amps or watts ??

I have a pellet stove that runs on 115 volts 60Hz 3amps
when calculating how long this will run off of 12 volt 450 ah battery pack do I use watts or amps draw ?
The reason I ask is I also have a couple of 12 volt 1.5 amp
bulbs that will run a very long time  drawing only 3 total amps
yet doing the math for the stove which draws only 3 amps the calculation comes out it will only run around 6 + or -
hours.
I know 115 volts X 3 amps = 345 watts
AM I doing this correct ?
I have used a couple different calculator one is system appliance run time the other is
Battery capacity calculator tells me ..12 volts 450 amp hour battery pack 345 watt draw will run the stove for 13 hours
Battery equipment run time calculator tells me 12 volts 450 amphours 345 watt draw will run stove for 6.52 hours
which is correct???
Thank You
Rene