Indoor Lighting - The Case For Inexpensive T-8 Fluorescent Fixtures

We'll get straight to the meat; It's easier, more cost effective, and flat out better to use fluorescent lighting for indoor bonsai than to use metal halide or high pressure sodium.

Before large-scale growers string me up, I'm not talking about commercial-scale greenhouses that need supplimental lighting during winter. I'm talking about small to large collections of indoor trees overwintered in basements, spare rooms, or modest-size greenhouses.

The four criteria to be addressed are;

Initial Cost:
Using my local Lowes and the 2008 Retail Catalog from Hydrofarm as pricing sources for pricing information, we'll use an apples-to-apples setup of lighting a 5'x5' (primary lighting) space. This area can be effectively covered with one 400 watt MH/HPS fixture or four 2-bulb T-8 fixtures (32 watts per bulb, electronic balast). We'll look at using a very inexpensive parabolic reflector for the MH/HPS bulb rather than a significantly more expensive air-coolable one.

MH/HPS
1ea Xtrasun™ Parabolic 42" reflector: $80
1ea Xtrasun™ 120V Halide 400W Ballast: $130
1ea GE/Philips/etc. 400W Halide Universal Bulb: $30 (36000 lumens)
... Total Initial Cost: $240.

Fluorescent
4ea 4', 2-bulb, T-8, electronic balast fixtures: $18/ea $72
4ea two-pack GE daylight bulbs: $7/ea $28 (2800 lumens each)
4ea two-pack GE warm bulbs: $7/ea $28 (2650 lumens each)
... Total Initial Cost: $128 (total lumens, 21800)

Round 1 goes to T-8 fluorescents with a savings of $112 or 47%. To further clinch this, MH/HPS is $0.007/lumen while fluorescent is $0.006/lumen.

Recurring Cost:
For this, we'll stick in a ballpark figure of $0.10/KWh for electric costs. You can adjust for your local provider as needed. We'll figure on running lights for 16 hours per day and use 30 days as our duration.

MH/HPS
400W / 1000 (KW) * 16 (hours) * 30 (days) * 0.10 (cost)
$19.20 per month.

Fluorescent
8 (bulbs) * 32W / 1000 (KW) * 16 (hours) * 30 (days) * 0.10 (cost)
$12.29 per month.

Is this better? Sure. Fluorescent wins. Is this savings significant? I'd say it depends on your personal finances. If you figure in maintenance costs, you still come out ahead. If you run the lights for 6 months/year (2400 hrs) for 2.5 years, you'll be at 1/4 of their rated 20,000 hr lifespan - approximately when any notable reduction in light intensity due to the breakdown of the coating of the tube - you'll go through two sets of fluorescent tubes in the same time as one MH/HPS bulb. This adds $56 to the total operational costs, leaving you still $56 ahead - enough for 3 extra fixtures if they happen to break (ballasts are more expensive than just replacing the entire fixture).

Light Intensity:
This is where the entire case for fluorescents can be made. MH/HPS lights are hot. Hot, hot, hot. The closest you can safely get them to the foliage is *maybe* 15", assuming you're using decent fans. 18" is a bit more realistic. Conversely, you can crank fluorescents down to within 3" of the foliage and not have any problems. They run cool. A small room fan you'd use anyway to prevent mites, powdery mildew, etc. is plenty sufficient to remove any residual heat.

This height difference is critical. The strength of light is a function of the square of the distance. Lumens are measured as the amount of light that falls on one square foot at a distance of one foot from the bulb. First off, since fluorescent lights are long, rather than spot, they offer a good, even amount of light along their length - except for the very ends of the tubes. MH/HPS are spot-source so the distance increases in the shape of a cone, with a longer length at the edges and a shorter distance directly below.

Using the square of distance, we can calculate the amount of light provided as follows:
Metal Halide, 400W bulb
36000 (rated lumens) / ( 18 (distance from light to foliage) / 12 (inches) )
24000 lumens to directly below the bulb.

Flurescent, 8 @ 32W bulb
21800 (total lumens) / ( 3 (distance from light to foliage) / 12 (inches) )
87200 lumens to directly below the bulbs.

This is 3.6 times (360%) more light - at less cost to purchase and operate! Even if you went with a 1000W MH bulb rated at 110,000 lumens and placed this at 18" above the foliage, you still would be shy by almost 14000 lumens. Not only would you still not have the same amount of light, but you'd be paying dearly to provide it. An extra $25 for the bulb, an extra $70 for the ballast, and an extra $28.80 per month for electricity. During a six-month lighting season, you could purchase a second set of four fluorescent fixtures and still have a spare $45 in your pocket - just on the electricity savings alone.

Light Color:
The fluorescent "daylight" bulbs are 6500degK and the "sunshine" bulbs are 5000degK color temperature. 6500degK is good for vegitative growth and 5000degK is good for blossoming. Mixing these by alternating bulbs within each fixture is a very good way to get an excellent spectrum for just about any tropical or coolhouse species you have. By contrast, most metal halides are between 3000degK and 4500degK, very much on flowering side. High pressure sodiums are completely incorrect for vegitative growth, operating at a color temperature between 2000degK and 3000degK.

Conclusion:
On every count, T-8 fluorescents beat out metal halide - initial cost, operational cost, amount of light, and color temperature suitable for vegitative growth and blossoming. Additionally, you can lay them out in a more flexible manner for shelving or other reasonble use of space. Metal halide bulbs have a place in agriculture, to be sure. That place just doesn't happen to be in most bonsai grower's tropical/indoor setups.

Notes:
We've seen some articles promoting the removal and relocation of the fluroescent ballasts for remote installation. DO NOT DO THIS!!!! THIS IS HIGHLY DANGEROUS!!!!. Why is this dangerous? It depends on the type of ballast/starter in your fixture.

If you're dealing with an older magnetic "rapid start" ballast, there is no voltage spike, but the design incorporates filament power windings which rapidly and continually warm the filaments/cathodes in the tubes. Since no inductive voltage spike is used, the lamps must me mounted near a grounded reflector so the arc discharge is properly initiated. If the ground is near the bulbs and the reflector but the ballast is a distance away ..and something or someone contacts the circuit between those two points, they become the ground. That's bad unless you're trying to light them up instead of the lamps.

If you've got an electronic ballast, it's a little more complex, but the operating properties of the fluorescent bulb remain. To strike an arc in the bulb and light it, the voltage must increase significanly (typically into the thousands of volts). Electric wire has a resistance. The smaller the diameter of the wire, the higher the resistance. This resistance causes a voltage drop at the far end (the bulbs). The ballast still must strike an arc, so it increases the voltage to overcome the voltage drop. Resistance causes wires to heat. Putting significantly more voltage over a distance of more than inches casues significant heating of the wire. Most wire available to the consumer is not rated to handle more than 600V. At that voltage, the distance is assumed to be very, very short (inches). The wire inside the fixture is made to handle higher voltages and provide less resistance for this initial starting voltage supplied by the ballast. So why is this dangerous? The threat of fire from overheated wires and melted insulation.

Please be safe - avoid risk of deadly electric shocks and/or fire. Do not try and remotely mount the ballasts for fluorescent fixtures. If that's not enough, then consider your insurance carrier probably won't cover your claim since you tampered with a device, made an unsafe modification, and you're not a licensed electrician. We'd just prefer you to be on the planet a little longer.