31 minutes
Reasonable load range: 15 minutes to 51 minutes
Calculation
300Wh × 85% usable efficiency ÷ (500W × 100% duty cycle) = 0.51 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 1000W continuous capability and about 1400W surge capability. Watt-hours alone do not prove compatibility.