Answer: Nothing beats sunlight! However, during the dark days of winter, or if you aren't lucky enough to have a sunny room, artificial lights will help your houseplants thrive. The important things when choosing a light source are that the bulb doesn't give off excessive heat, and that the quality of the light is as close as possible to sunlight -- that is, it covers the full spectrum.
Regular fluorescent lights are reasonably close to full spectrum, and are a good choice, especially if you are growing only foliage plants. Full spectrum grow lights provide a better quality light, and might be a better choice if you are growing flowering plants such as African violets. Avoid incandescent light bulbs -- they give off too much heat.
Because indoor lights aren't nearly as bright as sunlight, you need to be able to keep the lights 4-6 inches from the top of the plants, so you'll need to be able to raise and lower the lights as needed. Lights should be on 16 hours a day and turned off at night; putting them on a timer is helpful.
There's really no such thing as an "average" houseplant -- just like with outdoor plants, different houseplants have different needs. Some will tolerate very low light levels, while others need at least some direct sun.
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