Each 8' 75W bulb puts out between 4K to 5K Lumens each when new. This will degrade over time. So you would need one 4' fixture putting out 4K lumens each to replace each 8' bulb you have now to get approximately the same amount of light (if all the 8' bulbs came on). The long range cost is MUCH lower given the reduced energy use and the replacement costs of the fluorescent bulbs (not to mention of the pain in the patootie of getting 8' bulbs home - why I ditched mine).