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This study investigates the impact of including (or neglecting) the variable efficiency of hydrogen electrolyzers as a function of operating power in the modelling of green hydrogen produced from variable renewable energy sources. Results show that neglecting the variable electrolyzer efficiency, as is commonly done in studies of green hydrogen, leads to significant overestimation of hydrogen production in the range of 5–24%. The effects of the time resolution used in models are also investigated, as well as the impact of including the option for the electrolyzer to switch to stand-by mode instead of powering down and electrolyzer ramp rate constraints. Results indicate that these have a minor effect on overall hydrogen production, with the use of hour resolution data leading to overestimation in the range of 0.2–2%, relative to using 5-min data. This study used data from three solar farms and three wind in Australia, from which it is observed that wind farms produced 55% more hydrogen than the solar farms. The results in this study highlight the critical importance of including the variable efficiency of electrolyzers in the modelling of green hydrogen production. As this industry scales, continuing to neglect this effect would lead to the overestimation of hydrogen production by tens of megatonnes. • Variable efficiency electrolyzers are compared to fixed efficiency electrolyzers. • Fixed efficiency can lead to overestimation in hydrogen production by up to 24%. • Low fidelity time resolution has a modest impact on modelling hydrogen production. • Fixed efficiency electrolyzers underestimate the optimal size of hydrogen plant. • Results are validated using data from six renewable energy sites in Australia. [ABSTRACT FROM AUTHOR]