How much (and how quickly) will running appliances on my Honda Ridgeline's power outlet drain my battery?

I am looking into using two speakers that use around a total of 800-900W for about 2-3 hours. They will be plugged into a 120W adapter from my 12V power outlet in my 2007 Honda Ridgeline. If the vehicle is off and running this then how long will the battery last? Would it work if the engine was on? I'd love to see a formula as to how to equate this, but I have no clue what it would be.

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