425 hours
Reasonable load range: 170 hours to 850 hours
Calculation
20,000Wh × 85% usable efficiency ÷ (40W × 100% duty cycle) = 425.00 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 100W continuous capability and about 120W surge capability. Watt-hours alone do not prove compatibility.