Generator size for a well pump is the question that decides whether your generator is useful or ornamental, because on a well, losing power means losing water, and the pump is the hardest thing the generator will ever start.
The mistake is nearly always the same one: sizing against running watts, when the number that matters is the surge at start.
Why the pump stalls small generators
A submersible well pump is an induction motor, and induction motors draw several times their running current for the moment it takes to spin up. A pump that hums along at under a kilowatt can demand three, four, five times that for the first second. A generator sized to the running load meets that surge, sags, and either stalls or trips, usually with the fridge and the furnace already on it.
This is why the sticker math fails. Adding up the running watts of everything you want to power tells you about hour two. The pump decides minute one.
What actually sets generator size for a well pump
Three numbers, all findable without pulling the pump. The horsepower, on the pressure tank paperwork, the breaker label, or the well record. The voltage, almost always 240 for a submersible. And what else must run at the same time, because the furnace or heat pump air handler, the fridge and some lights are all realistically on when the pump starts.
From horsepower, the starting demand can be estimated closely enough to size honestly, with the surge capacity of the generator, not its running rating, carrying the pump start. As rough shape: the common 1/2 to 1 HP household submersibles land most owners in mid-size portable territory once the rest of the house is counted, and comfortably inside any properly sized standby unit. The right generator size for a well pump comes out of a short calculation we do from your pump’s numbers, and it is part of every generator quote we write for a house on a well.
Portable or standby, on a well specifically
A portable with a proper inlet and interlock covers the pump, the fridge, the heat and some light, at the cost of you being home to start it. A standby unit starts itself, which matters most exactly where outages run longest, in Fall River, Hammonds Plains and the other communities where the lines run long through trees.
Either way, the connection is the non-negotiable part. A generator feeding house circuits must go through a transfer switch or interlock, both because back-feeding can kill a lineworker and because the pump circuit is exactly the kind of 240 volt load people are tempted to improvise. Do not improvise it.
The five-minute version
Find the pump’s horsepower. Assume the start is the sizing event. Count what else must be on. Buy the surge rating, not the sticker. And have the generator inlet and interlock installed properly once, so the next outage is an inconvenience instead of a dry house.