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Bottom-up fabricated single-junction III–V nanowire array solar cells have shown efficiency up to 15.3%, which is approximately half of the conventional Shockley–Queisser detailed balance efficiency limit of 33.6%. Here, based on numerical and analytical opto-electronics modeling and analysis, we give guidelines for (i) geometry that gives strong absorption as well as (ii) the design of efficient p–n junction and electrical contacts in the nanowires to reach 20% and 25% efficiency. We exemplify the impact of eight different optical and electrical loss mechanisms in a 15% and a 25% design. We also provide an analytical equation for estimating the efficiency drop due to resistive losses in the top contact layer for varying cell size.