Today, hydrogen is overwhelmingly produced through natural gas reforming which involves significant carbon emissions. Green hydrogen production from water and renewable energy promises over 80% reduction in carbon emission, but the technology for large-scale (megawatt to gigawatt) solar- or wind-powered hydrogen production has yet to be developed. Technical barriers for green hydrogen production include engineering challenges associated with coupling direct-current (DC) solar power with DC electrolyzers as well as the low capacity factors due to intermittent solar and wind power. In this talk we will analyze three approaches for solar-powered electrolysis: 1) coupling a solar array and an electrolyzer through alternating current; 2) DC to DC coupling through a DC/DC power converter; and 3) direct DC to DC coupling without a power converter. We will also introduce the concept of maximum current point tracking (MCPT) and compare it with maximum power point tracking (MPPT) for solar-powered electrolysis. MPPT is practically used in all solar systems today except those direct-coupled systems, but MCPT is required to maximize the hydrogen output of a solar electrolyzer. We will also propose a solar + wind electrolytic hydrogen production system to improve the capacity factor of the electrolyzer to about 50% from 20% for a solar-only system.