Small air compressors are still units that consume a lot of electricity (of course, that depends on what you compared them to), and this information should always be visible on the label/product page.
Manufacturers are usually transparent regarding how much electricity their small air compressors need. That means you can calculate how much your electricity bill grows once you use that compressor for various tasks.
When contemplating buying a small air compressor, check to see its amperage. Lower amperage means less electricity consumption.
If you have a 15-amp compressor that works with the 220-volt current, it consumes around 3,300 watts per hour.