The total luminous energy outputted by a professional studio light set is about 100 to 1000 watts and will easily cost a few hundred dollars. The total luminous energy outputted by the sun and received by earth is about 174 petawatts or 174,000,000,000,000,000 watts and is free.
For Silhouettes Indoors
Pose your model or object directly in front of a window. Add some sheers or light-colored curtains to diffuse the light for more even back-lighting as well as to hide any distracting components visible outside the window.
Decrease your camera's exposure bias by -1.0 to -3.0 steps to ensure that the silhouette will be dark. Contrast adjustments will likely be necessary in post-editing to darken the silhouette as well as lighten the background and create better contrast.
To the right is a lighting diagram of the indoor lighting setup I regularly use for silhouettes with some results below.
For Dramatic Indoor Lighting
Use a window like a soft box for some dramatic lighting. Cover all windows except one with a dark curtain such that there is only one light source through one window. Do not cover the one light source window with any sheers in order to allow for harsh shadows and dramatic lighting. Pose your model or object sufficiently far away from the one light source window to prevent a silhouetting effect. Below is a lighting diagram of the dramatic indoor lighting setup I generally use with some results.
For Even Indoor Lighting
Let the sunlight shine through those windows! More windows, more light. Add some sheers and light-colored curtains to diffuse the light for more even lighting. Light-colored walls will also help create more even lighting by allowing light to bounce.
To the right is a lighting diagram of the indoor lighting setup I regularly use for even lighting with some results.
Try adding a backdrop for some color. No need for everything to be so white as in these examples.
But all outdoor natural lighting conditions, sunny or cloudy, in the early mornings or at high noon, can have great outcomes. So just go outside, experiment with natural light, and see for yourself! Some results are provided both above and below if you're still not sure.
Other Uses For Natural Lighting
Go experiment with natural lighting. Its uses are definitely not limited to just portraits or the results shown here. The possibilities are endless. Plus there's no cost to you! Use the power of the sun and have fun with it!