To determine what size UPS you need, you should consider the following factors:

• The power problems youre trying to avoid, such as blackouts, voltage fluctuations, or other power disruptions.
• The number of electronic devices youre looking to support with the UPS.
• The wattage of the electronic devices youre connecting to the UPS.
• How long you need the backup battery to run for, or runtime.
• Form factor/how much space you have for the UPS.

To calculate the size of the UPS you need, you can follow these steps:

1. List all the equipment and devices you want the UPS to protect.
2. List the amps and volts for each device. These ratings can typically be found on the label on the back of the equipment.
3. Multiply the amps by the volts to get the wattage for each device.
4. Add up the wattage for all devices to get the total load.
5. Multiply the total load by 1.25 to determine the UPS capacity you need. Your load should not exceed 75%-80% of your UPS systems capacity.
6. Plan for how much time you need to safely shut down your connected devices and account for this when considering how much capacity you need for your UPS system.

Its important to note that a UPS will never support its full capacity, and the closer you get to capacity, the less battery runtime you will have. Also, some UPS manufacturers offer extended battery modules (EBMs) to extend total runtime if that is a concern.