Destruction spell isn't some super-expensive, one-shot mega-blast. It's something that exploits Weakness effects in order to deliver exponentially more damage over several castings.

Edit: I think I may be mistaken about applying Weakness to Magic via a weapon, but in any case, that's not a very efficient way to do it.

That's because your character is not skilled enough. The costs go down dramatically if you get your skill to 100.

I stand by my statement that you can cast any spell you can buy in the game with under 200 magicka, given the OP's assumption of 100 destruction skill.

At 100 Destruction the cost of acsting spells is only 20% of the base cost. So with 200 Magicka you can cast any spell that has a base cost of 1000 or less.