Whether you need an electric or petrol pressure washer will depend how far away from your mains supply you want to work and what power you need. If you are going to be a fair distance away from a power supply you’ll need a very long cable or possibly a petrol model.
Electric pressure washers are low maintenance, easy to use, easy to keep clean and can be used indoors due to their lack of emissions. They are fine for domestic duties, cleaning your car, bike, barbecue etc, but would struggle with really big cleaning tasks,
Petrol pressure washers can be considerably more powerful and are potentially more portable so are able to be used anywhere, completely free of cable restrictions and complications. The bigger machines are perfect for tough, outdoor cleaning but they should not be used inside due to the hydrocarbons they emit.