Maintain a generator and store fuel legally

The two ways a stored generator fails

Almost every generator that lets someone down on the night fails in one of two ways, and both are entirely preventable.

The fuel went off. Modern petrol has a working life measured in months. Left in a tank and a carburettor for a couple of years, it turns to varnish, blocks the jets, and the engine will not run even though everything mechanical is perfect.

Nobody ever ran it. It was bought, tested once in the garden, and put away. Rubber perished, the battery went flat, the oil drained off the bores, the carburettor gummed.

The equipment did not fail. The habit did. Everything on this page is about the habit.

How much fuel you may legally keep

Thirty litres is the practical ceiling for most households, and it is worth being clear about what that buys you. A typical portable generator burns somewhere in the region of half a litre to a litre an hour depending on size and load, so 30 litres is roughly 30 to 60 hours of running.

That is one to two weeks of a few hours a day, and no more. Anyone planning on a generator as their primary power source for a long outage should look at that number carefully, because it is usually smaller than the assumption.

Which is the strongest argument in this whole domain for a battery bank. A generator charging a battery, run for an hour a day, stretches those 30 litres to a month or more and gives you power around the clock rather than only while it is running. See sizing a 12V system.

Where fuel goes, and where it must not

• A detached shed or outbuilding, ventilated at low level so vapour can escape rather than accumulate.
• Out of direct sun and away from anything that gets hot. Heat drives vapour off and pressurises containers.
• Off the ground, on a shelf or bunded tray, so a leak is contained rather than spreading.
• Away from ignition sources: no boiler, no pilot light, no electrics that spark, no welding, no smoking.
• Proper containers only. UN-approved petrol cans, correctly labelled, with intact seals. Not water containers, not milk bottles, not anything that once held something else. Petrol attacks the wrong plastics and the container will fail.
• A fire extinguisher outside the door, not inside, so you can reach it without entering.
• Tell the household where it is. Both so they avoid it and so the fire service can be told if it ever matters.

Diesel is far less volatile and much safer to store, which is a real and underrated argument for a diesel generator if you are choosing one. It does not produce an explosive vapour at ordinary temperatures.

Petrol goes off faster than you think

This is the fact that catches almost everyone, because the intuition is that fuel keeps.

Ethanol is the problem. Modern petrol contains it, and ethanol is hygroscopic, meaning it absorbs water from the air. Once it has taken up enough, phase separation occurs: the ethanol and water drop out of solution and form a layer at the bottom of the tank, with a fuel above that no longer has the properties it was blended to have.

What that does to a small engine:

• The water layer is what gets drawn into the carburettor first, because the pickup is at the bottom.
• The remaining petrol has lost octane and burns poorly.
• Ethanol attacks older rubber and plastic components, and small engines are full of them.
• The residue that dries out in the jets is varnish, and it is not something you can dissolve with a shake.

Buy fuel with less ethanol where you can. Higher grade petrol in Britain generally has less than the standard grade. Alkylate petrol, sold for chainsaws and garden machinery, contains no ethanol at all and stores for years. It costs several times as much and comes in small cans, but for a generator that must start after two years of sitting, it is genuinely the right answer for the first tankful.

Add stabiliser to fresh fuel, not to old fuel. Stabiliser slows degradation; it does not reverse it. Adding it to fuel that has already sat for a year achieves very little.

Diesel is different, and has its own problem

Diesel stores better, is far safer, and does not phase separate. It gets diesel bug instead.

Microbial contamination grows at the interface between the fuel and any water that has collected at the bottom of the tank. It forms a dark sludge that blocks filters, and once established it is difficult to remove.

• Keep tanks full to minimise the air space where condensation forms. This is the opposite of the advice for a petrol tank that is being laid up.
• Biocide roughly every six months for long-term storage.
• Drain water from the bottom of the tank periodically if it has a drain.
• Keep it sealed and at as stable a temperature as you can manage, because temperature swings drive condensation.
• Change fuel filters and carry spares. In a contaminated system the filter is what saves the engine, and it is the thing that blocks.

Rotating stock

Fuel is not something you store. It is something you circulate.

