Determine how many amperes your application needs from the battery and for how long. Multiply the two to obtain Ampere Hours required. Increase this by 20% for a safety cushion, and from our capacity charts, match a battery which will deliver this many AH for the required time, and voltage. Connecting batteries in parallel adds AH, and connecting in series adds the voltage. In either case the energy (WH) storage capability is increased by the amount of energy each additional battery provides. Click here for the U.S. Battery calculator