Redundancy is not paranoia, it is arithmetic
On grid, a failure is an inconvenience because somebody else's spare capacity covers you within hours. Off grid there is no somebody else. The question is not whether a component will fail but what happens the week it does, in the season when it is most likely to, at the distance you live from a replacement.
That reframes the spending. A second, cheap, independent way to do a critical job is usually better value than a more expensive version of the first way. Two modest heat sources beat one excellent one. A hand pump beats a better electric pump. The principle is independence: the backup should not share a failure mode with the primary, and above all should not share the electrical system.
Ranking failures by what they actually cost you
Rank by consequence and by season. Loss of heat in deep winter risks the building itself through frozen plumbing, so it ranks first in a cold climate. Loss of water ranks next, because the timescale is days and because a freeze can turn an outage into damage. Loss of refrigeration threatens food and, for some households, medication. Loss of lighting and communications is uncomfortable and matters most for safety and for calling out.
Then ask what a failure would cost in time as well as money: is the part on a shelf within an hour, a special order, or an item with a long lead time? Anything with a long lead time deserves a spare on the shelf, regardless of how reliable it is. That is the whole logic of the spares list on this site.
Finally, ask what fails together. A power outage takes the pump, the heat-trace cable, the furnace fan, and the internet all at once. Designing around single components while ignoring shared dependencies is the most common gap in otherwise careful systems.
Generators: the honest backup
A generator is the standard bridge for a run of dark days, for a heavy intermittent load, and for the recovery after a deep discharge. Inverter generators are quieter and cleaner for electronics; conventional units are cheaper per watt for heavy work. Dual-fuel models let you keep propane, which stores indefinitely, alongside gasoline, which does not.
The recurring failure is not the generator, it is the fuel and the neglect. Gasoline degrades within months without stabilizer. Carburetors gum up when a machine sits unused. The generator that has not been started since last winter is the one that will not start this winter.
Run it under load monthly, change the oil on schedule, keep spare plugs and filters and a spare pull cord, and store fuel safely and legally in appropriate containers. Size it to charge the battery bank efficiently rather than to run the entire house at once, since a large generator lightly loaded is inefficient and wears badly.
The spares that earn their shelf space
Electrical: fuses of every value in the system, at least one spare charge controller if the system depends on a single one, wire, connectors, terminal lugs, and a multimeter you trust. A spare inverter is expensive but worth considering where one unit carries the whole house.
Water: pump seals or a complete spare pump for a critical well, pressure switch, pipe, fittings, tape, clamps, filters in quantity, and the tools to pull and reset the system in the cold.
Heat: gaskets and rope for the stove door, firebricks, a spare chimney brush, and for propane appliances a spare thermocouple or igniter, which are cheap parts that reliably fail on cold mornings.
General: fuel filters, oil, belts, a good tarp, fasteners, and the specific fittings your systems use rather than the ones a general list suggests. The spares reference on this site is organized by system so you can print the sections that match what you actually own.
Skills and documents are also backups
The best spare part is useless if nobody in the household knows how to fit it. Walk everyone through shutting off water, disconnecting the battery, lighting the stove safely, and starting the generator. Do it once in daylight, and it will be available at two in the morning.
Keep a systems binder: model numbers, serial numbers, settings, wiring diagram, well report, septic as-built, dates of every service, and the photographs of the walls before they were closed. Keep it on paper as well as digitally, because a digital copy behind a dead router is not a backup.
Keep offline copies of manuals and one printed reference for each system. The download pack on this site includes a systems binder and a maintenance calendar you can print, and the maintenance reminders tool will email you the seasonal jobs so they do not depend on memory.
Testing, which is the part that gets skipped
Untested backups fail at exactly the moment they are needed, and they fail quietly until then. Put the tests on a calendar: start the generator monthly under load, light the backup heater at the start of each cold season, run the hand pump, check the fire extinguishers, test the carbon monoxide and smoke alarms, and exercise any valve you would need in a hurry.
Once a year, do a deliberate drill: shut off the primary system and live on the backup for a day. It is uncomfortable, it is educational, and it finds the missing adapter or the seized valve while the shops are open and the weather is fine.
Write down what the drill taught you and fix it before the season. A backup plan that has been rehearsed once is worth several that exist only on paper.
Communications, and being able to call for help
Connectivity off grid is a safety system, not just a convenience. Cell coverage at a rural address is frequently poor or absent, and the internet path that works beautifully in fair weather may be the first thing to go in a storm.
Plan two independent ways to reach the outside world, ideally on different technologies: satellite internet and a cell signal booster, or a landline where one still exists, or a radio option where geography defeats everything else. Keep the router and terminal on the battery bank rather than on a generator-only circuit, since the moment you most need to call out is likely to be during an outage.
Agree a check-in arrangement with someone off site, particularly if anyone is alone on the property for stretches. And keep a written list of emergency numbers that does not depend on a charged phone, including the non-emergency line for the county sheriff, the fire department, the power company if you have any grid tie, and your nearest neighbor.
Buying spares without buying a warehouse
The shelf can absorb an unlimited amount of money, so it needs a rule. A useful one is to stock the part when three things are true at once: the component is critical, the failure is plausible within a few years, and the replacement is either slow to obtain or specific to your equipment.
That rule buys thermocouples, pump seals, fuses, gaskets, and filters, and it declines to buy a second of everything. Generic hardware that any rural store carries does not need a spare; a proprietary controller three weeks out from a single supplier does.
Review the shelf once a year against what you actually own, because systems change and a spare for equipment you replaced is just clutter. Note what you used, replace it immediately rather than after the next failure, and keep the list in the systems binder so somebody else could find the part without you.
