I am replacing the SLA battery on my homemade bike setup. I have 2x20W halogen lights in my system, and am looking to replace the current battery with a smaller battery. Right now I have a 12V 7.0ah battery ( brick!). I am wondering if someone can confirm the formula I am using for calculating battery need is correct:

i.e

20W x 2 bulbs = 40 W x hours required ( 2) = 80 Watt hours

80 watt hours / 12 V = 6 amp hours.

Am I using the correct formula? If so, I should be able to get away with a 4.5 amp hour battery, considering that I run only one light for the majority of the time.

But what effect does temperature have on the number of watt hours a battery yields? I remember reading something about a percentage drain for every degree below zero, but can't remember what it was. ( I am planning on biking in winter, so this is important)

Thanks much!