How much more electricity would a fridge use outside?

If you keep it out of the sun, it probably won't be a significant addition to your electric bill. I have a large chest freezer in my garage, and it has barely added any cost even in summer weather. If you are concerned about the added cost, consider putting additional insulation on it.

Even assuming you'll have the internal temperature set the same, this still depends on environmental variables such as sun and wind exposure etc.

Keeping it in the shade where a breeze can remove excess heat will use less than keeping it on a sunny deck in an enclosed area.