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"100 watts" doesn't really state the intensity of the light required as different forms of lighting have different efficiencies, e.g. 100 watts of CFL bulbs will produce more light than 100 watts of incandescent bulbs. Light intensity is normally stated in lumens and bulb efficiency is measured in lumens per watt.
Anyway... the general rules of thumb is that if you can comfortably read a newspaper then the light intensity is sufficient.
If you want them to continue laying then they shouldn't see any reduction in the hours of daylight. The effect of natural daylight cycles is more pronounced in northern latitudes as there is a greater day-to-day difference in the hours of daylight as the days get shorter or longer and the hens respond accordingly.
Commercial operations control the day length of pullets to delay sexual maturity until the correct weight for the birds have been achieved, and thus correct egg size. Once the correct weight has been achieved the day length is increased to advance sexual maturity and bring the pullets into lay. In the absence of increasing day lengths (like at the equator or during times when the day length is decreasing, such as June through December) pullets will reach sexual maturity and start laying due to increasing body weights. That is the reason most BYCers see a great disparity in age at the start of lay, and why feeding extra protein helps to stimulate lay.
If you want your birds to continue laying through the winter, or stimulate those that haven't started yet, you need to start at the day length that exists right now at your location and add 20 to 30 minutes per week until you reach the desired day length. The desired day length isn't necessarily 14 hours, depending upon what you are trying to achieve. Since your birds aren't kept in a light-proof layer house their will always be some interaction with the natural day length.
If you do use 14 hours, that will hold them through the winter until the natural day length starts extending beyond 14 hours next spring. After late June the day length will start decreasing again and start to affect summer production unless you then gradually extend the day length to match the longest day of the year in your area (around June 22). If you want them to naturally molt the following fall you can start decreasing the day length at that point.
Clear as mud?
Wow, are you friends w/ resolution?