I have a 400w inverter on my truck for a laptop and printer. It will handle the printer just fine, but if I plug in the laptop and either leave it off to charge, or turn it on, the inverter will squeal like a pig. This is even with the printer off. So, obviously I need a bigger inverter. What do you guys use? And do you use a "pure sine wave" inverter or not? Seen any good deals on them? Brands?