Electric power was estimated in case of the large-scale flat thermoelectric panels exposed to two thermal fluids. The output powers of the proposed 15 systems were analytically deduced from heat transfer theory, and written by non-dimensional functions to reflect the characteristics of system design. The maximum output was the largest in the ideal isothermal systems. In the other realistic systems, it was the largest for the system of the counter flow with one panel. The multiplication of thermoelectric panels can shorten significantly the device area, although the output from the multi-panels decreases only a few %. The worse heat conductivity, l, of thermoelectric materials and the better heat transfer at the surfaces are commonly desired for these fluid systems, in addition to the better figure of merit, Z.