The pyromancers described here have found some very tall cliffs to jump off of, and they'd like to perform some experiments with gliders. They believe, or at least hope, that being able to magically heat substances at a distance might allow them to create personal thermal updrafts by heating the air directly beneath their gliders. But even if this is the case their ability to create heat is limited. To heat the air they have to burn kilocalories, and they can only produce so much wattage at a time. That's why before jumping off any tall cliffs they'd like to know how much thermal lift force they can generate per kilocalorie spent.
Going from there, I ... uh, these pyromancers, don't yet know a whole lot about the mechanics of flight, so if they can generate adequate thermal lift they'd also like to know if there are particularly energy efficient ways to use it, and if any particular kind of glider structure is best suited to their talents and limitations.
EDIT: Not to distract from the main question of lift, but I should mention that as this is currently a fairly low-tech world they don't have aluminum alloys or similar fancy materials to construct the glider. Pyromancy does allow for the cutting, welding and forging of materials very exactly however.
Heat transfer is pretty straightforward, as is converting the resulting joules to kcal. Air typically starts at 20 C° with specific heat of about 1, J/g °C, though that can vary. Assume that pyromancy is 100% efficient for simplicity, but keep in mind that probably won't be true for the final system. At that efficiency, a well-trained mage should be able to keep up 1000 W over an hour or so.
$ Q=mcΔT $
What's tricky (so far) is knowing how much heat generates approximately how much lift force, and how to most efficiently spread the energy under the glider, and at what range they should begin heating air; a range of within 30ft would be preferred for other aspects of the magic system, but that isn't yet finalized.
How much thermal lift can my pyromancers generate per unit of energy spent?
In response to @A.C.A.C.'s comment, and in light of the recognition that this may very well be impossible. If that holds true, I'd like to add that I would accept answers that give any of the following, in order of preference.
1) Answers that show how much thermal lift pyromancy could generate, even if it's inadequate for powering gliders.
2) Answers showing in detail how the above pyromancy could supplement gliding rather outright powering it.
3) Answers showing how this could work with a higher maximum wattage.