Actual wattage led grow light for 40 x 40 x 120 cm grow tent

That is a very small tent. I'm getting 15.7" x 15.7" x 47"?

It's not even two square feet. I use a 60 watt board based led in my veg area and the rated bloom coverage for it is 2 square feet. So, that 60 watt light is slightly overrated for your area. If you're using a 300w it's likely not as efficient as the 60 watt one that I have but still slightly overpowered.

If you plan to grow start to finish in that same space you'll need a less powerful light to get the plant started and hearty and then put that more powerful one in there once it gets enough size to not be cooked by it.

It would be more ideal to have a dimmable light for that space and one more around 50 watts or so if it's higher end efficient leds.

30-40 watts per square foot is rule of thumb for higher end leds. 40-50 for some cheaper models so you're right in the range but only for bloom.

I never veg at bloom light level. I veg with less then half if it's the same light type. If you use a CFL bulb you could probably veg with an 85 watt or so version and still have half the light levels or less.

 

Is your exhaust fan piped to the outside so that it pumps the hot grow room air outside and not in a circle in the room? If the answer is no that is part of your problem. The exhaust fan needs to dump outside.

 

I would get a larger tent. It's too small to have a decent crop in and get good light distance. The height of less then 4 feet and requiring at least 12" for a pot and 12-18" light distance along with the light hanging at least 4" or so from the ceiling. Add all that together and that's how much room you have for plant. I get 34" leaving room for a 13" tall plant.

3x3x6 is the minimum size I would run for veg and bloom. I would run two. It's double the harvests a year almost.

 

What you want is something with a dimmer so you can dim it down to the perfect level for each stage or to manage your temps. It's always a balance with lights. The highest you can run them and still maintain good temps in the garden is as hard as you should run them. You should never sacrifice the environment at the expense of running the lights harder.

 

Can you use a batheroom exhaust fan duct or over vent or dryer vent duct? You can also build a window vent in such a way that you can't tell it's a window vent from the outside and connect it to a duct. There's a good build of one 3 minutes in to this video. I use a window vent I made much more simple because I don't have to hide it where I am.

 

This board.
QB288 V1 & Slate 2 Single Combo

This driver and you'll have full dimming capability.
https://www.onlinecomponents.com/me...VUSCtBh0n4wBgEAAYAiAAEgKL5fD_BwE&gclsrc=aw.ds

 

Or one of these.
www.amazon.com/Quantum-Board-Grow-Bloom-4000K/dp/B076QDKVDZ/ref=sr_1_1?keywords=hlg+65&qid=1562615720&s=hi&sr=1-1-catcorr

Add this dimmer.
www.amazon.com/LPD-Dimmer-Inline-Switch-Ribbon/dp/B071NRKNJH/ref=sr_1_5?keywords=inline+led+dimmer&qid=1562615765&s=hi&sr=1-5

 

Adding a dimmer to an LED is best done on the driver output and with a commercially made light that doesn't already include a dimmer it's tricky to add one.

 

Can you link what light you have?

 

Those are garbage. Blurple LEDs are the past.