Exhaust fans are needed to ensure that your plants get enough fresh air, proper humidity and temperature, while also helping with the smell.
There are several ways of calculating the optimal power of a fan.
One is to calculate the volume of your grow area and multiple it by 20-40 - the air needs to be exchanged every five minutes.
For a one square meter box, 2 m high, that means 2 cubic meters, thus at least 40-80 m3/h exhaust fan will be an absolute minimum.
Another school says that to be safe, 1 m3/h for each Watt of HID (MH, HPS) lamps is required.
If you use a single 400W lamp, you should look for a fan with 400 m3/h.
Some growers bind the exhaust fan to the lighting controller, operating them in tandem.
It is best, however, to ensure some ventilation also during the "night".
One solution would be to use two-speed fans.
Another possibility is using a fan with temperature sensor, which turns it off automatically once the temperature in the box drops and on again when it rises.
If stronger fans are needed, or in the stealth setup, a fan's noise could be a nuisance.
This could be avoided by using silenced fans, with little or no compromise in terms of their quality.