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We formulate an adaptive power allocation problem where we minimize the energy-per-goodbit (EPG) of a system employing a multiple-input multiple-output maximum ratio combining (MIMO-MRC) scheme. Closed-form results are obtained for the optimum transmit power and minimum EPG as a function of the number of antennas employed and the quality of the channel. To assess the performance of the solution in a statistically varying channel, we obtain the cumulative distribution function (CDF) of the minimum EPG in closed form. We explore the energy-efficiency trade-off between enhanced diversity and the increased circuit power consumption of multiple antennas. In particular, we use the EPG CDF in a numerical example to find the most energy-efficient number of antennas for a given probability of outage. We consider both Rayleigh and Rician MIMO fading channels.