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Dryclean, you might want to check the inverter power consumption ratings.
Most inverters are efficient only when running at something like 90% of their rated continous power.
So, if your laptop draw 20W (which I assume means 40W if charging and running at the same time) you might find that connecting it to a 75W or 100W inverter, actually draws significantly less power than connecting it to a 250W inverter. Possibly 1/3 to 14/ less power!
Then take a look at a Kensington or other direct 12-volt adapter for your laptop, and you may find that again, using a single 12-volt "car and place" power adapter draws HALF the power that you use with an inverter plus your regular laptop power supply.
Total difference? Could be 50-75% less power being used if you just use one correct 12-volt power supply for the laptop.