The heat load is distributed between the ballast and the bulb. The heat
from the ballast can be used as bottom heat or could at least be
dissipated away from the plants. The heat load from the bulb is only
partly radiated heat. Much of it is convected heat.
Most of the reflector hoods for halide bulbs have vents around the bulb
to dissipate the convected heat without it getting to the plants. These
are baffled with reflected material so that little light exits via that
path.
The 1000 watt halide bulb is not necessarily more efficient than the 400
watt bulb. It varies with brand and technology. Generally LPS bulbs are
the most efficient at all wattages but are difficult to find above 175
watts and have a narrow yellowish sprectrum. HPS are next and are
available from 175 watt up to 1000 watt with similar efficiencies and
lifetimes but are biased toward the long end of the spectrum favoring
flowering over vegetative growth.
Next are the enhanced halides, which will have "agro" somewhere in their
names. They are more neutral and less biased towards the short end of the
spectrum. They are somewhat more expensive than basic halides.
Among the basic halides, the normal recommended lifetime for the 1000
watt bulb is half that of the 400 watt bulb. The better spatial
distribution of light from two 400 watt bulbs and the energy savings
makes two 400's a slightly better deal than one 1000 for a hobby lighting
area.
There is little evidence that the efficiency of the HPS or MH bulbs
decline much with age. The spectrum does change as does the power
consumption due to some of the radiant elements diffusing out through the
bulb envelope. It may be that the used bulbs are better for some purposes
even though their total luminance is lower.
Bob Cruder - bcruder@miaco.com