I'm starting my tomatoes under 10 Cree LED's mounted to a heatsink, with water-cooling on the heatsink running through a copper pipe in the water beneath the starts, so they get both light and heat. They're doing fine so far, is the best I can say: growing faster than last year's starts, which were getting full sun but no heat, whereas these are getting very indirect sun but 14 hours of LED per day. Because I'm a geek I have an arduino measuring/recording the temp of the LED heatsink, the water beneath the plants, and checking a realtime clock to decide when to turn things on and off, with control over the LED dimming (for overtemp) and pump speed (likewise). The Crees are cool white, putting out about 50 lumens each, which is a meaningless measure for growlights, but useful for comparison, I guess. Tomato starts at the edge of the array are growing much more slowly than the ones right under the lights (the LED's are packed all in one small array) so since the whole works is at the same temp, I presume that means the ones in the center are strongly benefitting from the LED light. If I did it again, I'd make an array that was planar rather than packed in at one point, for growing in a downstairs space with only small windows, but I'd stick with the single block of LED's for what I typically do, with them sitting in a south-facing window upstairs.