Depending on the light output and amount of time needed, contemporary fluorescent grow lights can consume between 40 and 432 watts a month. The cost is approximately £1.90 to £21 each month with constant use, at an average rate of ten cents per kilowatt hour.
Efficacy of Point vs. Linear Sources
Fluorescent grow lights are either compact fluorescent (CFL) point sources or tubular 5/8-inch-diameter high output (T5HO) lights, linear sources that can be grouped into an array of tubes to form a plane.
40w CFL = 2,500 lumens of light output = 63 lumens per watt.
125w CFL = 8,500 lumens = 68 lumens per watt.
24wT5HO = 80 lumens = 80 lumens per watt.
54w T5HO = 5,000 lumens = 93 lumens per watt.
Generally the larger the light source, the more efficacious it is.
Impact of Ballasts on Electricity Use
Compact fluorescents include an integral ballast, so their wattage is equal to their label--a 40w CFL consumes 40w. T5HOs use a separate ballast which may be rated lower or higher than 100 per cent to reduce energy consumption or boost light output. These ballasts generally alter the amount of electricity consumed by the fluorescent tube in a range of plus or minus 10 per cent.
Quality fluorescents with electronic ballasts are not only 25 per cent more efficient than old style magnetic ballasts, they also don't hum, have an indiscernable flicker and prolong lamp life.
Impact of Reflectors
Using grow lights with reflectors helps focus the available light toward the plants. In some cases this can triple the effective light at the growing surface.