How much light is enough per plant in terms of watts. Will it be 300 watts?

A customer has a question and I hope we can get some opinions on it, thanks

How much light is enough per plant in terms of watts. Will it be 300 watts?

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Define light and watts, as something claiming to be a 300w led grow light won’t be 300w and 300w of other lighting such as cmh will have a different effect to 300w of CFL.

Someone needs to be more specific in his question.

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You should be looking at how many umol/s/m2 a light puts out instead of watts for a plant, as this is what a plant uses. You’re looking for high PPFD, and low wattage, this will indicate a good light.
HID are around 1.3 umol/J and up and good LEDs can go all the way up to 2.5 umol/J.

Depending on your grow space you will need around
10-15k (lumens) (~150-220µMol/s) Early Vegging
25-30k (lumens) (~350-350µMol/s) Late Vegging
50-60k (lumens) (~750-900µMol/s) Flowering


You can grow a plant tiny or you can grow a plant huge. You can’t really say watts, lumens, or PPFD per plant. You have to figure out these per square foot of growing area.

A simple rule of thumb is to try for 40-50 watts per square foot. With very efficient COB or strip LED lights, 40 is enough. With average lights, go for 50. And that’s real watts drawn from the wall, not some marketing “equivalent” hype.

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Not enough info. Yeh 300 wats hps is good. But how big is your soace? Is it HID or LED. The basic formula is like 25 - 50 wayys a square foot. With a 300 hid… I wouldn’t go bigger 6x6. 4x4 is ok. It still need more info

That’s a very good question. 6 plants in week 3 of flower under 300 wall watts of light.

50 watts per square foot is what I’m using


I’m not aware of any 300 watt hid, but a 250 watt hps bulb is considered proper for flowering 2x2. For a 3x3 it would be 400 or 600 watt hps or 315 lec.

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I’m sorry dude. I meant the CMH (ceramic metal halide) 315 watt at Htg supply. Do yourself a favor and give them your biz. They even sell 400wt kits for $40+ and you can use a freaking metal mail box from Lowe’s cut out the bottom and make a reflector . Just need a long ext cord to. Wire the socket, socket wire can be long as hell then plug male end into ballast wire the other end and a $3 male plug to the ballast for power. Was my first grow light paid <$70 for it all the ballast gets hot just aim a small fan at it. Or spend about $117 on one of their complete 250watt kits . Multi ballast. Will run either bulb. Just search the name and sign for a catalog the catalog is much easier to use. I’ve dealt with this guy since he was on ebay. Super nice. Will make you happy

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Thanks for the tip, but I build my own led lights. I appreciate that you’re going the extra mile to save some cash though. I think you’ll be pretty happy with the 315.

I know nothing about LEDs except they are cooler and work well. But uber expensive. I cool my HIDs pushing a/c . air through the 8" duct. I a/c a 7x8 closet with a window unit. Push that air throigh all four 1000wt lights I’m the chain with 2 other 8" fans I’m the chain and exhaust in the attic. You can literally pick up the lamp it’s so cool. And I can get them 8 to 12" from the canopy . Before this. Heat was a night mare. But the central ac along with this keeps it bewtween 75 and 78. But these LEDs are making me think. I can’t do anything about the veg and grow rooms. But I may get some to keep mothers full of shoots. You say you build em? I’ve seen like 4 foot ones for $1000!!! Holy God. All 8 multilamp ballasts with bulds cost $2k so are you telling me I can flower a 10x10 room with enough light to penetrate with LEDs? The HIDs will pack the room wall to wall in what looks like one big monster can and football size buds.

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Sure, you can build them as big as you’d like. Or at least as big as your electrical circuits will handle. Or multiple lights too, whatever works best for your situation.

I definitely build them though, have helped several others here build lights too. Depending on what you need or like, we can make them fairly simple too. I would say most worry about the electrical end, but I can usually make that really easy. Having some fabrication skills will go a lot further than electrical skills anyway.

Not exactly cheap, but in terms of performance per dollar I think we do pretty good.


So say I wanna enough light to flower a 10x10 room packed like I can get with HPS
What kind of investment is this ? My grow supplier wants $3000 for one 4 foot LED. Holy shit.

