An inverter takes 12-volt DC power from your RV batteries and converts it to 120-volt AC to power your standard appliances, like your microwave or hair dryer. To size a power inverter you must first determine what your largest single output or load would be at a given time. For example, if you are running a 700 watt microwave and a 800 watt hair dryer (700+800=1500 watts) at the same time the total output wattage would be 1500 watts. To size an inverter properly, you would need to give yourself a bit more overhead above this value. A 2000 watt inverter would be recommended in this case
There are two different types of inverters sold. Modified and Pure Sine wave models. Modified inverters tend to not produce the same quality of an output sine wave as pure sine wave models. They are less expensive but may have issues running sensitive electronics.
Pure sine wave models are more expensive because they have been designed to run the sensitive electronics that the modified wave units will not. The sine wave is similar to what you see when plugging into a standard household outlet.
Examples of specific load sizes:
Examples of AC loads that don't require Pure Sine power: