You're right that there is an electricity cost to solving this type of captcha - the same as there is an electricity cost to loading 2MB of JS+images and clicking the pictures with the fire hydrants (and the infrastructure behind that). It's hard to estimate how they compare (and what value you assign to the human labor performed and privacy loss).

20 seconds would be a fairly high difficulty. It's up to the site owner to decide what makes sense for them.

If anybody comes up with a useful computational task with a small bundle size that can be verified cheaply that would be the holy grail - until then the computation is only there as a form of hashcash.