For any given load, UPS runtime or autonomy time is dependent upon battery capacity. A common misunderstanding is that higher power UPS provide more runtime than a lower powered UPS. This may be the case, only because the higher power UPS tends to contain more, or higher capacity, batteries than the lower powered UPS. However, it need not necessarily be so.
Most UPS operate using lead acid batteries which have a voltage rating (usually 12V, but 2V, 6V and 24V are commonly available), and smaller systems tend to be the sealed type, which means there is no danger of spillage. Lead Acid batteries are rated in Ampere Hours, and the ability to determine what runtime you can achieve for a given load is by checking what rating and quantity of battery is contained within the UPS.
It is not uncommon to see headline descriptions for a small UPS that state high levels of runtime, such as 30 minutes or an hour. However, seldom do such headline descriptions state at what load this runtime is achievable. Responsible manufacturers will state the runtime at full load, and perhaps half load. Do not be put off by descriptions that state full load runtime is only in the order of a few minutes. Chances are, the UPS runtime will be equal or better to those which don’t specify at what load the runtime is achievable.
|Units with the same battery will give around the same runtime|
Long runtime applications, defined as over 30 minutes with a reasonable sized load, will require an extended run UPS. Here, online double conversion technology is most appropriate, although line interactive sine wave systems can be a good choice. The UPS may, or may not have internal batteries. Where no batteries are in-situ, the unit will likely contain a higher capacity charger. A number of battery packs can be added and daisy chained together in order to meet the required runtime.
However, the more batteries that are added, the bigger charger is required in order to bring the batteries to full charge in a reasonable time frame. (Note that charging lead acid batteries is a chemical reaction and will take at least 4 hours to hit 90% capacity and 12 hours to fully recharge – assuming a suitably rated and professional charging circuit).