Alright, the answers above are talking about shooting real lightning, but let's scale things down a bit because if we're using real physics that's not really reasonable. Let's talk Tasers.
I gave up after 3 Google searches and settled on this answer for how many Watts a police taser uses:
"The TASER Advanced M26c and TASER X26c are the most powerful units, discharging 50,000 voltsat 26 watts."
According to this article, that's less than a quarter the average energy usage of a human being throughout the day, indicating that turning "on" the Magic Taser(TM) would likely not be much more tiring than a heavy jog.
A Magic Taser(TM) is probably going to be limited in range to whatever object is manifesting the electricity (i.e. the Wizard's skin), but you could easily channel it through a sword or a long pole of some sort (or even your own armor).
Now, I know what you're thinking. A guy in metal armor is going to be insulated and not going to feel the effects of a Magic Taser(TM) especially since the existence of the Magic Taser(TM) will encourage steel plate makers to design their armor i nsuch a way that there are nice smooth electrical connections to the ground. I don't have a clever solution to that. That's just a natural build up of the arms race that develops from magical armaments.
But wait, this whole thing implies that Wizards need to get into reach of their target doesn't it?
Actually, that I do have an idea for. Introducing the Arrow Taser (TM). A steel arrowhead can make an excellent half of a capacitor (with the ground being the other half). With enough control over the forces of nature, a skilled Wizard can force an arbitrarily large buildup of charge in the arrowhead that will be dispersed upon contact with the ground. Of course, it need not be an arrowhead.
Introducing the Ball Bearing Trebuchet Taser Mark 1(TM). Hundreds of small steel balls launched by Trebuchet and charged by a Wizard, channeling the energy directly out of a burning vat of oil. Each one delivers a potentially deadly static discharge that will rain down on the enemy forces.
Ok, but seriously
Let's talk about the greatest source of energy that no one ever thinks about when they talk about harnessing energy to fuel magic, thermal environmental energy. If the laws of thermodynamics no longer hold you down, why could you not just pull the heat energy out of an object around you to charge anything you want.
A single kilo of water at STP (standard temperature and pressure) contains around 4 kJ of untaped thermal energy if you could just suddenly cool it to absolute zero. If you needed to generate the 5GJ of energy required to create a bolt of lightning (as cited by Nuclear Hogie), you merely have to lower the temperature of 5 million cubic meters of air 1 degree Celcius or do the same thing with 5,000 cubic meters of soil.
I couldn't hazard a guess as to what kind of crazy weather phenomenons you'd create from repeated castings of this kind of spell, but it's something to consider.
Edit: Mass is energy, and any Wizard worth their salt can see that.
E = mc2 implies that you could generate a bolt of lightning off by turning a single object with a mass of around 0.000005 g into pure energy.