How to Choose a Grow Light
Wattage is the number on the box, but it is the number plants never feel. Two fixtures both labelled '300W' can deliver drastically different light to the same canopy depending on the efficiency of their diodes, drivers, and optics. Choosing a light well means ignoring the front of the box almost entirely and asking what it actually puts out, where it puts it, and at what hanging height.
Why wattage tells you cost, not output
Wattage measures electricity drawn, not photons delivered. The conversion between the two — photon efficacy, measured in micromoles per joule — varies enormously between fixtures: modern quality LED fixtures publish roughly 2.5-3.0 µmol/J, while older designs and cheap bar lights can sit well below 1.5. Two lights drawing the same 300 watts can therefore differ by nearly double in actual output. Marketing exploits the gap in both directions. Some fixtures sold as '300W' historically drew only about 150 watts from the wall, because the number described nominal LED-chip capacity rather than real draw; others advertise an 'equivalent wattage' versus an HPS lamp they cannot match at the canopy. Wattage has one legitimate use: it tells you what the light costs to run and roughly how much heat it adds to the room. As a proxy for growth it tells you nothing until you also know the efficacy — and the efficacy is on the spec sheet, not the box art.
Full-spectrum vs targeted spectrum, without the hype
Early LED grow lights used only blue and red diodes — the 'blurple' look — because photosynthesis peaks in those bands, and on paper that appeared efficient. In practice the case was oversold: plants use the whole PAR range, white full-spectrum LEDs (blue diodes coated in phosphor) drive excellent growth, and blurple's costs became obvious — a room lit in purple makes it nearly impossible to see pest damage, deficiency symptoms, or true leaf colour, and it skews camera-based measurement apps. Modern white fixtures dominate for good reason. Spectrum still does real work at the margins: more blue in the mix produces tighter, more compact growth, and end-of-day far-red shots can influence stretch — but UV is marketed far harder than its grower-visible benefits justify, and overdoing it carries real risk. For a single fixture that must cover every stage, full-spectrum white is the sensible default. Spectrum fine-tuning is an optimisation for growers who already have their DLI, hanging distance, and environment dialled in — not a fix for a fixture that is too weak to begin with.
Match the footprint to the tent, not the wattage to the room
A fixture's output only matters where it lands. Manufacturers rate coverage at a stated hanging height — a board that covers 90 × 90 centimetres at thirty centimetres may cover far less at the height you actually hang it, because intensity falls off steeply toward the edges of the beam. A powerful light in a small tent wastes its surplus on wall reflections; a modest light stretched across a large canopy delivers strong light only at the centre while the corners starve — and you have seen the symptoms before as uneven growth, stretched corner plants, and a canopy that leans toward one side. Two smaller fixtures spread across the space usually beat one large fixture of the same total wattage, because the overlap between them fills in the edge drop-off. Before buying, measure the canopy you actually intend to fill and compare it against the manufacturer's footprint rating at the height you plan to hang. Buying by wattage alone skips the only question that matters: coverage, at your distance, over your area.
What to actually check before buying
The single most useful document is the manufacturer's PPFD map — a grid of measured intensity readings at a stated hanging distance — because it answers the two questions wattage cannot: how much light reaches the canopy, and how evenly it is spread. If a seller publishes no PPFD map at any distance, treat everything else on the listing as decoration. Alongside it, check photon efficacy in µmol/J (2.5 or above is the modern bar), the real wall draw — ideally measured by independent reviewers rather than claimed by the seller — and dimmability, because a fixture you cannot dim cannot adapt as the canopy rises or your DLI target shifts by stage. Warranty and driver quality matter more than diode counts: drivers fail more often than diodes do. Ignore lumens entirely — that number describes brightness to human eyes, not to plants. And be sceptical of 'replaces a 1000W HPS' claims: the honest comparison is PPFD at your hanging height across your footprint, which is exactly the data most listings omit.
How GrowScope helps
Once the light is hung, GrowScope's light monitoring lets you verify the listing's promises with your own measurements: you log PPFD readings at points across the canopy, and the 3D light map — which models both direct output and reflected light from your walls — calibrates against them, showing the real hotspots and drop-offs in your actual space rather than on an empty showroom floor. Fixtures are entered by general type (quantum board, bar, COB, or custom) rather than a brand catalogue, so the tool works with whatever you ended up buying.
Key takeaway
Judge a light by its PPFD map at your hanging height and its µmol/J efficacy — wattage tells you what it costs to run, not what your plants receive.