The short answer
As a rough rule of thumb, about 0.5 watts per litre is low light and around 1 watt per litre is high light, with medium in between. But treat this as a loose starting point only. It dates from the fluorescent-tube era and doesn’t map neatly onto modern LEDs — the honest measure of planted-tank lighting is PAR, not wattage.
Where the rule comes from — and its limits
The watts-per-litre guide was handy when everyone used similar fluorescent tubes, because output tracked wattage fairly closely. Two things break it today:
- LEDs are far more efficient. A modern LED can deliver as much usable light as an old tube at a fraction of the wattage, so the same watts-per-litre figure means very different things depending on the fixture.
- Depth matters. Light weakens as it travels down through water. A tall tank needs more output at the surface to reach plants on the substrate than a shallow one of the same volume.
So the rule can put you in the right ballpark, but it can also mislead badly.
The better measure: PAR
PAR (photosynthetically active radiation) measures the light plants can actually photosynthesise with, at the depth they grow. It accounts for the fixture’s spectrum and how far the light has to travel. Manufacturers of good planted-tank lights usually publish PAR figures or a low/medium/high rating — lean on those.
Choosing a light in practice
- Low-tech, easy plants — aim for the lower end (roughly low light); it’s forgiving and algae-resistant.
- Demanding plants, carpets, reds — go high, but pair with CO2 and fertiliser.
- Judge by results — pale, stretchy growth means too little; algae on healthy plants often means too much.
For fixtures rated by usable output rather than raw watts, see our best light for a planted tank guide and the full lighting hub.