Here is a example from this forum, but I forgot from which thread and who is the author, maybe you can search with baterry number etc.
The example said:"
also battery will loose 5 to 10% charge during storage as they grow old.
the power consumption of the circuit is 0.625W+0.5W+1.35W = 2.475W
if it needs to run for 10 hours a day then the rating of the battery is 2.475*10 = 24.75WHr
Divide it by batt voltage of 1.2V you get Capacity in MaHr 20.625Ahr.
Take a 80% safety margin you get 25.78AHr.
so you will need 25.78Ahr/2100mAhr =~ 12Batteries.
one better choice would be to use cell phone batteries, which are avalable in 3.7V and up to 5000mAHr, and are very cheap.
So if you need high power, sam185 is right, when the size of battery is limited. But if you select high power battery, you need many DC/DC step-down converter. The conversion circuit size increase.
Anyway, the baterry voltage is high or low is up to your circuits and the budget of your project and the size limits etc.
Cheers.