That is correct. That is also why a V6 engine, although more powerful, can get similar gas mileage to a 4 cylinder. The V6 uses more gas to operate, but doesn't have to work as hard to do the same job.
Of course, that doesn't mean I'd put a 300 watt heater in there... :P
I have a 50 watt in my 10 gallon, and it is doing OK, although my basement is very well temperature controlled by the furnace and air conditioning.
38 Gallon (Freshwater): 1 Green Swordtail, 3 Yo-Yo Loaches, 1 Clown Pleco, 1 Blue Gourami, 4 Otocinclus, 7 Cherry Barbs.
46 Gallon (Saltwater): 60 pounds Lalo Live Rock, 60 pounds aragonite sand, 1 Coral Beauty Angelfish, 1 Peppermint Shrimp, 1 Mushroom Coral, 1 Emerald Crab, various snails and hermit crabs.
10 Gallon (Saltwater quarantine):