above: a 2Hr Tank in Green Effect, a freshwater tank retailer in Singapore. More difficult species such as the red Eriocaulon quinquangulare do better above 100 umols of PAR on the substrate.
Too low and plants are unable to thrive.
Too high and the tank becomes far more vulnerable to algae.
The relevant measure of strength for a planted tank is the amount of Photosynthetically Active Radiation (PAR) as used in horticulture, but this data is not often found on household bulbs (which generally show the Wattage (a measure of power consumption, not output) and/or Lumens ( a proxy for ‘brightness’ ).
The tricky part is that there is little correlation between PAR, Watts and Lumens...especially when it comes to LEDs bulbs (where there is huge variability in efficiency and efficacy). This leads the following:
We took a sample of 5 everyday household bulbs, with the question: given how bright these new LEDs appear, are they strong enough for a planted tank?
The conclusion is that the vast majority of household bulbs would look very bright to the human eye, but would be only suitable for fish-only tanks, or planted tanks with only shade-tolerant plants. To unlock richer hues, it would be far better to go for specialised aquarium lights. Read our review of the latest LEDs here.
Unfortunately there is no easy way. You cannot mathematically convert Watts and/or Lumens to PAR. The detailed specifications of household bulbs may contain Information about the bulb’s PAR. Professional aquarium lights generally publish PAR data.