How do you calculate Autonomy of a battery backup system?

These are the autonomy calculations

Autonomy = Battery Ah x Battery Volts = Battery Watt Hrs  * 50% battery discharge max = Battery Watt Hr Capacity / Ave Load in Watts = Autonomy Hrs

Ex:

Battery Ah = 180Ah

Battery Volts = 12V

Ave Load – 50W

180AH x 12V = 2160WHrs * 0.5 = 1080WHrs / 50W = 21.6hrs Autonomy