My Dell laptop came with two chargers, one 90 watt charger for fast charging and one 65 watt for travel.

Most laptops will exceed 70 watts only if you are running the computer and charging it at the same time.
One way to get around the 70 watt limitation is to run the laptop off the battery and then switch it off and charge it from time to time (for example during meal service) nand that way you can make it through the whole flight.
Some people remove the battery and run the laptop using the charger only to get below the 70 watt limit, but if there is a power glitch your computer will turn off instantly which could screw up your harddrive.

I also have an extra battery that goes into the DVD drive bay in my laptop so I usually get 6-7 hours of operation when running on the battery only.