{"id":11498,"date":"2026-10-08T14:45:04","date_gmt":"2026-10-08T12:45:04","guid":{"rendered":"https:\/\/libolux.com\/blog\/ppfd-dli-how-much-light-your-plants-need\/"},"modified":"2026-10-08T15:18:44","modified_gmt":"2026-10-08T13:18:44","slug":"ppfd-dli-how-much-light-your-plants-need","status":"publish","type":"post","link":"https:\/\/libolux.com\/en\/blog\/ppfd-dli-how-much-light-your-plants-need\/","title":{"rendered":"How much light your plants need: PPFD, DLI and why watts mislead you"},"content":{"rendered":"
Quick answer<\/span><\/p>\n

Plants do not see watts, they see photons<\/strong>. The unit that matters is PPFD<\/strong>: the micromoles of usable light landing on each square metre every second. Quick reference: 100\u2013300 \u00b5mol\/m\u00b2\/s<\/strong> for cuttings and seedlings, 300\u2013600<\/strong> in vegetative growth and 600\u2013900<\/strong> in flowering without added CO\u2082.<\/p>\n

To pick a fixture, multiply the PPFD you are after by the square metres you want to cover. That gives you the PPF in \u00b5mol\/s<\/strong> the fixture has to deliver \u2014 and that figure does appear on a serious spec sheet.<\/p>\n<\/div>\n

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Watts do not measure light, they measure your bill<\/h2>\n

A 600 W panel does not give \u201cmore light\u201d than a 450 W one by definition. It gives more consumption. What turns electricity into usable light is photon efficacy<\/strong>, measured in \u00b5mol\/J: how many micromoles of usable light come out for every joule going in.<\/p>\n

At 2.9 \u00b5mol\/J, 720 W becomes 2,088 \u00b5mol\/s. At 2.0 \u00b5mol\/J the same 720 W would stop at 1,440. Same electricity bill, 30 % less light over the crop. That is why the first figure to ask a manufacturer for is not watts: it is measured \u00b5mol\/s<\/strong> and \u00b5mol\/J<\/strong>.<\/p>\n

It is also why \u201cequivalent to\u201d claims deserve suspicion. If a spec sheet says \u201cequivalent to 1,000 W HPS\u201d and never states the \u00b5mol\/s, it is not giving you data \u2014 it is giving you a feeling.<\/p>\n

PPFD: the light that actually reaches the leaf<\/h2>\n

PPF<\/strong> (photosynthetic photon flux) is what leaves the fixture: \u00b5mol\/s, a single total. PPFD<\/strong> is what arrives at a given surface: \u00b5mol\/m\u00b2\/s. The same panel hung at 30 cm or at 70 cm delivers the same PPF and a completely different PPFD.<\/p>\n

That is where the two most expensive indoor mistakes come from. One: too much light too close, and the tips burn. Two: spreading a powerful panel over too large an area and ending up at 250 \u00b5mol\/m\u00b2\/s in flowering, which is vegetative light, not production light.<\/p>\n

Spread is never perfect. The centre of a tent always reads higher than the corners; on a well-designed bar fixture the honest gap is around 20\u201330 %. When a calculation says \u201c800 \u00b5mol\/m\u00b2\/s\u201d, read it as an average, not as the value at every point.<\/p>\n

DLI: a whole day of light in one number<\/h2>\n

PPFD is a photograph; DLI<\/strong> (daily light integral) is the film. It adds up all the light the crop receives in 24 hours and expresses it in mol\/m\u00b2\/day. It is the figure that best explains why two rooms with the same panel perform differently: one runs it 12 hours, the other 18.<\/p>\n

The sum<\/span><\/p>\n

DLI = PPFD \u00d7 light hours \u00d7 3,600 \u00f7 1,000,000<\/strong><\/p>\n

450 \u00b5mol\/m\u00b2\/s for 18 h \u2192 450 \u00d7 18 \u00d7 3,600 \u00f7 1,000,000 = 29.2 mol\/m\u00b2\/day<\/strong>. The same 450 \u00b5mol\/m\u00b2\/s at 12 h only reach 19.4.<\/p>\n<\/div>\n

This has a very practical consequence: in flowering, with the photoperiod fixed at 12 hours, the only way to raise DLI is to raise PPFD. In vegetative growth, with 18 hours available, you can hit a high DLI with noticeably less intensity and less stress on the plant.<\/p>\n

How much light each stage asks for<\/h2>\n

This is the summary we size installations with. The ranges are deliberately conservative: overshooting costs money on the bill and causes problems in the plant.<\/p>\n

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Light by growth stage<\/p>\n

Reference values<\/span><\/header>\n

\n
\n\n\n\n\n\n\n\n\n
Stage<\/th>\nPPFD<\/th>\nPhotoperiod<\/th>\nDLI<\/th>\nWhat for<\/th>\n<\/tr>\n<\/thead>\n
Cuttings and seedlings<\/td>\n100 \u2013 300<\/b><\/td>\n18 h<\/td>\n13.0<\/b><\/td>\nRooting and establishment without stressing the leaf<\/td>\n<\/tr>\n
Vegetative growth<\/td>\n300 \u2013 600<\/b><\/td>\n18 h<\/td>\n29.2<\/b><\/td>\nStructure, short internodes and broad leaf<\/td>\n<\/tr>\n
Flowering without CO\u2082<\/td>\n600 \u2013 900<\/b><\/td>\n12 h<\/td>\n32.4<\/b><\/td>\nProduction: the range that decides the harvest<\/td>\n<\/tr>\n
Flowering with CO\u2082<\/td>\n900 \u2013 1,200<\/b><\/td>\n12 h<\/td>\n43.2<\/b><\/td>\nOnly worth it with enrichment and controlled climate<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

PPFD in \u00b5mol\/m\u00b2\/s and DLI in mol\/m\u00b2\/day. Switch the photoperiod with the buttons to see how DLI moves at the same intensity.<\/p>\n<\/div>\n<\/div>\n

Above 900\u20131,000 \u00b5mol\/m\u00b2\/s the plant stops responding proportionally unless you add CO\u2082: you are paying for photons that turn into nothing. Without CO\u2082 control, 800 \u00b5mol\/m\u00b2\/s is a sensible ceiling.<\/p>\n

From \u00b5mol\/s to fixtures: the sum that actually decides the purchase<\/h2>\n

With a target PPFD and your square metres you already have everything. The sum is direct:<\/p>\n

The sum<\/span><\/p>\n

Required PPF (\u00b5mol\/s) = target PPFD \u00d7 area (m\u00b2)<\/strong><\/p>\n

A 1.2 \u00d7 1.2 m tent is 1.44 m\u00b2. Flowering at 800 \u00b5mol\/m\u00b2\/s: 800 \u00d7 1.44 = 1,152 \u00b5mol\/s<\/strong>. A 1,160 \u00b5mol\/s fixture is almost an exact fit.<\/p>\n<\/div>\n

Put your own measurements in here and we will tell you which fixture fits, what average PPFD you end up with and what the electricity will cost each month:<\/p>\n

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What each photon costs<\/h2>\n

There is one figure almost nobody publishes that sorts the market out in thirty seconds: how many euros 100 \u00b5mol\/s cost<\/strong>. Divide the price of the fixture by its PPF and you can compare a 240 W panel with a 1,000 W one without any sleight of hand.<\/p>\n

<\/div>\n

The other half of the cost is the bill. A fixture is not paid for once: it is paid for every month. With electricity prices moving the way they do, the sum is worth doing before and not after:<\/p>\n

<\/div>\n

Five mistakes we keep seeing<\/h2>\n