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That sounds crazy. A 10 by 10 room is 100 square feet. Just as an example, RapidLED has a 5’ by 5’ easy kit for $1089. You would need four of these. That’s $4356 plus sales tax and shipping. I’ve bought stuff from them and the shipping is very reasonable. Those put out about 900 watts each, so the room is getting about 36 watts per square foot. But these are Vero29 COBs so they are in the 160 lumens/watt range.

HPS would be cheaper to build, but only does about 130 lumens per watt, and the spectrum is horrible. I would go with CMH lights if you want to go that route. But they are not cheap either.

How would things come in buying the parts and getting help to build it yourself, there should be a fair bulk discount there, and the chances of not knowing someone who knows how to do the electrical “in black” is going to be slim.

The other thing to consider is the lifetime of these LEDs versus the costs of replacing these HID bulbs regularly as that will likely be an annual event for some, we’re looking at the extra ventilation and costs there, ballast breakdown, and so on. Long term I reckon a custom strip build would end up cheaper.

In terms of Mean Time Before Failure, any HID bulb made is only going to last a year or two. Magnetic ballasts will last a long time. Electronic ballasts will not. They all have big electrolytic capacitors in them and may only last five years or so.

With LEDs, the light emitters will last 10 or more years if you don’t drive them at max and keep them cool. But the drivers won’t last. Electrolytic capacitors again, I would plan on replacement every five years or so. They are usually the reason most modern electronic equipment fails. That’s actually one of the advantages of a DIY driver: Capacitors are easy to replace and pretty cheap, in contrast to throwing away the whole driver and buying another. The other parts in the driver last for decades.


So you take the price of all these HID lamps needed for 100 sq ft, there’s, what, say half a grand a year plus labour replacing bulbs alone.

Even allowing for only 5 years on a LED setup, there’s a potential 2,500 in costs that has to be considered in planning. Kinda puts a new aspect on the initial outlay when you consider these costs plus extra ventilation and cooling costs and, obviously, the extra electricity costs, doesn’t it.

As said, it’s not as simple as dropping seeds and selling the result, there has to be some serious long term planning done before anything is done

Anything bigger than a hobby grow needs a real business plan, or you WILL go broke. You have to account for everything, or you will discover at the end that you have been working for $1.50 an hour! You also need some security to prevent crooks from coming and stealing everything, and possibly hurting or killing you and yours. Security is expensive!

I had a friend when I was young (many decades ago). He smuggled weed from Tijuana and operated from a ranch in the back country. He would make a decent amount of money, but the fourth time people showed up at the ranch with guns to take everything, he decided he would do better with a fast food job!

The nice thing about a hobby grow for personal use, is you can keep it completely secret even if it’s legal. Then nobody knows to come rip you off. Robert’s grow bible says the first rule of growing, is to tell nobody. (Download it, if you haven’t: It’s free!)


I didn’t see your response sorry. Whatever your supplier has is not where you wanna be lol.

You have a lot of options in a 10x10. As pointed out you could certainly go with some kits that are designed for a pre determine space. Rapid led, timber grow lights, and horticulture lighting group all offer these much cheaper than what your supplier has, and these lights are probably better. If you can post what light he quoted that price on, I’d be happy to try and chase down the specs. And the timber and hlg lights can be purchased assembled and ready to use.

If you want to build lights, it would make sense to build them around how you use the room. I’m guessing a room that big has at least one area designated as a walkway or at least doesn’t have plants in it. No sense in life lighting that space if we don’t need to. So if you wanna go that route, could you post a pic or draw up a sketch of the how the room is used?

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Good point. My last grow had a 40 inch thick SCROG net and it was very frustrating because I’m not a goon with a 40 inch reach! Next time, I’m going to cut it down to 30 inches thick so I can reach everything.

With a 10 x 10, you want to have several rows. Keep the rows against the walls about 30 inches thick, but rows you can reach from both sides could be thicker. I had a good experience with a trellis outside. I trained the plant to grow about 8 feet wide but not very thick.

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My LED light draws 300 watts from the wall will it work in a 4x2 room