1020 hours
Reasonable load range: 510 hours to 2040 hours
Calculation
18,000Wh × 85% usable efficiency ÷ (300W × 5% duty cycle) = 1020.00 hours.
What can change the result
Actual device wattage, battery age, ambient temperature, inverter standby draw, conversion losses, power-factor behavior and cycling frequency can all reduce or extend runtime.
Check output before connecting
The battery or UPS needs at least 600W continuous capability and about 800W surge capability. Watt-hours alone do not prove compatibility.