I think getting more cores in new structure is the way that manufacturers going to boost up the run time of the device, as they are facing the issue of slow development in better battery. People like high performance handset with smoothness of OS that leads the phone take up more resources, while at the same time they want the phone can standby for 2 or 3 days...so making those fake-Octa cores (4 x A7 / 4 x A15) is one of the temporary methods to slow down the power consumption (or like Qualcomm putting in Quick Charge 2.0 to shorten recharge time) at this moment.
Power Consumption can be determined by different issues within the device itself, other than just the chips and cores (e.g. screen material / Size....) and yes in some criteria more cores can save battery as performance boosted up and shorten the run time like you said. It's why the big.LITTLE Octa-core chips concept came out by ARM promoting more cores with longer run time, adding 4 Cortex-A7 cores to consume even lower power in general use while 4 Cortex-A15 cores for premium performance.