My calculation, probably missing something, but here's what I did:
$$
1 \mathrm{\ \mu A} + (50 \mathrm{\ mA} \times 0.1\%) + 25 \mathrm{\ \mu A} =
76 \mathrm{\ \mu A}
$$
$$
\frac{76 \mathrm{\ \mu A}}{ 80 \, \% \mbox{ efficiency}} = 88 \mathrm{\ \mu A}
$$
Round up to \$100 \mathrm{\ \mu A} = 0.1 \mathrm{\ mA}\$
$$
\frac{2700 \mathrm{\ mAh}}{ 0.1 \mathrm{\ mA}} \approx 3 \mbox{ years}
$$
If you're using rechargeable batteries, they'll discharge on their own long before that. Or if any of your other calculations are off (like maybe it's a 98% instead of 99.9% sleep), that will affect it a lot too.
You are very close. The average power is a very accurate way to do this given that you are not pulling such a high current that the effective capacity of the battery fluctuates.
Batteries, Batteries, and More Batteries
There is one very important term, and that is the self discharge rate of the battery. This is dependent on chemistry, but lets say you get a nickel-metal hydride. The self discharge rate is
"20% or more in first 24 hours, plus 4% per day thereafter" if it is not a
low self discharge rate NiMH, which still discharges around 25 or so % a year.
Lithium batteries have some of the best characteristics for self discharge rate and my experience supports this fact. I think battery university has a great site to discuss many different battery characteristics and I often point people there to learn about batteries when they are starting to work with them. If you want to compare battery discharge rates they have an entire article discussing the phenomena.
This is a bit around the point, but I always try to make this point, when you measure battery voltage you need to have it under load. This varies with chemistry, but it is paramount in lithiums. I had a coworker placing bad coin cells in our devices and using them because the coin cells showed almost full voltage with no load. Under a load of any amount(10kohm aprox .2mA) they were flat dead.
Your Microcontroller and You
As you are dealing with using the manufacturer sheet on leakage current there are also many different issues you will have to deal with to keep to those specs that are probably also work thinking about. The biggest I have seen is a floating input. Many engineers will leave unused pins as inputs thinking, "Hey, what harm can this do?" Quite a bit if you are talking microamps. A floating input will have its transistors changing state constantly and the fluctuations cause a power draw difference. We once had a reduced lifetime in a product because we had an error that left 2 pins floating causing our standby current to more then double on our MSP430. You need to drive all of your pins to output and let them hold a state.
It is easy to miss when doing these calculations things like wakeup time. I seem to remember our MSP430 had a non-negligible wakeup time if you were doing it very often. It also had a larger power pulse for just a moment as it came online. Our little homespun RTOS had to try to take this into account and if the shutdown was less then X milliseconds we skipped it with NOPs and saved some power.
If you are looking at a very long life product, you are going to need conformal coating. The oils in your skin are not an issue immediately, but with time they form a lightly conductive material on your board. Conformal coating protects your board from this little current sucking side affect.
Read any app notes they have about low power operation, it probably covers issues like the pins need to be held as output and many other important and useful facts.
Last but not least, Dont let yourself get relaxed just because you have read the app notes and everything seems okay after a week of running your product, you have to do as clabacchio says, you must measure and make sure. You debug your code normally, this is part of it, you need to find out if you made a mistake that is causing your idle current to be mAs instead of uA or even just if you did what we did and a pin is floating on accident. Make sure you use buffered measurements when you do this, if you have a large leakage on your device taking the data you can make a mountain out of a molehill when testing. Also, never forget about pullups, they are little power hogs if you are not careful.
Best Answer
10 years =~ 87650 hours.
1 uA drain will require 87.75 mAh in 10 years.
With som shelf life degradation that's close enough to
= 10 mAh / uA / year or
= 100 mAh / uA / 10 years
So your cited 163 mAh battery will supply 1.63 uA mean.
Pushing technology, size and luck may get you to say 5 uA mean.
There are 86400 seconds/day. There are 1440 minutes/day.
You will find that eg alarm use is much restricted in the allowable use to get 10 years. If 1 uA of the drain is for alarm use then you get 24 uA.hr/day or 86400 uA.seconds or 86 mA.seconds. That's about 240 mW seconds at 3 V. Or say 5 x 50 mW x 1 second burst/day.
An LED can provide ample lighting at 1 mA. Use it 5 times/day x 1 second = 5 mA.sec = 5000 uA.sec or "only" 5000/86400 = 0.06 uA mean drain. Increase as desired and allowed.
Can you run a time keeping IC on say 1 uA?
Probably yes.
So overall it all falls in the area of "notionally possible if really really really clever and careful".
Casio can be expected to be quite clever by now.
Note that if any sort of energy harvesting is being used then all bets are on. Harvesting a uA or few sounds doable.
REAL WORLD EXAMPLE:
There are many others.
In September 2012 user Hli commented:
The link he then provided is now broken, so:
EFM32 "Gecko" family are M0+, M3, M4 ARm Cortex microcontrollers from Silabs
Silabs EFM32 search
Wonder Gecko
EFM32™ Wonder Gecko 32-bit ARM® Cortex®-M4 Microcontroller Silicon Labs’ EFM32™ Wonder Gecko 32-bit microcontroller (MCU) family includes 60 devices based on the ARM® Cortex®-M4 core, which provides a full DSP instruction set and includes a hardware FPU for faster computation performance.
Wonder Gecko MCUs feature up to 256 kB of flash memory, 32 kB of RAM and CPU speeds up to 48 MHz. The MCUs incorporate highly differentiated Gecko technology to minimize energy consumption, including a flexible range of standby and sleep modes, intelligent peripherals that allow designers to implement many functions without CPU wake-up and ultra-low standby current. With the lowest active and standby power consumption, the Wonder Gecko is the world's most energy friendly Cortex-M4 MCU.
Other xxx-Gecko variants M0+, M3, M4
Digikey listings of "Gecko" - legion
Lowest cost in 100's with LCD EFM32TG822F32-QFP48T$US2.03/100 Digikey
Lowest power useful mode with RTC running - EM2 - deep sleep
In EM2 the high frequency oscillator is turned off, but with the 32.768 kHz oscillator running, selected low energy peripherals (LCD, RTC, LETIMER, PCNT, LEUART, I 2C, LESENSE, OPAMP, WDOG and ACMP) are still available. This gives a high degree of autonomous operation with a current consumption as low as 1.0 µA with RTC enabled. Power-on Reset, Brown-out Detection and full RAM and CPU retention is also included.
EM1 - sleep
In EM1, the CPU is sleeping and the power consumption is only 51 µA/MHz. All peripherals, including DMA, PRS and memory system, are still available
EM0 - running
In EM0, the CPU is running and consuming as little as 150 µA/MHz, when running code from flash. All peripherals can be active.
So running in EM0 for 1 ms/s adds 0.15 uA to the EM2 standby load.
Overall, operating in EM2 at around 1 uA mean plus EM0 as required would allow the 10 years / 163 mAh example target to be met.
___________________________________
Energy harvesting:
Vibration and motion may well be possible energy sources.
A silicon solar PV/solar panel seems viable.
Very roughly power available is 150 Watts/m^2 at 1 sun = 100,000 lux.
A 10mm x 10mm "panel" at 10 lux at those ratings would provide ~= 150 Watt x (0.01m x 0.01m) x 10lux/100000lux = 15 microWatt.
10 lux is dim roomlight - at the level where colour fades into monochrome. Dim!
If that level of sensitivity can be maintained at such low light levels (as it quite possibly can with other 'chemistries') the light powering looks viable.