Old-style fluorescent tubes made wattage a rough but usable proxy for light output, but LED fixtures vary enormously in efficiency, meaning two fixtures with identical wattage can produce wildly different actual light intensity at the substrate. PAR (Photosynthetically Active Radiation), measured with a PAR meter at substrate level, is the metric that actually correlates with plant growth.
As a rough guide, low-light plants like Anubias and Java fern do well around 15-30 PAR, moderate-light carpeting plants and stems generally want 30-50 PAR, and demanding high-light plants or those grown for intense red coloration often need 50-80+ PAR, typically paired with CO2 injection to keep pace with that light intensity.
More light without more CO2 and nutrients doesn't grow better plants. It grows better algae.
Spectrum matters alongside intensity — a fixture weighted toward blue and red wavelengths, in the range plants actually use for photosynthesis, will grow plants more efficiently than a fixture that looks bright to the human eye but is heavy in green wavelengths plants use poorly.
A photoperiod of 6-8 hours daily is a safer starting point than the 10-12 hours many new keepers assume tanks need, since excess light duration without matching nutrient uptake is one of the most common triggers for algae outbreaks in new planted tanks. It's easier to extend photoperiod gradually than to fight an established algae bloom.
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