The thermal characteristics of matrix-addressed smectic liquid-crystal cells are studied using the temperature dependence of the resistance of the heater and detector lines and compared to analytic solutions. The cell behavior is most strongly affected by the thermal properties of the substrate near the liquid crystal if the heater is at that interface. Cell features further away from the heater have a smaller influence. The cell power efficiency improves if the thermal flow through the substrate is reduced relative to the liquid crystal. The cell addressing speed can be increased if the substrate has two layers composed of a thin insulator on a conducting layer. Increasing the insulator thickness improves efficiency but reduces speed.