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I will suggest to you to use a 175w or 250 watt inverter.
They cost about $20 (or less) and will have less loss then a 800w inverter.

The small size is a plus.

Most laptop power supplies use about 65w. But with a already charged laptop and low power use (screen brightness turned down mid way and low % CPU use) the charger will only drain about 18w to 24w.
Or so I have noticed with my laptop when monitoring it with a kill a watt meter. They seem to trickle charge while you are using it lightly.
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I use one

I have a 250w inverter mounted under the drivers seat , its wired into the car with a on/off switch plus its own self contained on/off switch on the inverter body.
It is always there ready but never seen by anyone.