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you'd need 520W to power a tandem with a total weight of 175kg along at 54kph (15m/s)
I think it's a bit more than that, otherwise our tandem 10 PB would be around 18 minutes, not just under 21. It took Wiggo something like 450W on his super-aero solo track bike, after all. You're not an order of magnitude out, though, so that 10% hill is still going to need over 1.5kW of braking to hold that speed.
gbj_tester
Good science but let's try and account for GPE (neglecting air resistance first, but then trying to approximate that).
175kg going down a 10% hill at 35mph (v=15m/s), so they are losing elevation at a rate of 1.5m/s.
So the GPE difference for 1.5m elevation difference is 175 * 1.5 * 9.8 = 2572J, since this is per second it's a healthy 2572W.
Of course, some of the energy 'gained' from GPE is eaten up by air resistance and rolling resistance.
A quick play with http://www.kreuzotter.de/english/espeed.htm shows that you'd need 520W to power a tandem with a total weight of 175kg along at 54kph (15m/s), so that is a reasonable approximation as to how much power will be sapped by air resistance and rolling resistance at 15m/s.
This leaves ~2000W that needs to be constantly dissipated (without overheating) in order to keep the speed at a constant 35mph down a 10% hill for a 175kg tandem. Oof.