How this is calculated. Runtime is usable capacity divided by
average
current, and the average is the duty-weighted mix of the two states:
Iavg = Iactive·D + Isleep·(1−D). The
counter-intuitive part is that
sleep current usually decides the answer: a device awake
1 % of the time spends most of its charge in the other 99 %, so shaving the sleep floor from
10 µA to 2 µA often does more than making the active burst faster. The split above tells you
which one you are actually paying for. Capacity is derated because a cell's rated mAh assumes a
gentle discharge to a low cut-off — you will not get all of it, and less again in the cold.
This ignores self-discharge, which for multi-year designs can matter more than the load.
More than half the energy budget is going into sleep. Reducing the sleep current will extend runtime more than optimising the active burst.