It really depends on what you're going to be powering with the inverter.
My laptop powers off one of those small 75watt inverters fine. I ran a PS2 and a laptop off of a 550Watt one plugged into a power port/cigarette lighter with no problems.
One thing to keep in mind, because the higher wattage you go, it will depend on where you plan to draw the power from. For example, the power ports will handle up to 20 Amps of current....if you consider the battery provides 14 volts when the engine is running, then that equates to 280 Watts of total available power. With the engine off, the battery voltage drops down to 12 volts....so that's 240 Watts of available power. You can't get more Watts out of the inverter, than what you put in to it. So if you get anything above 280 Watts, you will need to directly connect to the battery to be able to use all that available power.
Last edited by Rednroll; 02-28-2010 at 12:12 AM.