battery power inverter for how long

battery power inverter for how long?
battery power inverter load acting through time: work length = battery capacity × battery voltage × 0.8 × 0.9 / load power
For example, a battery for a 12V 60A 220V 100W light bulb:
Working time = 12 (V) × 60 (AH) × 0.8 × 0.9 / 100 (W) = 5.18 (h)
Which 12V-- battery voltage; 60 (AH) - battery capacity; 0.8-- battery discharge coefficient (constant); 0.9-- inverter conversion efficiency; 100 (W) - Lamp nominal power; 5.18 (h) - calculated 100W bulb with 60AH battery chargers work time.