Electrolytic hydrogen produced using renewable electricity can help lower carbon dioxide emissions in sectors where feedstocks, reducing agents, dense fuels or high temperatures are required. Several standards are being discussed to certify that the grid electricity used is renewable. The standards vary in how strictly they match the renewable generation to the electrolyser demand in time and space. In this paper, we compare electricity procurement strategies to meet a constant hydrogen demand in a computer model for selected European countries in 2025 and 2030. We compare a case where no additional renewable generators are procured with cases where the electrolyser demand is matched to additional supply either on an annual, monthly or an hourly basis. We show that local additionality is required to guarantee low emissions. If no storage is available to buffer the hydrogen, the electrolyser must run at full capacity at all times. For the annually matched case, constant operation means using fossil-fuelled generation from the grid for some hours that results in higher emissions and increased electricity prices compared to the case without hydrogen demand. In the hourly matched case, emissions and prices do not increase, but baseload operation results in high costs for providing constant supply if only wind, solar and batteries are available. Buffering the hydrogen with storage, either in steel tanks or underground caverns, reduces the cost penalty of hourly versus annual matching. Hydrogen production with annual matching can reduce system emissions if the electrolysers operate flexibly or coal is phased out and the renewable generation share is above 80%. The largest emission reduction is achieved with hourly matching when surplus electricity generation can be sold to the grid.