If the wattage is too low, the Dells sometimes refuse to charge. You will see the charge light blink instead of staying a steady color. If I use a standard 90 watt unit on my m6400, which uses a 250 watt brick, it will work only now and then. Sometimes it blinks, other times it will charge and the laptop gives me a warning that the unit isn't charging properly and I need the proper one. Make sure the output matches the rating of the old unit.