In Harry Potter and the Order of the Phoenix, Dumbledore and Voldemort face off in an epic wizard duel. The two exchange lightning and fire before Dumbledore magically pulls the water out of a nearby pool and forms a massive spinning water ball, trapping Voldemort inside.
Water is heavy: an eight-foot sphere of it would weigh over seven tons. Dumbledore seems to struggle to telekinetically lift the orb, which seems to match other observations of Potterverse power levels. To lift a seven-and-a-half-ton object five feet in the air requires 112 kilojoules, though the rotation and shape make the actual energy requirements higher (see below).
Voldemort's escape of the ball seems to be not a propelling of the water outward but rather a dispelling of the magic that holds it together, as the water merely drops as he escapes.
One weird thing is that the orb, while holding its spherical shape, still sheds water. This is confusing—why is some water pinioned and some not? This difficulty can perhaps be resolved by positing that Dumbledore's spell effectively turned off gravity in the immediate area of the ball. Just above the average surface the water becomes affected by gravity again, leading little swells to exit the gravity negation field and fall to the ground (still affected, however, by their initial spin).
In this case, lifting the water itself is not what is causing Dumbledore's mental strain, because the water is weightless (the ball would still have a lot of inertia, which means some energy would still be spent in lifting it). Perhaps spinning it is the difficult part—the moment of inertia of the ball is about 4000 kg⋅m^2, and the rotational kinetic energy at 20 rpm would thus be about 8kJ.
Or maybe it isn't the physics of the spell at all, and merely casting the spell requires intense concentration. Whatever it is, this clearly is a difficult spell.
Now, if wizards can shoot lightning, or turn off gravity in a localized area...
They can create clean energy! Instead of sitting around being mysterious and stuff, they could solve the energy crisis by repeatedly lifting and dropping a heavy weight attached to a generator.
However, it's also possible that Potterverse magic can't create new energy—it merely uses stores already available (much like how antimatter does). In that case, Dumbledore's rotating water orb spell would probably burn off a fifth of a saltine cracker (closer to a full cracker if we take into account that he has to lift the ball into place first). On the other hand, the lightning blasts mentioned earlier would consume several Coca-Cola cans' worth.
This is enough of a burn to be good exercise (IF the wizards regularly shoot lightning multiple times a day) but not close to Steve Rogers' caloric needs of 16 lbs of bananas a day (for those interested, that's roughly 80 bananas, eating which would double his daily intake of [mostly harmless] ionizing radiation).*
*This is ignoring the fact that much of a normal person's daily radiation exposure is already from foods, but hey.
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