I have one of those emergency
starter units on my boat - battery clamps, 2x12V outlets and a 400W inverter with 2 standard ac outlets all in one compact unit. Bought it from Sears, $125 as I remember. The battery provides 10A/hr.
I keep it onboard as a backup for the starting battery, as well as to power any power tools, etc.
If Stillraining's calculations are correct, it would power 3K lights for 1 hour, or 1K lights for 3 hours - plenty for the parade I've signed up for mid-Dec, and all without messing with the house/starting batteries onboard.