Placing a good surge protector on our Fleck 4650 Control Valve (used as well water softener)
It's probably very low power, but you can be sure its less than 15 amps since it runs off a normal 120VAC 3 prong plug. So, get a normal (but quality) plug-in surge suppressor (single port if this is the only thing you are trying to protect) and plug it in - don't overcomplicate the process. While you are at it, buy a spare so that when the surge suppressor gives up the ghost you can replace it quickly. The manufacturers of quality suppressors often claim to cover damage to connected equipment, though it's probably a pain to get them to pay up in practice.

You might also want to go over the grounding connections of the power to the well shed, and check for basic details like "if the well casing is steel (or other metal, which would be uncommon) is it tied into the grounding electrode system properly"