A 20 high tank is 24 inches long and 16 inches high. The length means you will have to use 20, 40, 60 watts of T12, or 16, 32, 48 watts of T8, etc. You can't pick a wattage and buy a bulb or bulbs that are 24 inches long with that wattage.
The 16 inch height defines how much light each type of bulb produces at the substrate level. A T12 or T8 bulb produces about 15 micromoles of PAR, while a T5HO bulb produces about 100 micromols of PAR. The first is very low light, and the second is high light. But, the T12 bulb wattage would be 20 watts, and the T5HO would be 24 watts. That should tell you that wattage is irrelevant.