How to Winterize a Portable Generator: Step-by-Step (2027)

Winterizing a portable generator means two different things depending on your plan: prepping it for winter storage (fuel, oil, battery, clean and covered) or prepping it for winter operation (right oil weight, fresh fuel, and a storm-ready setup). Most “my generator won’t start” spring stories I hear trace back to skipping this 30-minute job in the fall. Here is the full procedure for both tracks.

I live where winters are real, and I have made the classic mistake: parked a generator in October with a half tank of untreated gas, then spent a Saturday in January rebuilding a varnished carburetor in a cold garage. Never again. The winterizing routine below is what I do every fall now, and my generators start on the first or second pull every spring.

Track 1: Winterizing for storage

If the generator is going to sit from roughly November to March, do all of these. Budget 30–45 minutes.

1. Deal with the fuel (the critical step)

You have two good options. Pick one — do not half-do both.

Option A — Stabilize: Add STA-BIL Storage fuel stabilizer (1 ounce per 2.5 gallons of gas) to a full tank, then run the generator for about 10 minutes so treated fuel reaches the carburetor and fuel lines. STA-BIL claims up to 24 months of protection; in my experience, 6–9 months of winter storage is well within its comfort zone. A full tank also minimizes air space and condensation.

Option B — Drain completely: Turn the fuel valve off, run the engine until it stalls, then drain the carburetor bowl (there is usually a small drain screw at the bottom of the bowl — put a rag underneath). Some fuel always remains after stalling, so the bowl drain is the step people skip and regret. Drained dry, there is nothing to varnish.

I stabilize when the generator might be needed mid-winter (storm country), and drain when it is truly done for the season (camping-only units). Both work. What does not work is a half tank of untreated gas — that is a carburetor rebuild waiting to happen.

2. Change the oil

Change it before storage, not after. Used oil contains acids and moisture from combustion; letting the engine sit for months on dirty oil etches bearings. Drain it warm, refill with fresh oil — and if the generator will see any winter use at all, make it 5W-30 instead of 10W-30. Our oil change interval guide has the full schedule, but the winter rule is simple: thinner oil starts easier in the cold.

3. Service the air filter

Pull it, tap out the dust, wash foam elements in warm soapy water, dry completely, and re-oil lightly if it is a foam type. A clogged filter makes cold starting noticeably harder.

4. Protect the battery

Electric-start models: fully charge the battery, clean the terminals (baking soda and water, then a light coat of dielectric grease or petroleum jelly), and either connect a smart battery maintainer for the winter or plan to charge it monthly. Cold slows the chemical reactions in a lead-acid battery and increases self-discharge — a battery that held a charge all summer can be flat by February. I keep a maintainer on mine all winter; it is the single most reliable no-start prevention I know.

5. Fog the cylinder (optional but smart)

Pull the spark plug, squirt a short burst of fogging oil into the cylinder, and pull the recoil slowly a few times to coat the bore. Then reinstall the plug. This prevents surface rust on the cylinder wall during months of disuse. It takes two minutes. I do it on every unit I store longer than three months.

6. Close everything up

Fuel valve OFF. Choke off. Clean the exterior — mouse nests in the cooling shroud are a real spring surprise; I stuff a rag in the exhaust outlet and the air intake to discourage tenants (remove them before starting, obviously — I tie a bright ribbon on mine as a reminder). Store the generator upright, covered, in a dry, ventilated spot: a shed or detached garage is ideal.

Track 2: Winterizing for winter operation

If your generator is your storm backup and might run in January, winterizing means making sure it starts in January:

• 5W-30 oil, fresh. Cold oil is the #1 cause of hard winter starts. If your manual allows 5W-30 (most do), switch for the season.
• Fresh, treated fuel. Fill with fresh gasoline plus stabilizer now, not in the middle of a storm. Ethanol-blended fuel older than 30 days is suspect; I date my gas cans with a marker. Size your winter stockpile with our fuel storage planning guide so you are not hunting for gas mid-blizzard.
• Monthly test runs. Start it on the first Saturday of every month, run 15–20 minutes under a moderate load (a space heater works). This charges the battery, circulates oil, and — most importantly — proves it starts before you need it.
• Stage the setup. Know exactly where the generator will sit: outdoors, at least 20 feet from doors, windows, and vents per CDC guidance, exhaust pointed away from the house. Keep a long, heavy-gauge extension cord or your inlet cable accessible — not buried behind the Christmas decorations.
• Cold-start technique. Full choke, no load, and give it 2–3 minutes of warm-up before plugging anything in. Cold engines stumble under sudden load; a brief warm-up prevents the stall-surge-stall dance.

Propane and dual-fuel units

Winter is where propane earns its reputation. Propane does not go stale, varnish carburetors, or care about 30-day shelf life — it stores indefinitely. If you run a dual-fuel generator, winter is the season to favor the propane side: fill the gas tank with stabilized fuel as a backup, but plan to run on propane. One caveat: in very cold weather, small 20-lb cylinders can struggle to vaporize enough propane for a big generator under heavy load — the tank gets cold, pressure drops, and the generator starves. If you heat with propane anyway, a larger tank solves this; for portables, keep the cylinder out of the wind and do not expect full rated output from a frosty 20-lb tank.

Spring recommissioning (2 minutes)

When winter ends: remove the exhaust/intake rags, check the oil level, confirm the fuel valve opens, and start it up. If you stabilized, the fuel is fine. If you drained, fill with fresh gas. Run it 10 minutes under load. Done — this is the payoff for the fall work.

Should I run my generator out of gas before storing it for winter?

Either run it dry (fuel valve off until it stalls, then drain the carburetor bowl) or stabilize a full tank and run 10 minutes to distribute the stabilizer. Both work. What fails is storing it with untreated, partially full fuel.

What oil weight should I use in my generator in winter?

5W-30 — conventional or synthetic — for temperatures below about 40°F. It flows much faster on cold startup than 10W-30. Check your manual, but nearly all portable generators approve 5W-30 for cold weather.

How long does stabilized fuel last in a generator?

STA-BIL Storage claims up to 24 months. For practical winter storage of 4–6 months, stabilized fuel is completely reliable as long as you added the stabilizer to fresh fuel and ran the engine long enough to get treated fuel through the whole system.

Should I disconnect the battery for winter storage?

You can, but a better approach is leaving it connected to a smart maintainer, or charging it monthly. A disconnected battery still self-discharges — slower, but it still goes flat over a winter. Store batteries cool (not freezing) and fully charged.

Can I store my generator in the garage over winter?

A detached garage or shed is fine. Never store a fueled generator in an attached garage or in the house — gasoline vapors can travel to ignition sources like water heaters and furnaces. Drain the fuel if indoor-adjacent storage is your only option.

Thirty to forty-five minutes in the fall — fuel handled, fresh winter-weight oil, battery on a maintainer, unit cleaned and covered — is the difference between a generator that fires up on the first pull in March and one that eats your Saturday. Do it once, make it a habit, and winter stops being generator-killing season.