In this study, we experimentally evaluated electrolysis of water using a short stack of six solid oxide electrochemical cells (SOEC) at temperatures ranging between 700 °C and 800 °C. A model of the SOEC stack was verified and then used to evaluate thermal management of the stack operating below the thermoneutral voltage, which at these temperatures requires additional heat input into the stack to maintain the operating temperature of the stack. High quality heat supplied from two different sources were evaluated in this work. Heat generated from concentrated solar power (CSP) can reach the required SOEC operation temperature and, when scaled properly, can provide enough heat to sustain sub-thermoneutral voltage operation of an SOEC. CSP output follows a diurnal pattern and the peak output generally coincides with the time of day when solar photovoltaic power may need to be curtailed. Heat generated from nuclear power was also considered and offers a relatively constant supply of heat to an SOEC stack. The SOEC model is spatially and temporally resolved and was used to evaluate the performance of a system operating at sub-thermoneutral voltages with CSP and nuclear heat as two separate and distinct heat inputs to maintain the stack temperature and temperature gradients. Dynamic performance of the system was evaluated with respect to thermal management and the simulation results of various system configurations are presented.