The shelf light is for fitting in a shelf of an article of furniture. The power controlling apparatus also includes a power conversion controller which transmits a load request power information signal based on power information requested by a load to the fuel cell controller, analyzes the power state information signal transmitted by the fuel cell controller, and controls a power convert amount based on the analysis. When the voltage dip occurs in an electric power system, a desired combination is selected from the plurality of voltage compensation circuits P1-P3, N1-N3. The trader is also able to interactively respond to visible prices in counterparties’ orders by changing the prices in their own order. The system and method utilize measuring means arranged at the secondary side of at least one chosen distribution transformer for reading voltage, which voltage in turn is used for controlling the power transformer so that desired voltage at the secondary side of the chosen distribution transformer isare achieved. The wavelength of the light being such as to make the fovea of the eye resolvable.