In order to calculate your power requirement add up the wattage's of the tools appliances and items you want to use with your inverter. Keep in mind there are items that have resistive or constant loads and those that have reactive loads. For constant load items simply add up the Amps and multiply by 240 to find out the Watts you need. Reactive load items require additional power when they start up. The product's nameplate should list both the operating amps and the maximum amps.
When you do not have access to the nameplate for reactive load products then as a guide use the rated Amps or Watts multiplied by 7 to arrive at the total required at start up. For example - a 200W power tool will need a 1400W inverter a 60W fridge will need a 420W inverter and so on. It is also recommended to get an inverter with at least 30% more than your needs.
Australian Standard AS4509.2 states a surge factor of 7x the continuous power should be used for electric motors water pumps fridges air conditioner and washing machines. A surge factor of 3x should be used for kitchen appliances halogen and LED lights and other electronics equipment.Read more