202 hours
Reasonable load range: 121 hours to 347 hours
Calculation
10,000Wh × 85% usable efficiency ÷ (120W × 35% duty cycle) = 202.38 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 200W continuous capability and about 1000W surge capability. Watt-hours alone do not prove compatibility.