What kind of car power inverter do I need if I want to run a laptop and two hard drives?

Question

I can't find how many watts they use. Can I just plug in a surge protector into a 400w inverter?

Answer 1

My laptop power supply says 144W in (120V x 1.2A) for about 80W to the  laptop (19V x 4.22A).  One of my hard drive supplies is 120W (120V x 1.0A) in and 30W out (24V x 2.5A).

This looks like a 144+2x120 = 384W need for a laptop + 2 hard drives.  I bet the real draw is lower but would be more than 80+2x30 = 140W.  Adding margin for efficiency would mean you need between 170W and 460W.  You should be able to find the correct numbers for your case on the UL stickers.

The 400W inverter will draw more than 400W, say 480W (depends on efficiency).  At 12V this would be 40A.  Plugging this into the cigarette lighter will blow the 15 or 20A fuse.  It has to be wired directly to the charging system unless you already have a 40A connection available.  200W is probably the limit if you're going to plug into a cigarette lighter (yes, I know it's not the cigarette lighter these days.  I just can't remember the name)

There are two types of inverters.  The cheap ones have a lot of distortion and the power supplies may not work well with them.  I don't know if this is a problem here, but otherwise you have the right approach.

Answer 2

If you mean a USB case with hard drive in it it should come with its own power supply

This is the only way you can run external hard drives