Is there a scientific/engineering formula to figure out the needs of a battery backup & POE switch/cameras?

[Holbs]

I never know what is appropriate when it comes to battery backup needs. My default for just about everthing is the number 1500 (Cyberpower, APC, etc).
But now is the time I should narrow down "expected" battery backup life in case of main power outage.
All will be on 110VAC outlets.
I do have one of those WATT watchers that keeps track of watts used for a device.
1.) I will have a Blue Iris windows machine with appropriate sized power supply (400w or such).
2.) 20-24 POE powered Dahua IP cameras (possibly 2 SDx series PTZ)
3.) Ubiquiti 24port POE switch

Is there a formula to figure out length of time for battery backup once I gather all the watts & amps?

The ubiquit POE switch I use right now for my POE cameras ranges from 2.5-3.5 watts between turrets & bullet 2MP, 4MP, and 8MP.
SDx series PTZ is up around 10-12watts.

 

[sebastiantombs]

I total the number of watts for every device, camera, switch, whatever, then add another 25% for "SWAG" and headroom to get a total. The load usually gets spread across at least two UPS supplies though both to prevent overload as well as redundancy.

 

[DsineR]

I never know what is appropriate when it comes to battery backup needs. My default for just about everthing is the number 1500 (Cyberpower, APC, etc).
But now is the time I should narrow down "expected" battery backup life in case of main power outage.
All will be on 110VAC outlets.
I do have one of those WATT watchers that keeps track of watts used for a device.
1.) I will have a Blue Iris windows machine with appropriate sized power supply (400w or such).
2.) 20-24 POE powered Dahua IP cameras (possibly 2 SDx series PTZ)
3.) Ubiquiti 24port POE switch

Is there a formula to figure out length of time for battery backup once I gather all the watts & amps?

The ubiquit POE switch I use right now for my POE cameras ranges from 2.5-3.5 watts between turrets & bullet 2MP, 4MP, and 8MP.
SDx series PTZ is up around 10-12watts.

No need to figure out all cameras load, go with the max load from your POE switch & you're covered, Plus PC, external HDD, etc.
Here's a decent calc from APC