1. It’s always the key question when people are considering purchasing a UPS. To select the right UPS and battery, we need to consider load type and load power.
    1. Load type: resistive or inductive load. Resistive load is straight to estimate peak power and average power and select the proper UPS accordingly. While the inductive load is different, it takes peak power several times of rated power, and the peak power might  or might not be addressed in the related documents. Motors are the most popular inductive load, the peak power could be 4 to 7 times to rated power, some even higher. So, some devices have soft start control circuits to reduce the peak power. 
    2. Peak power: most devices run at several different modes, such as running, idle, sleep and wakeup. Further mode, even in running, it could be different power consumption at full speed, half speed and so on. If the UPS is planned to supply several different devices, the situation becomes more complicated. The safe way is to sum up peak power of all devices, then select a UPS whose rated output power is 10% more than the peak power of load. 
    3. Average power: the main factor of backup time estimation is average power. If the peak power time is very short, it even can be ignored for the estimation. 
    4. Sometimes the average power does not express the power consumption situation as the words expressed.

      If we use the average for 24 hours of a day, we get around 5500W for this example. However, from 8:00 to 12:00, the power is much higher than average. If the mains goes down in the morning, the backup time will be much less than the expectation. So, how to select the average range is complicated too, needs to consider the demand.
      After considering the complete situation of the application, you can get suitable peak power and average power. Use peak power to select the UPS and average power to select battery.
  2. Why is the calculated backup time not accurate?
    The calculated backup time is based on certain fixed parameters and they vary time to time in the real world. 
    1. The load power is not exactly shown on the document and changes from time to time depending on operation modes
    2. Operation environment, especially temperature impacts the battery capacity
    3. As we know, the UPS is purchased to secure the power supply when main is gone. In practice, people purchase the UPS with backup time more than the target. Considering this, it’s not necessary to get a very accurate calculated backup time