Building a speed control?

Suppose your pump is 1500W 120V. Current is 12.5A

A 10 ohm potentiometer rated at least 2000W is needed to install between the pump and converter 120V output to control it.

What is needed is the current drawn by the pump, or the wattage rating, from which you can calculate the current.

And, is this 120 VAC or 240 VAC?

I can tell you now that resistive potentiometers don't work very well as speed control for motors. You need a large expensive power potentiometer, as much as 100 watts, even larger if this is a big pump. And that can cost $50-100 easily.

You do not use a potentiometer to control a motor. You use a rheostat. Even a small pump is going to draw a heavy current, so your rheostat is going to be a fire hazard.

Think of something else. Efficiency is not something you casually disregard, it is the major engineering problem in this project.

you are just on to wrong track man. potentiometer can not handle power drawn by your pump as you mentioned. i will suggest you to make some circuit with power control device like TRIAC, with enough power rating and then use can easily use a small handy potentiometer to control the triac triggering. there are lot of circuits you can find for triac like SEARCH FOR TRIAC DIMMER.. I am leaving inductive load tpc onto you!