I'm going to try and approach this from the most scientific position I can, so no alternate universes, no mystical crystals, no nothing besides thermodynamics.
You said not to worry about how the wizard controls the energy, right? Good, because I'm not even going to try to figure that one out. For now, it's magic.
We know we can neither create nor destroy energy-mass, correct? Well, it's got to come from somewhere. We could use:
thermal energychemical energynuclear energyEinsteinian energy
Let's start with thermal energy. (If I screw up the thermodynamics, it's cause I'm not a chem major) Luckily for our wizard, there is a fortunate lack of stuff at 0K on planets, particularly habitual planets, which is where I assume our wizard would be operating. So, why don't we just drop something to 0K and use the energy from that? If we dropped one cubic meter of atmosphere at 20C, at sea level, to absolute zero, our wizard would have 355 kilojoules of energy to play with. (This is a rough estimation, things get weird around 0K, models stop being linear, we get phase changes, bluh bluh bluh. This is really really rough, actually. Data pulled from here.) That's about 83 food Calories. Not much.
Also, we run into another problem, the reason why nobody ever gets to 0K. Energy won't really move unless it's to a place with less energy. This pretty much rules out freezing type of things, and getting fire type spell from the energy gotten from said freezing type spells. However, we don't know the range within which the wizard can transfer energy back and forth. If it's practically unlimited, why couldn't they draw energy directly from the sun or transfer it to Pluto? It'd give an interesting thing as well, you'd probably get "Sun Wizards" and "Moon Wizards", who use those bodies to manipulate the temperature. But let's try something else.
Good old chemical energy - the most commonly used method of storage and manipulation of energy by spongy organic creatures. Say our wizard is a morbidly obese wizard with 300kg of fat on him. Assuming he doesn't suffer heart failure, if he goes on a magical diet, that fat will give him 11,100,000kJ [source], much much better when trying to cast super-powerful science spells. Of course, this assumes there's a McWizards devoted to getting wizards the raw magical energy they need to preform their wondrous acts.
Nuclear energy! Antimatter! Science/magic! Nuclear, like chemical, relies on extracting the energy that's stored in bonds, except nuclear bonds are strong force bonds, not electromagnetic. So: lots more energy, but you already knew that. Einsteinian energy is based on the fact that E=MC^2; it's the direct conversion of mass to energy. However, mass won't just spontaneously flip to energy, you need antimatter and matter to annihilate for that to happen. Of course, if you could do that, you'd have stupid amounts of energy to play with. "Potions", possibly, stored in magnetically shielded vials, perhaps?
This all assumes that the wizard can safely manage this much energy, which I assume they can, given that they will be using it for spells and such.
The issue I see as the largest, aside from however the heck you're going to manipulate this energy, is somehow storing it for use. I mean, there's energy everywhere, that's no big issue. But what if you don't want to have to open a portal to the sun every time you want to light a match with your finger? What if the wizard wants to collect their energy and use it slowly, over time. This question is really linked to whatever the mechanism is for using the energy, something I'm going to have to avoid completely due to it being magical.
Chemical storage makes the most sense, and it's rather humorous to imagine a wizard putting on hundreds of pounds before a battle. Antimatter is also an effective storage. Special electrically charged crystals or totems, held in "magical" airtight, magnetic containers, paired with another one, seem like effective and classic-ish ways to do that. Something else to realize is that if there's any wizard capable of containing the energy that would be released by a goddammned annihilation would be more than capable enough of absorbing whatever explosions or weapons somebody else could dish out, and then fire that back at them.