Low lights are between 1 - 2 watts per gallon (0.3 - 0.5 watt/litre)
Medium lights are between 2 - 3 watts per gallon (0.5 - 0.8 w/l)
High lights have 3 watts per gallon or higher (0.8 w/l or higher)
How to calculate the light level:
Divide the total aquarium light wattage with the aquarium volume (gallons or litres) to get the light level per gallon/litre.
e.g. Lets say the aquarium is 48 gallons (180 litres) and has 2 x 30 watt fluorescent tubes.
2x30 watt = 60 watts in total over the aquarium
60 watts divided by 48 galls = 1.25 w/g, or
60 watts divided by 180 litres = 0.33 w/l
meaning this is a low-light set-up.