The right amount of light

Posted by GrowDave 18/09/2018 1 Comment(s)

There is a very simple equation when growing


More light = more growth = higher yield


Making sure you are delivering the correct amount of light to the grow area at the right time is key. In this article we look at the best way to calculate how much light energy you will need.


Q1. What is the floor grow area in m² (1.2m x 1.2m = 1.44m²)


Q2. What grow rate do I want?

We use PAR to measure the light requirement of the plant. A low light plant would be 250-600 PAR. Medium would be 500-750. High, above 750. Lettuce would be around 350 PAR so your light requirement based on Q1 above would be 1.44m² x 350 PAR = 504 PPFD.



PAR is Photosynthetic Active Radiation. PAR is only the light which drives photosynthesis, it defines the quality of light to enable plant growth.


Example 2. Growing a high growth plant like tomato would be 900 PAR. So for tomato in our tent we would need 1.44m² x 900 PAR = 1296 ppfd

Example 3. Growing a super high growth plant where you have added COenabling more photosynthesis = 1.44m² x 1300 PAR = 1872 ppfd



PPFD is photosynthetic photon flux density. PPFD measures the amount of PAR that hits the plant and it is measured in micromoles per square meter per second (μmol/m2/s).



Q3. What light type will I use?

All types of light have a different efficency. We have tested many of them here but most are known and available online. All lights generate heat but it is the amount of heat they generate that causes the light efficiency to drop. LED’s are cooler than CFL’s so they must be more efficient as they don’t waste enerry on heat.

The other aspect of light efficiency is the reflective quality of the grow areas. If an area is very well lined with highly reflective material then more light will be reflected back to the grow area, increasing efficiency.


Here is a simple efficiency chart :

CFL + white 1.2
CFL + mylar 1.3
HPS + white 1.4

HPS + mylar

Double Ended HPS 1.6
CDMH 1.7
Blurple Box LED 1.9


To get a rough idea of how much light energy you need for the grow area you need to divide the PPFD number by the efficiency.

Example, earlier we calculated our high growth plant needed 1296 ppfd for our 1.44m² grow room. 

CFL + White has an efficieny of 1.2 so 1296/1.2 = 1080w.

CFL + mylar has an efficieny of 1.3 so 1296/1.3 = 996w.

HPS + white has an efficieny of 1.4 so 1296/1.4 = 925w.

HPS + mylar has an efficieny of 1.5 so 1296/1.5 = 864w.

Double Ended HPS has an efficieny of 1.6 so 1296/1.6 = 810w.

CDMH has an efficieny of 1.7 so 1296/1.7 = 762w.

Blurple Box LED + White has an efficieny of 1.9 so 1296/1.9 = 682w.

COB LED + White has an efficieny of 2.0 so 1296/2.0 = 648w.

Clearly the more efficient the light the less power needed. 


So how do I find out the PAR I need for my particular plant?

Search google for the PAR requirement of the plant you are growing and then apply it to the above calculations.




Tags: led cdm ppfd lumens

1 Comment(s)

Ofir oren:
22/10/2018, 06:24:56 PM

Looking for led blubs 700 nm E40

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