On a Concept2 (and most ergs), watts = 2.80 × (speed in m/s)³, and speed = 500 ÷ split seconds. So a 2:00 split (120 s per 500 m) is 500 ÷ 120 = 4.167 m/s, which is 2.8 × 4.167³ ≈ 203 watts. Going 10% faster takes about 33% more power — that cube is why the last split of a 2k hurts so much.