If you need 1000W for a fast brew it doesn't matter if it's 12V or 120V, you'll still be drawing 1000W.
So, 12V input, 1000W output means 1000W/12 = 83Amps
Use an inverter to get 1000W output and it will draw 1000W (actually more due to conversion losses) from your 12V, so still 1000W/12 =...