G'Day Arthur,
Calibos beat me to it.
I was going to suggest a 12V air/travel plug pack to go direct from the 12V battery to the lappy and skip the inverter and normal plug pack.
19.5V x 4.7A / 12V = 7.6A ... say 8 or 9 A with losses.
9A x 15h = 135 Ah, but that draining the battery to 100%.
You probably want to drain the battery no more than 50% to give it reasonable life, so that means about a 270 Ah battery!
What sort of battery life are you getting out of your laptop battery? 2-3 hours?
Let's say 3 hours on the lappy battery and assume 8A @12V (might be a bit light on
) then 8A x 12h / 50% = 192 Ah... say a 200 Ah battery.
I have just killed my 37Ah battery for the lappy after about 2 years. I probably only got about 40 or 60 recharge cycles out of it because I obviously used it well below 50%...
... so I have been doing some homework on making the observatory solar powered, but in the end I have decided that because of the lappy power drain, I'll just put the power on...
Al.