The magic number is 1-2.
For general area lighting you will need at least 1 watt of lighting per square foot. This will be adequate light for cleaning and most tasks.
For more detailed work, 2 watts per square foot would be appropriate. Ceiling height also has to be taken into consideration, as does the reflectance of the interior surfaces.
Determining light levels, based on foot candles, typically is done by a computer program, operated by someone with experience in lighting requirements and specific lamp/fixture performance. Not something accurately done by an amateur. Frequently even the "experts" screw it up.
My recommendation is to plan for 2W in the area of the workbench, where you may be reading manuals or working on small parts such as carburetor jets, with 1W throughout the remainder of the space. Use supplemental portable lights if you require additional light in the remainder of the garage.
Plan for lighting patterns to overlap, minimizing shadows. 4 foot tubes are much easier to handle than 8 footers. Decide early on how important color rendition is for you. Some fluorescent fixture lamps really don't work for color sensitive projects.
Lighting costs, both to purchase as well as to operate. Correct lighting comes from proper planning, not simply adding as many fixtures as you can. Take your garage plan to a lighting rep or consultant and get a professional opinion. Typically they are happy to provide their input, no charge, especially if you are shopping with them.
American made products seem to have better ballasts and lamps will last and perform better.
Offered only as opinion
