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A reproducible calculation of ideal single-junction photovoltaic efficiency is presented using the ASTM G173 AM1.5G terrestrial spectrum and the full ASTM E490 AM0 extraterrestrial spectrum. The analysis separates incident-spectrum effects from the assumed optical emission boundary and compares a Lambert-W maximum-power solution with numerical maximization of the same idealdiode model. At 300 K, the one-emitting-side AM1.5G case reaches 33.693% at a bandgap of 1.3365 eV; equal front and rear emission reduces this result to 33.080%. With the full E490 integrated irradiance of 1366.091 W m−2, the one-emitting-side AM0 case reaches 30.243% at 1.2450 eV, corresponding to 413.15 W m−2 electrical output. Analytic and numerical maximum-power values differ by approximately 10−15 in relative power over the tested bandgaps. Bandgap steps of 1 meV or finer change the terrestrial maximum by less than 0.001 percentage points. A deterministic sensitivity study combining temperature, irradiance scale, and spectral resampling gives 33.568–33.797% for one-sided AM1.5G emission. The contribution is an auditable implementation and comparison of specified modeling conventions, not a new efficiency limit or experimentally validated device model.