Abstract-We propose two bit allocations for a powerline OFDM channel to minimize a total transmission energy. The first is the optimal bit allocation derived for a deterministic channel model. The optimal number of bits allotted for each subchannel depends on channel gains, required data rate and transmission duration. For the second allocation, an equal number of bits is transmitted on a subchannel whose ratio between a noise power and squared channel gain is less than certain threshold. This onoff threshold-based bit allocation is also analyzed for a random channel model for which the closed-form expression for the total energy consumption is obtained in a large system limit. Numerical examples show that the optimal bit allocation could results in 83% reduction in transmission energy, compared to a uniform bit allocation and that the on-off allocation performs close to the optimum when a transmission time is short.