The performance of Schottky barrier varactor multipliers is compared, with emphasis on generating power at frequencies above 1 THz. Measured efficiencies at 800 GHz are greater than 3 percent at and decrease to 0.9 percent at 1.13 THz and 0.25 percent at 1.4 THz. The power available from the penultimate multiplier in a chain imposes a limit on the output from the last stage. Further work is also required to overcome losses in multiplier structures and to understand better velocity and voltage saturation effects in varactors.