Then what is a more suitable way of determining sufficient lighting? I know 6700K is the right color temperature. But if 27 watts is good for a 5 g, wouldn't 2 27 watts be good for a 10g? They cover the larger area and the tanks are of similar depth. And for that matter would that be low, medium or high lighting? Oh...lighting can be SO confusing! Thanks for your help.
First of all, 6700K is only ONE of many "right" color temperatures. Anything between 6500K and 10000K is considered appropriate.
Second of all, don't fall into the trap of thinking that watts-per-gallon is a law of physics. It's just a guideline, one which notably does not apply very well to small tanks (anything under ~20g or so?). That being said, in keeping with the guideline theme, YES, if 1x27w lamp works well for a 5g, then 2x27w lamps would work well for a 10g. If you do the math, you end up with the exact same 5.4wpg in both scenarios. But because this guideline is not linear, it's not really accurate to say they are getting EQUAL lighting in respect to their size. I would say that the 5.4wpg on a 10g aquarium is on the lower end of what could be called "high light," whereas the 5.4wpg on a 5g is on the upper end of what could be called "medium light."
Did that make sense?
In other words, when dealing with small tanks, the wpg guideline is not very dependable. It is also said that the same applies to extra-large tanks as well.
One last thing, too. It is highly contested that water depth plays much of a role in the lighting calculations. In indoor terrestrial gardening, distance from the bulb plays a visible role, so I don't know why aquatic gardening would be any different, but many people with light meters claim that there is no discernible difference in lighting a 3 ft tall tank vs. a 18" tall one.
Hope some of that helps ya. I know it's confusing, but you'll get it. Good on you for doing your homework ahead of time!