A UPS protects valuable equipment from black- and brownouts.
An uninterruptible power supply (UPS) is an electrical device that provides backup power to other electrical devices during power outages. UPSs are plugged into a power socket, and the devices protected by the UPS are plugged into the UPS. If main power goes down, the UPS will continue supplying power to the devices plugged into it, until its internal batteries run out of charge. The time during which the UPS will provide power to its client devices depends on the UPS's capacity and on the power requirements of the clients. Typically, and especially for computer equipment, the power needs are specified in watts. You can use a simple calculation to compute for how long a given UPS will continue to provide charge. A minor variation will tell you how big a UPS is needed to keep the devices powered up for a given minimum time period.
Instructions
1. Find out the amp-hour rating of the UPS. This information should be in the UPS's specifications. If you cannot locate it, it is written on the lead-acid batteries contained inside the UPS. Open the UPS (with much care and wearing rubber gloves) and read the amp-hour rating from each of the batteries. Add them up (if there is more than one battery) to get the amp-hour rating for the whole UPS.
2. Divide the amp-hour rating from Step 1 by 0.8. This is the usable amp-hour rating of the UPS. You should not drain all the charge from lead-acid batteries unless you want to dramatically shorten their life spans. A divisor of 0.8 corresponds to draining the batteries down to 20 percent charge, but no further.
3. Add up the watt power requirements for all the devices that are protected by the UPS. For instance, if the computer monitor requires 100 watts and the desktop computer requires 350 watts, the total requirement is 450 watts.
4. Look up in the UPS specifications the voltage of the batteries contained inside it. Typically, it is 12 volts. Divide the total watt requirement by the voltage to obtain the nominal amperage the devices need.
5. Look up the efficiency of the UPS inverter, which should also be in the specifications. Multiply it by the nominal amperage to obtain the real amperage needed. For the example, multiply (450 watts / 12 volts) x 0.85 = 31.875 amp, assuming the UPS inverter is 85 percent efficient.
6. Divide the usable amp-hour rating from Step 2 by the real amperage needed from Step 5 to get the number of hours during which the UPS will power up the devices.
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