If you have a campervan and want to use appliances that need electricity, you will need to pick the right size for your inverter. An inverter is a device that converts the electricity in your van from DC into AC which is required with certain appliances. With inverters you have to make sure you choose the correct voltage your system. For example, if you have a 12V inverter, it means it works with a 12-volt system.
When you look at Victron inverters, you will see two numbers. The first one is about the voltage (like 12 for a 12V inverter), and the second one is about the most electricity it can handle, which is measured in watts. To figure out the right size of inverter for your van, add up the watts of all the appliances you want to use at the same time. It's a good idea to add a little extra to make sure your system will work well in the future.
If you only have small appliances, a 500W inverter is a good choice because it doesn't cost much more than smaller ones. Make sure that you are careful not to use more electricity than the inverter can handle, or it might break.
One important thing to note is that some companies use watts to talk about inverter power, but Victron uses volt-amperes. This can be confusing, but it's important to understand the difference. Watts tell you the real power, while volt-amperes describe the apparent power. If you want to know more, you can read an article that explains these differences and how they affect your choice of inverter or inverter/charger.
However for simplicity 1VA = 1W.