Every UPS will supply power to a load (such as a computer, telephone switch or medical equipment) when main power fails. It may also condition the power and prevent spikes, brownouts, interference and other unwanted problems from reaching the supported equipment.
As long as you want, providing you buy enough batteries and the charging system is up to it. After about four hours it's usually more cost-effective to buy a generator, with a short runtime UPS to bridge the generator start-up gap.
Most plug-in UPS are good for at least five years. We'd advise you to change the batteries every two to three years. With larger equipment (and more substantial investment), the lifetime of the equipment increases.
There are three simple methods:
Never overload your UPS.
Never connect any home electronic devices such as cooling fan to your UPS. This may cause malfunction of your UPS.
Discharge the battery in a consistent interval. The best way to do it is once a month or once two months. The simplest discharge way is to turn on the UPS without connecting the mains.
In this type of UPS, the primary power source is line power from the main utility, and the secondary power source is the battery. The battery charger is using line power to charge the battery, and the battery and inverter are waiting "on standby" until they are needed. When the line power fails, the transfer switch changes to the secondary power source. When line power is restored, the UPS switched back
Line Interactive UPS under normal condition smooths and to some degree regulates the input AC voltage by a filter and a tap-changing transformer. The bi-directional inverter/charger is always connected to the output of the UPS and uses a portion of AC power to keep the battery charger. When the input power fails, the transfer switch disconnects AC input and the battery/inverter provides output power. Its typical efficiency is 90~96%. This type is currently the most common design in 500 VA / 5000 VA power range.