From memory (I don't row much) on a concept C2:

At 1:35 / 500m pace, 31 s/min, Calorie burn was 1700/hr or so.

That's about 28 cal/minute. So 20 calories burns up in a little less than 40 seconds, in which time I pulled 20 times.

Seems too convenient - one SDHP per calorie.