A hyperthermophilic deep-sea methanogen, Methanocaldococcus strain JH146, was isolated from 26°C hydrothermal fluid at Axial Volcano to model high temperature methanogenesis in the subseafloor. Emphasis was placed on defining growth kinetics, cell yields and growth energy demand (GE) across a range of conditions. The organism uses H2 and CO2 as its sole carbon and energy sources. At various temperatures, pHs, and chlorinities, its growth rates and cell yields co-varied while GE remained uniform at 1.69 × 10−11 J cell−1 s−1 ± 0.68 × 10−11 J cell−1 s−1 (s.d., n = 23). An exception was at superoptimal growth temperatures where GE increased to 7.25 × 10−11 J cell−1 s−1 presumably due to heat shock. GE also increased from 5.1 × 10−12 J cell−1 s−1 to 7.61 × 10−11 J cell−1 s−1 as NH4+ concentrations decreased from 9.4 mM to 0.14 mM. JH146 did not fix N2 or assimilate NO3−, lacked the N2-fixing (cluster II) nifH gene, and became nitrogen limited below 0.14 mM NH4Cl. Nitrogen availability may impact growth in situ since ammonia concentrations at Axial Volcano are < 18 μM. Our approach contributes to refining bioenergetic and carbon flux models for methanogens and other organisms in hydrothermal vents and other environments.