I grow under lights too (t5's). If your plants get natural light through out the day then you can use the artificial light for only a few hours as supplement lighting but if your plants get hardly any artificial light you can use the t5 all day as your main lighting. I leave my lights on 16 hours a day during the summer, 14 hours for fall/spring and 12 for winter. You will hear a lot of people say that plants need to be 6" or closer to the lights but i have find that when using t5's, this is not the case. I put my vanda about 4" away from the light any closer than that and it starts yellowing, dendrobiums 6", catts/phrags 6-8", Oncidiums 8-10", phals/paphs/AV's and any other low light plants 10-14". I have tried putting alot of this plants closer to the lights but alot of them turn yellow or purple.