The average load of a thermal mass slab depends on the size of the room and the desired temperature. Plumbers recommend using a mini-split system to heat the room as it would cost three times less to run than a radiant floor. Alternatively, 1000-1500 watts of radiant cove heaters mounted at the crown-molding level would cost less than $500 and wouldn't use any power until/unless the place was occupied. The cove heater approach works by radiating heat at the humans and objects rather than convecting the heat to the air first.