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they use very little energy, but if you don't keep it on all day, you'll be buying fish on a regular basis.
more water volume requires a bigger pump to be able to aerate it.
The deeper it has to push the air the more power it needs. You can run a small pump on a large system if air line doesn't run very deep. As with heaters the next size larger is always better IMO.
Not sure if this helps but this is what I do. Again this is just me.
I run 2 Rena 300's on my 75g. I'm running 1 12" air wand and 1 4" round air stone I leave them on all the time. I just go with what the manufacturer says for like the 300 is for up to 75 gallons and that's what I have. To me anyway it's not like a heater where I would go +1 or a filter where I would go +2 or sometimes more depending on your stock and size tank.