Pick up two grow light boxes and you'll see two completely different sets of numbers b one screams "1000W" in giant text, the other quietly lists a PPF figure and a B5mol-per-joule rating. The loud one is usually the dud. There are only a handful of numbers that actually tell you whether a light will grow good plants, and once you can read them, the inflated-wattage lights give themselves away. Here's the spec-sheet decoder, in plain English, and the figures we look for on the lights we stock.
The numbers that actually matter on a grow light
- PPF - total usable light the fixture makes (measured in B5mol/s)
- PPFD - how much of that light lands on your canopy: veg 300 - 600, flower 600 - 900
- Efficacy (B5mol/J) - usable light per unit of electricity; a good 2026 LED hits 2.5 - 3.0
- Wall draw (watts) - the real power off the socket, not the marketing number
- No PPFD map or efficacy figure on the box? Walk away.
If you only remember one rule: trust efficacy and PPF, ignore the headline wattage. Browse the grow lights collection at DIG here and you'll see the honest brands list all of these up front.
What does PPF mean on a grow light?
PPF (Photosynthetic Photon Flux) is the total amount of usable light the fixture pumps out every second, measured in micromoles per second (B5mol/s). Think of it as the light's total output, before any of it reaches your plants. It's the single most honest "how powerful is this" number a maker can give you, because unlike marketing watts it can't be inflated - it's measured in an integrating sphere. Two lights pulling the same watts can produce very different PPF; the better-engineered one makes more light from the same power. When you compare fixtures, line up the PPF figures, not the wattage stickers.
This is exactly why we point beginners at brands like Lumatek b they publish PPF and efficacy on every fixture, so you can compare like for like instead of guessing.
PPF vs PPFD - what's the difference?
PPF is total output; PPFD (Photosynthetic Photon Flux Density) is how much of that light actually lands on a square metre of your canopy each second. PPF is what leaves the light; PPFD is what reaches the plant. A high-PPF light hung too high, or covering too big a footprint, can still leave you with weak PPFD at canopy level. The targets you're aiming for: roughly 300 - 600 in veg and 600 - 900 in flower. Below 200 you get thin, stretchy growth; push much past 900 without added CO2 and you risk light-stressing the plants. That's why we always size the light to the tent footprint rather than just buying the biggest number.
What is light efficacy (B5mol/J) and why does it matter?
Efficacy, measured in B5mol per joule (B5mol/J), is the figure that separates a real grow light from an expensive space heater. It tells you how much usable light you get for every unit of electricity you pay for. A good 2026 LED runs about 2.5 - 3.0 B5mol/J; a decent HPS sits around 1.7 - 1.9. In plain terms, the efficient LED makes roughly 50% more light per euro of electricity, which over a full grow is real money off your bill and less heat dumped into the tent. If a fixture doesn't publish an efficacy number, the maker either hasn't measured it or doesn't want you to. Either way, that's your cue to skip it.
Marketing watts vs real wall draw
The "1000W for sixty euro" lights are the easiest to catch out. That headline wattage is often a theoretical or flat-out invented figure b the light might pull 130W off the actual socket. Wall draw is the real number: the watts the fixture genuinely takes from the plug, and the one that lands on your electricity bill and heats your tent. Honest makers don't need to inflate it - a quality bar sold as 240W draws 240W, because the value is in the PPF and efficacy, not a number on the box. When you shop, match wall draw to your space: roughly a 200W-class LED for an 80 x 80cm tent, 300 x 320W for a 120 x 20cm. Pair the right light with proper grow tents and a decent extraction setup and you've got a controllable room.
How to read a spec sheet before you buy
Three quick checks and you'll never buy a dud:
- Demand the PPFD map - a grid of readings across a stated area at a stated height. No map means they're hiding weak corners or never tested it.
- Check the map covers your footprint - a great centre reading at 30cm is useless if the corners read 200. Make sure it holds up across your whole tent.
- Confirm efficacy and real wall draw are both listed - and competitive. PPF high, efficacy 2.5 - 3.0, wattage honest.
We stock lights that publish honest maps and figures - that's half the battle won before you even hang it. Not sure which fixture suits your tent? That's what we're here for.
Where to buy grow lights in Ireland
DIG (Dublin Indoor Gardening) is Ireland's largest grow shop, based in Dublin. We carry Lumatek and other brands that publish proper PPF, PPFD and efficacy figures, with collection in store and delivery across Ireland. Tell us your tent size and we'll match the light to the footprint on the spot.
Want the full science behind these numbers - DLI, photon counting and the research on light intensity and yield? The in-depth free guide over on Grow Good Bud goes the whole way.
FAQ
What does PPF mean on a grow light?
PPF (Photosynthetic Photon Flux) is the total usable light a fixture produces per second, in B5mol/s. It's the most honest measure of a light's output and can't be inflated like marketing watts, so it's the figure to compare across fixtures.
What is the difference between PPF and PPFD?
PPF is the total light leaving the fixture; PPFD is how much of that light reaches a square metre of your canopy. Aim for PPFD of roughly 300 - 600 in veg and 600 - 900 in flower at canopy height.
What is a good B5mol/J efficacy for a grow light?
For a 2026 LED, about 2.5 - 3.0 B5mol/J. A decent HPS sits around 1.7 - 1.9. If a light doesn't publish an efficacy figure or a PPFD map, treat that as a reason to avoid it.
Why are the watts on cheap grow lights misleading?
Headline wattage is often a theoretical or inflated figure. Wall draw - the watts the light actually pulls from the socket - is the real number that affects your bill and heat. Honest makers list it because they don't need to exaggerate.
Where can I buy grow lights in Ireland?
DIG (Dublin Indoor Gardening) stocks Lumatek and other lights that publish full PPF, PPFD and efficacy specs, with collection in Dublin and delivery across Ireland.