T5s are thinner (duh!!) and so are capable of producing a more intense light. They are invariably a modern quality design, whereas T8 and T12 shoplights are often older less effective designs. T5s are slightly more efficient than the best T8 and T12 lights. T5s are also readily available in "High Output" (overdriven) formats which are very useful for providing high intensity light for plants.

On the downside, T5s are slightly different sizes to T8 and T12 tubes, which can be a nuisance. They are also rather expensive, or very expensive in the US where it is difficult to find them except as specialist plant lights. The non-High Output format tubes are extremely efficient but designed in relatively low powers for their size (eg. 28W in the four foot length) and so produce no more light than fatter four foot tubes.

So they don't really have a downside as a plant light compared to T8 and T12 fluorescents, except that because of the initial cost you would only generally buy them if you cannot get sufficient light intensity from other fluorescents. For starting seeds you should be fine with your existing lights, unless they are a particularly old and inefficient design (eg. magnetic ballasts). Running very old fluorescent fittings is just pouring money down the drain when you can get something almost twice as efficient for ten or twenty dollars.

shrubs_n_bulbs,

How can one tell if my current lights have the magnetic ballast?

So it sounds like the only down side to the t5 lighting is the cost. It is a better and efficent light just a bit expensive. Do any of you folks have them? Any suggestions on a good brand. I noticed the Sun System brand seems the most expensive. My other issue will be getting then up here without breaking the bulbs.

Thanks for the help!
Phil in Alaska

How can one tell if my current lights have the magnetic ballast?

If it has a starter, that is a dead giveaway!

For comparison, high frequency electronic ballasts give out about 40% more light than the same power magnetic ballast. The high output T5s, as sold by Sun System (they are all the same with a different sticker on, and maybe a different reflector), are no more efficient than a good T8 or T12, the four foot tubes produce about 50% more light and use 50% more power. Standard T5s are about 10% more efficient than the others, the four foot version produces the same light as a four foot T8 or T12 but uses less electricity. Shorter tubes are always less efficient, you can get very good results using eight foot tubes if that suits your growing space and you can get the things home ;)

I just ordered one from eBay. I'll report in as soon as it gets here and gets plugged in!

watergal,
We're still waiting on that report :)

Sorry for the delay. I've got way too many things going on right now. I like it a lot so far. It is very, very bright (unfortunately I have misplaced my light meter). It also gets quite warm but not enough to worry about setting anything on fire. I have noticed better growth since I put it over my tropical waterlilies, although some of that may be due to the extra warmth of the water. The lilies go outdoors in June and then I will try it on some other types of plants.

There are 4 tubes in a reflector roughly 11 inches across (going from memory here, too tired to go measure). It puts out enough light to cover an 18x48 shelf very nicely. This particular vendor makes the fixtures a tad under 48" long so that they can fit under a 48" shelf without hanging over.

The eBay seller I bought it from used to be "dinosore777", now their name is "greatlights4less!" I thought the price was very good compared to the other systems available. The aluminum edges of the reflector are a little flimsy, but if you unpack with care it should be OK, and once it's hanging up, it doesn't really matter if the sides are strong as long as they aim the light where you need it.