Inside a glass bulb, electricity heats up a wire filament, causing it to glow and give off light. Of course, electrical heaters work in much the same way, and that's why more than 90 percent of the energy produced by incandescent lights is heat, not light. As a result, incandescents are inefficient light sources. The heat they produce can drive up your electricity bill in hot weather if your home or office is air-conditioned.