We use a go power 300w pure sine wave to power a heat pad
IIRC Idle no load power for the GP is 0.600 amps yes 600 milliamps
Any way 25 amps ...has NOTHING to do with it
5~6 amps plus 10% inverter conversion loss, figure 8 amps load at max heat for whatever hours you use it. 6 hours at 8 amps = 48 amp hrs or about 30% of your bank
To stay above 50% discharge, you will need to recharge your batteries everyday
If your heat pad is old and has the manual three position selector resistive style switch, you can use a MSW inverter if it's new with digital control then you need PSW inverter