To maintan the output dc voltage constant with resistive load you have to required PI controller.
with resistive load when you change the solar irradiation then small change on PV voltage is occure
and your duty cycle will remain same(mppt operation will not working) because of this output voltage will be change.
whenewer you have replace the resistance with battery then your voltage will be maintan constant because solar system will not feed the power to the battery or SOC of battery will not very.
Pemendra Kumar Pardhi