To determine the approximate power drain on your battery you will want to estimate the reserve power (amp/hour) and the amps the inverter will require to meet the continuous load demands of the appliance.

Calculating UPS/INVERTER Battery Backup:

Before calculating the Battery Backup, let us know a few factors that vary battery backup.
For and UPS with 875Va we can us a maximum load of 640 watts
677Va we can us a maximum load of 540 watts
The main thing we have understand is that what ever may be the UPS wattage the battery backup will not vary. The battery backup will only vary depending on the battery Ah and the Usage Load.

To calculate UPS backup, We have a simple formula.

UPS Backup = Battery Ah * (Volts/Load) * (1/Powerfactor)

Example:        Let us calculate the backup for a system UPS

Load is the usage power, suppose we are running a PC then the load is around 300 watts

Power Factor varies for device to device, the average power factor is 1.4

Voltage is the voltage of the battery.
For a single battery the voltate is 12v.
If the batteries are connected in a series   Voltage = 12 * no. of batteries.

Battery Ah is the battery ampere used for the ups,
System UPS battery AH is 7

Then   Battery Backup =  7 * (12/300) * (1/1.4)
=  7 * (0.04) * (0.7)
=   0.19 hours
=    19 Minutes

Note:    You can use the same formula for calculating the battery backup for home UPS or an INVERTER.