DoD is the percentage of battery capacity used before recharging. Using only 50% DoD doubles battery life compared to 100% DoD. For lead-acid batteries, 50–80% DoD is recommended.
Required Ah = (Load in W × Hours) / (Voltage × DoD × Efficiency). For 500W, 2hrs, 12V, 80% DoD, 85% efficiency: Ah = 1000/(12×0.8×0.85) = 123 Ah.