Direct current (DC) power distribution has recently gained traction in buildings research due to the proliferation of on-site electricity generation and battery storage and an increasing prevalence of end uses operating internally on DC. The research discussed in this paper uses Modelica-based simulation to compare the efficiency of DC building power distribution with an equivalent alternating current (AC) distribution. A variety of parametric simulations determine how and when DC distribution proves advantageous. This work shows that using DC distribution can be considerably more efficient than AC: a medium office building using DC distribution has an expected baseline of 11% savings, but may save up to 17%. In these results, the baseline simulation parameters are for a zero net energy (ZNE) building with enough battery storage to act as an islanding microgrid. DC is generally most advantageous in buildings with large solar capacity, large battery capacity, and high voltage DC distribution. In addition, based on the efficiency modeling results, a comparison of the economic performance of DC vs. AC distribution systems in commercial buildings is conducted. The results for the baseline scenario show that DC distribution systems in buildings can be cost effective when PV generation and battery storage are included in the building.