The efficiency of hydrogen production is largely unaffected by scale. Electrolyzer efficiencies are normally quoted at their rated input level, but the efficiency at power levels below the rated level can actually be higher. Commercial electrolyzers are quoted in a range of 40-60 kWh/kg (55-85%).
The capital cost of electrolyzers tend to drop rapidly with scale—the larger the project, the lower the cost. Electrolyzers of 100+ MW scale were in service from the 1920s and although most commercial electrolyzers are in the 1-20 MW scale range today, 100 MW electrolyzers are in development by various manufacturers as demand for them increases.