My strobe will run a lot longer than the few minutes my exterior siren does.
I did a detailed power budget analysis of my system and found that the exterior speaker and the Uplink 4500ez GSM radio were the two big power hogs. I definitely did not want those powered by the main ELK M1G system, so I installed a separate Altronix eFlow4nx8 (12 VDC; 8 fuse-protected, Class 2 power-limited outputs. 2.5 Amp 12 VDC outputs; 3.5 Amp 120 VAC input, ELK 8 AH backup battery) to drive those two loads. I don't have a large system (1,680 sq foot house), but I needed two power supplies. I calculate 6 hours on the main ELK battery and 4 hours on the Altronix battery. These numbers are based on active current consumption, but obviously the outside siren has a cut-off time and the GSM radio doesn't broadcast continuously, so my real backup battery operating time would probably be a lot longer. The ELK strobe consumes 0.21 A activated and the exterior speaker consumes 0.7 A. The speaker cuts off after 3 minutes. The strobe would stay on a lot longer.