I was planning on building a greenhouse 10x12x8 ( SF bay area) for this up coming winter. I know during winter time, ill have about 11 hours of light. I was thinking of supplementing the green house with LED lights to keep my plants in veg for 6 hours. What is the minimum wattage I need to keep my plants in Veg? does it even matter? I want to use the least amount of light, I just need my plants to stay in veg and not flower. Thanks for any help.
Philips have bulbs specifically for the task. Less than 100 watts would easy cover that area, believe it or not. GreenPower LED flowering lamp - Philips Lighting
You can keep your plants from flowering during the winter by just turning the lights on for a few minutes during the dark period. Download and read the attachment for details.