Just picked up two 400w power inverters from Canadian Tire (one for each car).
Neat little things... I've had a 350w one in the 91 for a while, but these new "digital" ones can give you a number of readings, including...

-displays amount of watts the attached electronic device is drawing
-amount of voltage remaining in the battery (main reason for buying them)
-and fault codes

They also have an autoshut off, so it won't kill your car battery.
They come in 200, 400 and 800 W power levels.
I bought the 400w one because it had 2 plugs.

Here'* the link for the 200w one incase anyone is interested...
http://www.canadiantire.ca/assortmen...rtment=primary

I have an inverter in my truck and absolutely love it. I use it to charge batteries for my cordless drill and for a laptop used for navigation.

I like the digital one you described. Battery voltage is important to know. Can we get a pic of the readout?