There are two ways to change the intensity at the canopy. The first, and traditional, method is to vary the height of the light above the canopy. The figures
@SmokeSara gave above are for this method.
The second method is to keep the light at a constant distance above the canopy throughout the grow, and adjust the intensity of the light itself. This is becoming more popular since you can realize substantial power savings. A light meter is used to set the intensity at the canopy. You can buy one for about $30 USD through the big on-line malls, or you can get an app for your cell phone. If you want to use power, you can do that, but first you need to measure how much is required for each stage of growth. The figures are usually given as PAR values, but with white light such as the Mars Hydro SP and TS series of lights emit, you can use Lux. The conversion is 1 PAR ≈ 67 Lux. Here are the values for the stages of growth:
Code:
Stage: PAR Lux
Seedlings 150 = 200 10,000 - 13,500
veg: 200 - 500 13,500 - 33,500 increase level over 3 weeks
Flip 450 - 800 30,000 - 54,000 increase level over 2 weeks
flower 750 - 950 50,000 - 63,500 increase level so max at 1/2 flowering time
Note: As these measurements are taken at the canopy, they are not dependent on the height of the light.