How long will laptop run from UPS?
Re


The laptop has input: 18.5V, 2.7A
The UPS output is: 220V, 6.82A
and also something about 1500AP


These are informative but not directly related.  

Input is to laptop by charger.
 This IMPLIES that the cells are &lt; 2.7 Ah each - say probably 2.2 Ah LiIon 18650 cells
 BUT this is uncertain.  

6.82A is notional UPS output max current.
 They Are REALLY sayin that it is a 1500 VA or Watt inverter and the 6.82 was got by dividing 1500 by 220 . It will be approximate.



Question has many factors affecting it.

This is the general formula that applies and may be approached in several ways.


Time = Available_energy / rate of using energy
     = (UPS_battery_capacity_in_Wh x UPS_efficiency) / Laptop_Power_use



You can get a good measure of  (UPS_battery_capacity_in_Wh x UPS_efficiency) by operating a known load on a full charge and seeing how long it ran.

BUT you may guesstimate that 

 UPS  energy capacity ~~= Battery amp-hour rating x battery voltage x 0.8


or If battery Watt hour capacity is known.

 UPS  energy capacity ~~= Battery Watt-hour rating x 0.8


In both cases the 0.8 is an assumed efficiency of 80% from battery output to end use device.

__

The 18.5V, 2.7A is the laptop charger raing.
 More useful is the battery amp hour (Ah) or Watt-hour(Wh) capacity. ither or both are almost always shown on the battery.

Determine N from N x 3.6V ~+ battery voltage.

Then laptop battery energy capacity = either

energy capacity = Watt.hours shown on battery  


or

energy capacity = N x 3.6 x Battery amp hours capacity 


Run laptop from fully charged battery using conditions similar to those to be used when UPS is in use. If you do not know thse use conditions then you cannot answer the question.

Laptop Run Hours = LRH.

Then

Laptop_power = energy usage rate = Battery_energy_capadity (from above) / LRH


Then 

**UPS runtime ~~= UPS_battery_capacity_in_Wh x UPS_efficiency / Laptop_power**




EXAMPLE:

Made up data.

UPS Battery  = 12V, 30 Ah.  

UPS available energy = 12 x 30 x 0.8 = 278 Watt.hour


Laptop battery = 14.6V.

So N = 4 as 3.6 x 4 ~~ 14.6


Laptop battery capacity = 4Ah (on battery).

Laptop  battery energy capacity = 3.6 x 4 x 4ah = 47.6 Watt.hour -  say 48

Laptop is run from this battery and achieves 4.5 hours operation.

Laptop power use = Laptop battery energy / hours operaing   
 = 48/4.5 = 10.666 atts = say 11 Watts


UPS runtime - UPS energy/Laptop power  

 = 278 Wh/11 W ~= 25 hours


This is longer than a UPS will usually opera a computer system.
 This is due here to goodish battery in UP =30 Ah and rasonably low thirst laptop.

NB - note that I have taken UPS and laptop batery capacities as rated. If they are not new this will not be true.