Low voltage (LV or 12v) downlights are wired to a transformer, which is then wired to the mains supply. Although the transformer wastes approximately 10% of the power through stepping down the voltage to 12v this is more than compensated by the improved performance of low voltage downlights.ow voltage (LV or 12v) downlights are wired to a transformer, which is then wired to the mains supply. Although the transformer wastes approximately 10% of the power through stepping down the voltage to 12v this is more than compensated by the improved performance of low voltage downlights.