Some thoughts:
1. They have this little thing for batteries where you press on it, and it colors depending on how much charge you have left. There are also a very simple pressure gauge for tires. Have the wizard hold a lightbulb in his hand and see how bright he can make it.
2. I'm not a biologist, but I seem to recall that the energy of the body generally comes from burning sugars. There's a very simple way to measure blood-sugar level (people with diabetes do it all the time), so maybe drip a bit of blood on some scale.
3. Look at Calories. Pretty much everything you eat has numbers on the side. There are also guidelines for the amount of calories each type of activity uses up. You can keep track, but there's no real way to know for sure how much you currently have. (Maybe spells have been more carefully measured?)
4. If the wizardry you have is an exact science, then I would expect more than "light spell==1 mp". More likely that at one point the scholars perfected Magic-to-Energy conversion rules (maybe even E=Magic*C^2), in which case, your basic unit of energy would something along of "The amount of Magic to heat up a Liter of water by one degree", or somesuch.
Oh, and the chance that a spell takes an exact whole amount of MP to cast should be the same chance that a human being is *exactly* a whole number of inches tall (close to zero).
5. Units:
Thaum: (as in Thaumaturgy - from discworld)
QuasitPower: (equivalent to horsepower) - The amount of magic a quasit can produce in a second.
Take a look here:
http://en.wikipedia.org/wiki/List_of_unusual_units_of_measurement