The Ultimate Guide to Help You Choose the Correct LED Wattage for Your Grow Space (during flowering)

*Article updated Jan 15, 2017

Are you new to the world of LED growing? If so, the charts below will help you determine how much energy (by wattage) your cannabis plants require for flowering.

Disclaimer: Wattage is a relic of the HID (MH and HPS) days. Wattage should not be used to determine the coverage area a fixture or the usefulness of an LED grow light. Wattage should only be used as a simple method for approximating the correct fixture size for your grow. 

PPFD (umol/j/m2) and DLI is the correct metric for measuring useful light intensity over a given area. Coupled with the correct spectrum for your plant type and phase of growth, PPFD (and spectrum) are the keys to a successful grow. However, for the purpose of estimation, we will report wattage calculations in this article.

Grow variables

It is important to understand that the number of actual watts that you need for your grow will vary depending on:

    • Plant type (high light or low light)
      • High light plants - i.e. cannabis, tomatoes, peppers, etc
      • Low light plants - i.e. herbs and leafy greens (such as basil lettuce)
    • Growth phase
      • Seedlings, vegetative, and flowering plants all require a different amount of light
    • Total area of your grow
      • A larger grow area will require more LED watts

Watts per square foot

    The average LED grow light draws about 32 watts to cover 1 sq ft for flowering cannabis. Compare this to a typical HID fixture which rings in at 40 watts per square foot (assuming a  5’ x 5’ area, or 25 sq ft coverage area). Using an HID in a 4’ x 4’ will bring the power coverage up to 62.5 watts per sq ft. (The energy saving that growers can expect from using LEDs instead of HID is around 38%. This energy savings only takes into account to savings growers gain when using LEDs fixtures. It does not account for other environmental factors where growers can save energy. For example, less ventilation and air conditioning may be used, or foregone completely, depending on the size of the setup and ambient temperatures. Total power savings may rise above 50% in certain scenarios.)

    LED Wattage Chart Explanation

    The wattages listed below assume you are flowering your plants. If vegging, you might consider reducing your wattage by 50% since vegging plants only require about half the light intensity. As always, it is best practice to follow the manufacturer's recommendations.

    Using the charts

    In each chart you will find the following columns:

        • '# of Plants' to be grown
        • Square footage that the respective number of plants cover
        • Recommended LED wattage for the number of plants grown in the respective space

    Remember, you can scale these values up if you are growing more than the number of listed plants or if your grow space is larger than we suggest below. Just be sure to use 32 watts/sq ft as a baseline for determining the fixture wattage and quantity.

    Note: The listed wattages are actual LED wattages, not advertised wattages. See our discussion on the difference between advertised and actual LED wattages (second heading on page).

    Cannabis wattage chart

    Assume about 32 watts/sq ft (actual LED wattage) for flowering

    Quick Chart

    1' x 1': 32 watts

    2' x 2': 128 watts

    3' x 3': 288 watts

    4' x 4': 512 watts

    5' x 5': 800 watts

    6' x 6': 1152 watts

     

    Assuming 1.0 sq ft/plant

     # of Plants   Square Feet   Recommended LED Wattage
    1 1 32
    2 2 64
    4 4 128
    6 6 192
    8 8 256
    10 10 320

     

    Assuming 2.0 sq ft/plant

     # of Plants   Square Feet   Recommended LED Wattage
    1 2 64
    2 4 128
    4 8 256
    6 12 384
    8 16 512
    10 20 640

     

    Assuming 4.0 sq ft/plant

     # of Plants   Square Feet   Recommended LED Wattage
    1 4 128
    2 8 256
    4 16 512
    6 24 768
    8 32 1024
    10 40 1280

     

     

    Shop LED Grow Lights for Cannabis ›

      

    Are you a beginner grower?

    You might also be interested in the following books (click to buy):

      

       

    Related: Why LED watts do not really matter to a plant and why PPFD and DLI do

    Related: See what else you should consider before purchasing a LED grow light

    Disclaimer: All LED grow light 'Descriptions' and 'Specs' stated by LED Grow Lights Depot are the claims of the original manufacturer and/or supplier and not LED Grow Lights Depot. We do our best to ensure the information is accurate and correct.

    Notes: Please be aware that the cultivation of certain plants may or may not be legal within your own specific region or country. LED Grow Lights Depot does not encourage or condone any illegal activity and advises each individual / user to inform themselves of the relevant laws within their own region/country.

    The book links are affiliate links. Therefore, we make a small commission if you decide to purchase these products from Amazon.