I'm planning on having a computer in my car, but it has a 300w power supply. So I may have to use a power inverter to power that thing. Does anyone know if having a power inverter on for long periods of time would do harm to the car?

 

Depending on the draw you may need to hook it up directly to the battery with some big wire. Some inverters come with 2 sets of hookups. Once for the lighter/power outlet and another that can be directly connected to the battery.

Since the cheap (less than a few hundred $) inverters we use in cars put out pretty noisy sign waves (Modified Sine Wave) there can be some applications that are not appropriate. For example, those little plug in NI-CAD chargers that plug directly into the wall aren't supposed to be used with cheap inverters. You may want to check about your computer's "brick" power supply and if others have used inverters successfully with it. That's more for a computer BBS.

Also, read the inverter manuals. Mine was pretty good about do's and don'ts and when the bigger wires were required. In the end, you may blow some fuses if you overload the lighter/power outlet.

If you really need all 300W you will need to run some wire and watch the battery. If you do that right you should be OK.

 

Adobeman is right,
If you are going to require 300w you should definitely hard wire direct to your battery. Also mentioned, look for a good "Pure sine wave" inverter for cleaner power - better for computer applications etc.

The only harm it could cause is to your battery if left operating with the car not running. The obvious - dead battery would/could result. Many of the better inverters have auto shut-off when they sense a battery drop to around 10-11 volts. Most will at least give you an audible tone when your car battery is getting low.. Never try to start your car if your inverter is on and in use. Using it while driving/the engine running is perfectly safe etc. (unless you're surfing Acurazine at the same time...)

Overall,
--Try to get a pure sine wave inverter
--Check your computer power brick etc. and ensure the inverter "exceeds" the computer requirements. IE, if your computer requires 300 watts at 110 volts I would look for an inverter with at least 350-400 watt capacity. Many inverters will claim 300 watt peak or surge but will not work if expected to provide that amount continuously. Just read/comprehend the specs on both devices.
--Even though direct wiring it to the battery, always install the appropriate inline fuse. Once again, as mentioned by Adobeman, a 30 watt hi draw capacity fuse will be required for 300 plus watts continuous use. You can't just use a typical 30 amp car fuse. Research the hi draw fuses used in car audio/large amp installs. They work great and are relatively cheap.

Good luck