we are building a shop building , 40feet X 100 feet X20foot ceioings. How many watts of bay lighting will we need? Also what type of lighting would you recommend?

The watts per square foot is to size the electric system if the actual load is not known.

Better to know what the task is, what type of work will be done, and determine the amount of footcandles of illumination needed. If a warehouse , you may only need 10-30 FC. If assembly or fine work will be done, may need 50-80FC.

With this known then the amount of the appropriate light fixture can be chosen.

I will most likely recommend fluorescent high bay.

Light output is not measured in Watts but in Lumen. The rate of Watts to Lumen depends on the type of lamp and construction of the ornament.

The amount of light measured on the work plane is measured in footcandles (fc or lux in metric). A footcandle is actually one lumen of light density per square foot; one lux is one lumen per square meter.

Other influences are the ceiling and wall color, ceiling height, distance to shop displays, other light sources in display areas, etc.

Standard light bulbs waste Watts in the production of light.

Fluorescent tube light is more efficient than bulb light. Electronic fluorescent light is even more efficient than the older ballast type. But make sure not to get the strobe effect - when there are rotating machines or tools.

"Downlighters" give a nice effect - specially in a shop - but the amount of light depends on the type of bulb used. Special light bulbs/ornaments :

Mercury vapor - lots of (efficient) light, but rather cold in color.
Metal halide - lot's of (efficient) light, bluish in color.
High pressure sodium - extremely efficient light, slightly orange.

Most efficient shop lighting per consumed Watt : fluorescent lighting.
Required average shop lighting is 40-60 foot candles measured at work surface.

As a very rough rule of thumb use the following general calculation : 2 watts fluorescent per square foot of work surface.

But if you provide more details on the local situation and the details on the shop itself and data involved it is better to (re)calculate that once more !

Success !

:)

I will most likely recommend fluorescent high bay.

Based on my observations, it seems fluorescents are back in popularity replacing metal hallide and mercury vapor. Yes?

Yes, the T5 system is the fixture of choice now. Much more efficient than HID lighting, typically HPS and Metal Halide.

Mercury vapor is the granddaddy of HID source, not used in new installations any longer.

Plus the fluorescent has other advantages, such as instant start and good color rendering.

I figured as much.

When I was in school, the gyms and larger rooms had mercury vapor, so I remember them!