Here is a picture of the battery.

I intend to use this battery as a backup power source for my PTCL DSL modem. The modem which I possess is rated for 12V 1A DC. The actual consumption which I have measured using a digital multimeter is around 230mA. I'm trying to calculate the minimum capacity of a battery which would power the modem for 8 hours of load shedding.

Reading articles over internet, I've did some calculations. Please review

Capacity needed for powering up the modem for one hour = 230 mAh

Capacity needed for powering up the modem for 8 hours = 230 mAh * 8 = 1.84 Ah

Since a lead acid battery should not discharged more than 80%, thus capacity needed = 1.84 / 0.8 Ah = 2.3 Ah

Is this 2.3 Ah the required minimum capacity?

Assuming the power goes out for 8 hours consecutively, will a fully charged 2.3 Ah (rated @ 20hr discharge) SLA battery power the modem for 8 hours?