378 hours
Reasonable load range: 212 hours to 680 hours
Calculation
20,000Wh × 85% usable efficiency ÷ (45W × 100% duty cycle) = 377.78 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 80W continuous capability and about 100W surge capability. Watt-hours alone do not prove compatibility.