I definitely do not think it a good idea to run an inverter on a bike, but I imagine if someone needed their laptop, or something similar, having it charge while riding the bike would be fine. Would not want to use one above 350 watts. I'm just saying, anytime an inverter is used the vehicle preferably should also be running and above 1500 rpm's.

As mentioned by others, anytime one is plugged in it uses usually .5 -1 amp, with no load. Fortunately most do have a shut down that activates around 9.5 volts, and usually start warning you around 10.5 volts.

I am also pretty sure that the charging system (at least on mine) is 50amps/3phase, and the battery should be a 12v/28Ah. Others are less.

 

I have a few inverters from 140W to 3500W. I have actually used the 140W on my bike a couple of times to charge laptop, electric razor, and cell phone while riding. Never had a problem. Make sure that you wire it to a switched lead so it shuts off all power when ignition switch is off. Every time you start the bike it is likely that you will have to cycle the on/off switch on the inverter because of the voltage drop when you hit the starter button.
The cheaper modified sine wave inverters hardly ever will do anywhere near what they claim for output for more than a nanosecond. Their efficiency is not great either ~ 80% or so. They also like ventilation. A few issues, none insurmountable, just temper your expectations accordingly.
Captsluggo

 

My 2008 has a 50 amp alternator, or 625 watts.

To get 625 watts out at 120 volts, the converter needs at least 625 watts in. 625 watts in, at 12 volts, is 50 amps.

This is not engineering, it is just a formula, watts is amps times volts, amps are watts divided by volts.

 

Why not just get a DC power adaptor for your laptop? A lot smaller.
I bought one when I got my notebook for use in our truck camper.
This was back when WIFI wasn't readily available. We'd sit in parking lots of shops and motels and upload/download e-mail.
Works as well as AC.

 

Your formula is entirely correct. The engineering aspect comes in when you figure the consumption from everything else involved, Fuel injection uses more power than carburetors do, all of the lights consume power, when you hit the brakes you use more power still, and even a short piece of wire has losses involved. Add that a 50 Amp alternator probably won't last as long fully loaded all the time and that manufacturers routinely overstate the real world capabilities of their products and all of a sudden you're an engineer.

 

Go and google Ohm's Law. It will make all of this a lot easier once you see the math and the relationship between current and voltage. Most people don't realize that 90% of the stuff we have in this world runs off of low voltage DC. Almost everything has some type of a power inverter in it or some type of rectification to get this low voltage where it is needed. Problem solved. Take your inverter and trash it and buy the proper 12 V adapters and all will be fine. Simple fix.

 

I'm just going to drag a generator behind me on the bike. Problem solved.