High output - usually used for describing a class of smaller fluorescent tubes (t5 HO) which can take a higher power input than others.

T5, T8 refers to the diameter of the tube. A T5 tube is 5/8" in diameter, while a T8 tube is 1" in diameter.

The easy way to figure out the number of fixtures is as follows
for example
30 lumens per sq ft , area of garage - 24 * 24 = 576. Total lumens required is 576 * 30 = 17280

Assuming a standard T8 output of ~ 3000 lumens per tube and each fixture has 2, you get 2.88 or 3 fixtures (rounding up).

T5 HO will have a different figure ~ 5000 lumens per tube.
Assuming 2 tubes per fixture, total fixtures - 2.

Here is a link that might be useful: description

T5HO lights are typically used as grow lights or where the lights are placed high (frequently mentioned figure of 20') as they can be intense.
You may want to see the lights in action first.

An alternative could be the Cree CR24 LED.

I think we're going to stick with three of the t8 fixtures for now.

It seems like the HO T5's are geared more for high bay (high ceiling) garages or industrial use (factories and such) although on the Lithonia website it does say they can be used in the home for closets which kind of went against all that I've read about them on other sites. That struck me as odd.

Knowing the total lumens we should have is very helpful and I appreciate your feedback. Thank you!

Would love to go with LED's but the LED fluorescent replacements are just too expensive still. (Although we have replaced a few bulbs in the house with LED's and they are just wonderful.)

T8 will serve you well in a the garage. Get program start ballasts if you will switch the lights frequently. If you turn them on and leave them on for hours, instant start is fine.