What rating inverter should I buy?

Determine Continuous Load and Starting (Peak) Load: You need to determine how much power your tool or appliance (or combination of them that you would use at the same time) requires to start up (starting load), and also the continued running requirements (continuous load).

What is meant by the terms "continuous-2000 watts" and "peak surge-4000 watts" is that some appliances or tools, such as ones with a motor, require an initial surge of power to start up ("starting load" or "peak load"). Once started, the tool or appliance requires less power to continue to operate ("continuous load")

Helpful formulas: 
To Convert AMPS to WATTS:
Multiply: AMPS * AC (voltage) * η (efficiency of Inverter) = WATTS 
This formula yields a close approximation of the continuous load of the appliance 
To calculate approximate Startup Load:
Multiply: WATTS * 2 = Starting Load
This formula yields a close approximation of the starting load of the appliance, though some may require an even greater starting load.

NOTE: Induction motors such as air conditioners, refrigerators, freezers and pumps may have a start up surge of 3 to 7 times the continuous rating.

Most often the start up load of the appliance or power tool determines whether an inverter has the capability to power it.