68.0 hours
Reasonable load range: 34.0 hours to 136 hours
Calculation
800Wh × 85% usable efficiency ÷ (10W × 100% duty cycle) = 68.00 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 20W continuous capability and about 25W surge capability. Watt-hours alone do not prove compatibility.