The first thing you need to know is that 2.5 amps at 20 volts is the MAXIMUM draw, it might average as little as 0.5 amps at 20 volts.
Only one way to be sure.. Meter it.. In this case I recommend using a Kill-a-watt meter on the wall plug side.. This is about a thousand dollars worth of meters in a 30 dollar box (A quick google search found it ranged 10-40)
Measures
Volts
Amps
Frequency
Volt-Amps (Not the same as watts)
Watts
Power factor (The relationship between the last two items)
And Think one more thing (Running time)
Oh KWH (Killowatt watt hours or watt hours that's the other thing)
To figure out what the battery needs to provide assume 10 volts after losses
So, if the computer draws 50 watts.. or 50 volt amps (use the larger) the batteries need to push 5 amps to the inverter.
