How to Calculate UPS Battery Backup Times
As a safeguard against losing work to blackouts and other power failures, many computer users are turning to uninterruptible power supplies (UPS), which consists of a battery integrated into a surge protector or other voltage adapter. Each UPS has a specific volt-ampere (VA) rating, which is an expression of the maximum AC power it can supply. The length of time for which the UPS can support a particular appliance's electrical load depends on the efficiency of its DC-to-AC inverter, its effective output voltage and the capacity of the battery.
Determine the efficiency or "power factor" for your UPS's DC-AC converter. This information is normally found in the "Technical Specifications" section of the UPS-enabled device's Owner's Manual. If you can't find the efficiency there, try searching the manufacturer's website or send on inquiry to their customer service email address.
Determine the capacity (in ampere-hours or "AH") for each of the UPS's backup batteries. To approach the voltage levels of an electrical outlet, most UPS models will link several 12-volt, sealed lead-acid batteries in a series. Some manufacturer's will include the capacity of each battery in the Owner's Manual under "Specifications." If you cannot find the battery capacity listed in any of the UPS's literature, you'll have to carefully remove one of the panels from the UPS's housing to see the capacity printed on the batteries themselves.
Add together the voltages from all of the batteries from Step 2. This will give you the combined DC voltage output for the UPS. Note: do not add together the capacities, as that value in unaffected by the series circuit configuration.
Multiply the combined DC voltage (from Step 3) by the UPS's power factor (from Step 1). This result will represent the effective AC voltage output by the UPS.
Determine the voltage and amperage ratings for the appliance that the UPS will be backing up. These values are typically found either printed on the back of the appliance or published in the "Technical Specifications" section of the Owner's Manual. As a last resort, you can search of this data in the "Products" section of the manufacturer's website.
Multiply the appliance's voltage (from Step 5) by its amperage (from Step 5) to calculate its power consumption in volt-amperes (VA).
Divide the appliance's power consumption (from Step 6) by the UPS's AC output voltage (from Step 4). This will give you the current (in amperes) the appliance will draw from the UPS.
Divide the single battery capacity (from Step 1) by the current draw (from Step 7). This will give you the ideal maximum backup time for the UPS, in hours.