Lumen is a measurement of the intensity of light; one lumen is equal to the brightness of a candle at 1 foot away. Just to get an idea on a clear summer day by the time the light from the sun hits reaches us on earth it has an intensity of up to 10,000 lumens. If you wanted to simulate sunlight in your house you just need to hook up 6 (100 watt incandescent or 29 watt CFL) light bulbs and stand one foot away.
Fortunately plants only need 100 to 4000 lumens to sustain vegetative growth depending on the variety, so indoor growing can be setup fairly inexpensively. For example, with our strawberry plant in a grow box experiment we can get away with just 2 CFL bulbs and provide enough light to provide the 1500 lumen required to allow vegetation and flowering. All of this for about $1.00 a month of electrical usage, at $5.00 a pint for strawberries maybe I can make this profitable.