Ohm's Law my man.

840 watts / 14.4 volts = 58.33 amps

You should have about 30 extra amps for things like charging the battery, running lights, etc. I don't know about your car but my stock alt is only 65 amps. Also, 58 is a conservative rating because amplifiers are not 100 percent efficient! If your amp's current draw isn't specified in the documentation that came with it, look and see if there are any fuses on the side. Add up the values of all the fuses and that will give you an idea of the current draw you're dealing with.

BTW, is that 840 watts peak or RMS?

If that's a peak rating, you're looking at the following:

((840 watts) / square root of 2) / 14.4 volts = 41.2 amps

I have 2 amps, 3 subs, and a 1 farad cap in my truck (300 watts RMS total) and I went from the stock alt of 65 amps to an aftermarket high output alt of 130 amps and I was amazed at the performance increase in the bass response of the system. Then again, I don't have a digital mono sub amp.