the highest u can aford. most place like homedepo or lowes don't cary anything higher than a 35 watt (150 watt eqv) daylight blub. an 42watt warm light bulbs. (daylight for veggie, warm light for flower) i whent to a lighting wherehouse, an got 4 65watt daylight bulbs for 20 bucks each. i all so have one 85 watt daylight, an 85 watt warm light. thats 85 true watts, not the reg incadisent evelivent. the reason most places like that above do not cary them is cuz they are pricy for a light bulb. cuz they have a self ballest. wich makes it possable to convent the ac power supply in to DC to run the light. make sure when buying a bulb it has a self ballist an is not just a replacement bulb. with no ballist, cuz then it won't work in your socket. u can all so go online to google, an type in 85 watt cfl bulbs, an there are a few sites that sell them for about 22-28 dollors + s&h