With the 2015 MacBook, Apple has provided us with a lightweight solution for people with average computing needs, and a preference to use a keyboard to get their tasks done. I have not seen yet a breakdown where the average 5W power draw goes, but we have three main components that draw the bulk of power:
Screen needs to power the backlight, and the individual pixels in the LCD. Most of the consumption is with the backlight, typically dominating the entire power budget at the highest setting, under low loads.
DRAM Memory can draw a few watts of power when actively accessed, while the self refresh is less than 10mW per GByte. More memory increases power consumption mainly because chips not accessed in a transaction still increase parasitic power draws on the connections. The best strategy to reduce power consumption is to not access the memory, and this means software optimized to utilize the RAM caches. Intel Iris Pro provides a 128 Mbyte extra cache which can also greatly reduce main memory accesses, even though currently its own consumption eats up any power benefits.
CPU/GPU are the classical consumers, but have now become so powerful that they tend to sleep almost all the time. When we actually need to do some processing, it is worth remembering that power consumption is proportional to f U^2, the frequency times the supply voltage squared. Since with a higher frequency we also need a higher voltage so that the transistors can switch fast enough, we can compute more with a given charge by running slower, up to a point.
So when Marco Arment wants more battery life, we can actually configure a computer that can run long on batteries, and fast when plugged in. We can set up the latest processors to disable Turbo Boost, even reduce its TDP to reduce cooling requirements, so that we do not need waste energy spinning a fan. The external GPU can be completely disabled while on battery. One can change the system scheduler to reduce the number of active cores when it needs to boost a single threaded load. The envelope to throttle down the available power to increase battery life into the 12 hour plus range is there, it is up to Apple to actually implement it.