4w/gallon | 1W/litre is what I wish I had but I only have 160W/300 litres (75 gallons) which = basically 2W/gallon or 0.5W/litre. I have easy to grow plants: ambulia, java fern, some palmey plant I'm not sure which it is, wisteria, rotala, and a few grassy ones in the foreground. I tried some red plant but it didn't survive even with ferts, not enough light I'd say, or perhaps it was outcompeted by the other fast growing plants.
My tubes are grow-lux, 4 X 40 W bulbs, 2 aqua one units that house 2 bulbs each.