I was wondering if anybody had a guess as to just how much horsepower that kite must have...
This addresses your question in some detail. I wrote it five years ago, about force, pressure, energy and power:http://kiteboardbc.com/phpBB3/viewtopic.php?f=1&t=178
And here's an excerpt from it:
Power is the rate at which a device can convert energy from one form to another, and is usually measured in watts or horsepower. It can be a confusing term, because the relevant types of energy have to be defined. A 100 watt lightbulb converts electrical energy to light energy and heat energy at a rate of 100 watts, but if the lightbulb is only 20% efficient, that means it only converts electrical energy to light energy at a rate of 20 watts.
A bouncing rubber ball, and a jumping kiteboarder, both convert energy alternately between kinetic and potential forms, but because they aren't sustainable processes, power ratings aren't relevant in these examples.
In some circumstances, power can be quantified by multiplying a force by the speed at which that force is exerted. Here are some examples of power calculations based on specific examples:
1. Heavy(ish) kiteboarder on freestyle board going 25 mph with 30 lbs resistance (independent of wind speed and kite size): power = 2.0 hp
2. Light kiteboarder on high L/D ratio board (eg Spleene Session) going 18 mph with 20 lbs resistance: power = 0.96 hp
3. Same kite as in example (1.), attached to a Honda Civic with tires overinflated to reduce resistance, on a broad reach on an airport runway, with wind blowing like !@#$%, chicken loop mechanically fastened to car (pilot couldn't possibly hold it), going 50 mph with 40 lbs resistance (rolling resistance only, apparent wind over the beam): power = 5.3 hp
A kite alone does not generate power. Power is generated when a traction kite and surface vehicle interact as a system, whether that vehicle is a kiteboard and rider, a buggy and rider, a car, an ice boat, or whatever.
The maximum achievable power is based largely on what the vehicle can handle. A given vehicle will have a maximum practical upper limit to power generation. If the vehicle is a particular kiteboard and rider, they may have an approximate upper end of 1.5 hp, regardless of whether they achieve it with a big kite in light wind or a small kite in high wind.
The heavier and stronger the rider, and the more efficient the board, the higher the maximum power will be. A 200 sq m monster kite wouldn't help a kiteboarder generate more power than with a normal kite, but a freighter or a heavyweight ice boat could generate huge power with the monster kite.