Yes, Watt-Hour is a unit of ENERGY, and 300 Watts/hour is DUMB. Power divided by time doesn't make any sense. Anyway 300 watts of solar panels even on earth is quite a bit. Since Mars is further away from the Sun, even more area is needed to make that much power.

I suspect that the proper term is about 300 Watt-hours/(mars)day [sol], or the solar panels are doing about 25watts (in sunlight for 1/2 day[sol]) for the 12+ hours they have sunlight.