Electric pump motors require extra power to start spinning—up to four times the power used to just run. Many pump controllers have linear current booster (LCB) circuitry, trading volts for amps (and vice versa) to run the pump at its most efficient rate for the amount of power the PV array is producing. On an overcast day, a pump with no LCB may be sitting idle, while a pump with an LCB can overcome the required startup load and then run all day, albeit at a slower rate, proportional to solar intensity.