• Buy it, use it in the car or the mower, and replace it. Every six months at the outside for petrol.
• Date every container in marker on the side. Not on a label, which falls off.
• First in, first out.
• Keep a note somewhere you will see it, on the same reminder as the alarm tests in smoke and CO alarms.
• Old petrol is not waste. Diluted heavily into a car's tank, a quarter of old fuel to three quarters fresh, it burns off perfectly well. Do not put it into the generator, which is the small fussy engine you actually need to work.

Never pour fuel down a drain or onto the ground. Take genuinely dead fuel to a household waste site that accepts it.

The service schedule

Figures vary by manufacturer, so your manual beats this table. As a general shape:

Oil level is the one that kills engines. Small engines hold very little oil and consume it, and many have a low-oil cutout that stops the engine without explaining why. A generator that stops on its own and will not restart is very often simply low on oil. Check it before you conclude anything is wrong.

Keep the consumables in stock: oil, at least two spark plugs, two air filters, and a fuel filter if it has one. These are cheap, they store indefinitely, and they are exactly the class of item that will be unobtainable when it matters. Store them with the machine, not in a drawer indoors, so they are found by whoever needs them.

Keep the manual with the machine too, in a plastic bag, and print it if it only exists online. A manual on a website is not available in the situation this equipment exists for.

The monthly exercise run

Once a month, run it under load for at least 30 minutes. Diarise it.

Why it matters:

• Circulates oil over surfaces that would otherwise corrode.
• Charges the electric start battery, if it has one.
• Keeps seals and diaphragms from drying and perishing.
• Moves fresh fuel through the carburettor, which is where the varnish forms.
• Confirms it works, which is the real point. A generator you have not started in a year is a generator of unknown condition, not a resource.

Under load, not idling. Plug in a heater, a kettle, anything that draws real current. Idling does not get the engine hot enough to drive moisture out of the oil and the exhaust, and in a diesel it causes wet stacking.

Write the date and hours in a notebook kept with it. Memory is not a maintenance record, and in a year you will not know whether you did this in March or imagined it.

Putting it away for months

Two valid approaches, and the mistake is doing neither, or half of each.

Option A, drain everything. Best for long storage.

1. Turn off the fuel tap and run the engine until it stops on its own. This empties the carburettor, which is what matters.
2. Drain the tank.
3. Drain the carburettor bowl at its drain screw as well, because running dry leaves a residue.
4. Change the oil while the engine is warm, so the contaminants come out with it.
5. Remove the spark plug, put a teaspoon of oil into the cylinder, pull the starter slowly a few times to spread it, refit the plug.
6. Cover it, and store somewhere dry with airflow.

Option B, fill and stabilise. Better if you might need it at short notice.

1. Fill the tank completely with fresh fuel and the correct dose of stabiliser. A full tank has almost no air space and therefore very little condensation.
2. Run it for 10 to 15 minutes so stabilised fuel reaches the carburettor. Stabiliser in the tank alone protects nothing where the problem actually happens.
3. Change the oil.
4. Cover and store.
5. Still run it monthly.

What not to do is leave a half-empty tank of untreated petrol in it over a winter, which is what most people do, and which is the single commonest reason a generator will not start when it is finally needed.

Do not seal it in plastic sheet. Trapped condensation rusts everything. A breathable cover, or a loose sheet with airflow underneath.

It will not start

In rough order of likelihood.

1. Fuel tap off, or the vent on the cap closed. Embarrassingly common. Check both.
2. Old fuel. Drain it, put fresh in, try again. This alone fixes most cases.
3. Low oil, and the cutout has operated. Check the level before anything else mechanical.
4. Choke wrong. Full choke cold, off once running. Flooded engines want the choke off and a few pulls with the throttle open.
5. Blocked carburettor jets. The main event after long storage. The bowl comes off with a few screws and the main jet is a brass screw with a very fine hole. Carburettor cleaner and compressed air clear it. Never poke a jet with wire, which enlarges the orifice and changes the mixture permanently.
6. Spark plug fouled or dead. Pull it, look at it. Wet and black means fuel but no spark. Dry means no fuel. Clean, re-gap, or fit the spare.
7. Air filter blocked, so it cannot draw air.
8. Flat starter battery on an electric start model. There is nearly always a recoil starter as well.

The diagnostic that sorts most of it: an engine needs fuel, air, spark and compression. Establish which of the four is missing before touching anything. Pull the plug and look at it, and it will usually tell you.