It is the actual load watts, not the inverter rating that counts. So a 1500W inverter with a 500 Watt load would be 50 (25) Amps, not 150 (75) Amps. The same inverter with a 1200 Watt load would draw 120 (60) Amps. How long will my battery last with an inverter load of 1000 Watts?