Hi Jill! I was/am in a similar situation - I had a really old T12 2 bulb fixture. But I find that it works really well with 2 40W bulbs for phals and other low light growers, if mounted around 4 inches above the plants. Obviously things become problematic when they start spiking, but for growth, it's great, especially if you have a long growing area like a shelf or something.
T5s put out about 4-5000 lumens of light each bulb so you're looking at a whopping 20,000 lumens! But remember that light decreases by an inverse square as you move away from them, so you can simulate much lower light levels by just moving your lights away half a foot. I'm assuming you're going to hang these anyway, so the best control you could hope to get would be to adjust the height gradually and check the light levels falling on your leaves with your light meter. That way, you can make a completely smooth transition to the higher light conditions by decreasing the distance by an inch or so each week.
Where are you getting your T5s from?
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I use compact fluorescents and I find them cheaper and versatile, but I am definitely attracted to the high light output of those T5s!
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