The short answer
PAR stands for Photosynthetically Active Radiation. It measures the usable light energy plants can actually photosynthesise with, in the roughly 400–700 nm band. For a planted tank it’s a far better guide than watts or lumens, because it tells you how much of the light reaching your plants is light they can genuinely use.
Why PAR beats watts and lumens
The old measures each miss the point for plants:
- Watts measures how much electricity the light draws — nothing about the light it produces.
- Lumens measures brightness as human eyes perceive it, weighted toward green where plants are least sensitive.
- PAR measures the actual photon energy in the wavelengths plants photosynthesise with — the thing that matters.
Two fixtures can pull the same wattage yet deliver wildly different PAR at the substrate. That’s why serious planted-tank lights are rated in PAR, usually measured at a given depth below the light.
Using PAR to choose a light
Match PAR to the plants you want and whether you’ll run CO2:
- Low-light plants (mosses, anubias, java fern): modest PAR is plenty.
- Medium plants (many stems, crypts): a mid-range PAR.
- Demanding carpets (dwarf hairgrass, HC): high PAR, and almost always CO2 to match.
More light isn’t automatically better — high PAR without matching CO2 and nutrients just fuels algae. Aim for enough PAR for your plants, then keep the photoperiod sensible. Many modern fixtures are dimmable, letting you tune PAR down if algae appears.
For choosing a fixture see our best light for a planted tank guide and browse aquarium lighting. Related equipment answers: how much light do plants need?, what colour light is best? and do LED lights cause algae? High PAR often pairs with CO2 — see CO2 for beginners.