The power-to-ammonia process requires flexible operation due to intermittent renewable energy supply. In this work, we analyse three-bed autothermal reactor systems for design and off-design performance for power-to-ammonia application. The five reactor systems differ in terms of inter-stage cooling methods, i.e., direct cooling by quenching (2Q), combination of indirect and direct cooling (HQ and QH) and indirect cooling (2H) with variations. At optimum nominal operation conditions, the inter-stage indirect cooling (2H) reactor systems result in the highest NH3 production. For off-design performance analysis, NH3 production is minimised or maximised by varying one of the following process variables at a time: inert gas, feed flow rate or H2-to-N2 ratio. For each variation, the effect on H2 intake, recycle stream load and recycle-to-feed ratio is also analysed. Among the three process variables, the H2-to-N2 ratio provided ca. 70% lower NH3 production and 70% lower H2 intake than at nominal operation for all five reactor systems. Operation of autothermal reactor systems at significantly lower H2 intake makes them reliable for power-to-ammonia application; as during energy outage period, shutdown can be delayed.