A 10 Ah would probably be sufficient but a 10 Ah would discharge close to an 80% level which would decrease the life expectancy of the battery. You'll end up purchasing another 10 Ah before too long.
If you get the 15 Ah you'll only need to charge it once a day and hit the 65%-75% discharge level. A 15 Ah will be cheaper for you in the long run and in case you need to make a trip other than just to work and back then you'll have the extra power.
What are the wind speeds like on your commute to work and back? Riding against the wind will slow you down and eat up the battery power as well.
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