The TDP (thermal design power) is the maximum power consumption in steady-state conditions, when all the CPU cores and/or the GPU are 100% busy and the power consumption is limited to avoid overheating. It is used to dimension the cooling system, together with the maximum working temperature of the CPU.
The TDP has nothing to do with the battery lifetime in normal usage. Any smartphone or laptop will drain completely its battery in a couple of hours at most when running at the power limit (you will never see a laptop configured for an 8 W TDP also having a 99 Wh battery; such batteries are normally used for laptops where the CPU is configured for a 45 W TDP).
For the battery lifetime what matters most is the power consumption when the CPU is idle (which should be a fraction of a watt, so that the total idle power consumption is dominated by the display power consumption) and also how big is the additional power consumption caused by light CPU loads, like Internet browsing, movie watching, document reading and editing etc.
The TDP has nothing to do with the battery lifetime in normal usage. Any smartphone or laptop will drain completely its battery in a couple of hours at most when running at the power limit (you will never see a laptop configured for an 8 W TDP also having a 99 Wh battery; such batteries are normally used for laptops where the CPU is configured for a 45 W TDP).
For the battery lifetime what matters most is the power consumption when the CPU is idle (which should be a fraction of a watt, so that the total idle power consumption is dominated by the display power consumption) and also how big is the additional power consumption caused by light CPU loads, like Internet browsing, movie watching, document reading and editing etc.