Number of Lamps x Lamp Rating in watts (w)
Eg. 8 Uplighters fitted with an 80w bulb and 2 x 5 arm chandeliers fitted with 60w bulbs:

Uplighters 8 (lamps) x 80 (watts) = 640watts
Chandeliers 10 (lamps) x 60 (watts) = 600watts

640watts + 600watts =1240watts. In this example you could either use one of the 2500 watt dimmers or two of the 750 watt dimmers./ (One for the chandeliers, one for the Uplighters)