Stationary batteries of all chemistries are usually rated in watts or Ampere-hours (Ah) which indicates the amount of power (current) that can be delivered over a period of time. These ratings are dependent upon operating temperature and depth of discharge.
They are also rated in terms of expected years of useful service under ideal conditions which is never usually achieved under actual service conditions in the field. This is because of various things such as electrical noise, number of cycles, type of load, temperature and humidity. In North America, in order to have a way of comparing battery life between various products and manufacturers, a somewhat rough estimated life expectancy in terms of years was introduced. The main categories are 5, 10 and 20 year life batteries. In reality these times are never usually achieved except with top of the line Vented Lead-Acid (VLA) and large format Nickel-Cadmium (NiCd).