Solar concentrator cells are typically designed for maximum efficiency under the AM1.5d standard spectrum. While this methodology does allow for a direct comparison of cells produced by various laboratories, it does not guarantee maximum daily, monthly, or yearly energy production, as the relative distribution of spectral energy changes throughout the day and year. It has been suggested that achieving this goal requires designing under a nonstandard spectrum. In this work, a GaInP/GaAs tandem solar cell is designed for maximum energy production by optimizing for a set of geographically-dependent solar spectra using detailed numerical models. The optimization procedure focuses on finding the best combination of GaInP bandgap and GaInP and GaAs sub-cell absorber layer thicknesses. It is shown that optimizing for the AM1.5d standard spectrum produces nearly maximum yearly energy. This result simplifies the design of a dual-junction device considerably, is independent of the optical concentration up to at least 500 suns, and holds for a wide range of geographic locations. The simulation results are compared to those obtained using a more traditional, ideal-diode model.