A 40x40 is the smallest growing size there is: under a fifth of a square metre, usually one plant. The range matches it, because little is made specifically for this size. That is not a problem as long as you choose something you can turn down.
The ViparSpectra P600
At this size there is the ViparSpectra P600 at 95 watts with a dimming knob. It is rated small enough for a 40x40 tent and usable up to around 60x60, and that second figure is the interesting one: you are buying a lamp that still works at the next size up. Move to a bigger tent and it comes with you rather than being replaced.
Dimming is the whole trick
Because nearly anything you can hang at this size is on the generous side, a dimmer matters more here than wattage. You set it low for the first weeks and bring it up once the plant can take it, and it never has to run at full power. A lamp without that forces you to hang it higher, and in a tent 40 cm across height is exactly what you do not have. More on turning lamps down is under Dimmers & Controllers.
Heat is the limit here, not light
In a volume this small the air warms quickly and the lamp sits close to the crop whatever you do. So you run into temperature before you run into too little light, which is the reason not to buy heavier than you need. Extraction for a small tent is under Climate & Ventilation.
For cuttings this is too strong
Ninety-five watts above a propagator dries your cuttings out before they have roots. For the first weeks hang the cool fluorescent from Fluorescent just above the tray. This size is meant for finishing one plant.
Frequently asked questions
Why is there only one lamp here?
Because almost nothing is built for 40x40. Most panels start at 60x60, and you only hang one of those here if you can turn it well down.
Can I use a lamp meant for 60x60?
You can, provided it dims and you have the height to hang it further away. The choice at that size is under For 60x60.
How many plants fit in here?
One. On under a fifth of a square metre, two plants are mainly two plants taking light from each other.
