series to work with the car's 12 volt electric system so I would get 2.4 watts for 50 bucks. Regardless, 2.4 watts would only extend my time a few hours. Total draw while the car is off is nearly 20 watts. I should probably just work on the efficiency of my existing setup... like killing the USB devices on the PC when it's off. No matter what measures I take to extend battery life, I will still have the auto-start on low voltage feature as a backup.