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PPFD, DLI and distance, in plain English

Listings throw around PPFD, PPF, DLI, lux and "equivalent watts". For a shelf in a flat you need two of those ideas, and you need to know which numbers are measurements and which are slogans.

PPFD is the light at the leaf

PPFD (photosynthetic photon flux density) is how much plant-useful light arrives on a square metre in one second. The unit is micromoles per square metre per second (µmol/m²/s). It is a reading at a place, not a property of the box. The same lamp reads higher 15 cm under the middle than it does 40 cm out at the corner.

PPF, without the D, is the total output of the fitting in micromoles per second. A higher PPF means the lamp emits more. It does not tell you what one leaf receives until you know the distance and the spread.

Lux and lumens describe light as human eyes see it. They are a poor way to compare a horticultural lamp with a desk bulb. "300 W equivalent" on a 15 W LED is a marketing comparison with an old lamp, not the power you pay for. You pay for the watts on the electrical rating.

DLI is the total for the day

Daily light integral (DLI) adds the PPFD up over the hours the light is on. In mol per square metre per day:

DLI = PPFD × hours × 3,600 / 1,000,000

A worked example, which is arithmetic rather than a target: 100 µmol/m²/s for 12 hours is 100 × 12 × 3,600 / 1,000,000 = 4.32 mol/m²/day. Double the hours, or double the PPFD, and the daily total doubles. A brighter lamp for fewer hours can land on the same daily total as a dimmer lamp left on longer. The plant still needs a night, which is covered in hours and a dark period.

Distance is where people burn leaves or stretch them

Light drops quickly as you move the lamp away. A chart that says "30 cm" is for that model at that setting. It is not a universal houseplant rule.

Maker charts we checked on 13 November 2026 already disagree with each other, which is the point:

We haven't tested this kit. Specifications are as the maker states them (checked 13 November 2026).

• The SANSI 15 W E27 bulb listing says to mount it 18–24 inches above the plants (about 45–60 cm) and not to put it in an enclosed shade.
• The SANSI 40 W E27 bulb listing says at least 30 cm.

A slim bar is often used closer than a bulb because it runs cooler, but "often" is not a measurement. Read the sheet in the box.

The RHS artificial lighting guide describes a simple home setup of two 60 cm T5 tubes about 60 cm above a small group of plants. That is a fluorescent example, not an instruction to hang every LED at 60 cm. It does show that a reputable UK source expects you to place the lamp deliberately, not rest it on the foliage.

What this site will and will not tell you

The shelf tool gives an indicative distance band for a format (bulb, bar, panel, gooseneck). The band is labelled indicative because:

• it is not a reading from your shelf
• a phone app is not a lab PAR meter
• two lamps with the same wattage can throw very different PPFD

If leaves bleach or curl and the compost is not bone dry, the lamp is probably too close. If new growth is long and pale, it is probably too far, or on for too short a day. Move in small steps and give the plant a week.

For the money side of leaving it on, see running cost.