The design of the power metering system is described. Both theoretical analysis and simulations are made to examine the influence of the sigma-delta Analog to Digital Converter(ADC) on this system. It points out that the reference voltage in sigma-delta ADC is a key factor in power metering system. As an offset of x% in sigma-delta ADC, reference voltage results in a 2x% bias in power measurement and x% in RMS(Root Mean Square) measurement. In addition, the resolution of sigma-delta ADC also affects the performance of the system. This is especially true in current RMS calculation channel because this channel adopts the current input, whose SQNR(Signal-to-Quantization-Noise Ratio) is low when the current is small.