Can You Run a Generator Indoors or in a Garage?

No. A petrol generator must never be run indoors, in a garage or in a shed. Its exhaust contains carbon monoxide, a gas you cannot see, taste or smell, and the Health and Safety Executive warns that using petrol powered equipment indoors or in an enclosed space can create levels that kill even after a few minutes¹. The same applies to diesel generators and other fuel-driven equipment².

The danger is not the smell of petrol or the noise. It is the exhaust. Carbon monoxide levels can quickly kill without warning, and the fire and rescue services say plainly that petrol or diesel generators used indoors can produce fatal levels of the gas¹. An open garage door, a propped shed window or a "just for an hour" run does not make an enclosed space safe.

The safe rule is simple: the generator stays outside, in well ventilated open air, with its fumes dispersing away from the building and away from any door, window or vent¹. If you need power inside during a cut, a battery power station is the safer route, and our guide to using generators and power stations during an outage sets out the choices.

Where a generator must never go

The list of places a generator must not run is longer than most people expect, because "enclosed space" covers more than a living room. The Health and Safety Executive says petrol powered equipment must never be used indoors or in enclosed spaces unless the ventilation needs have been fully assessed¹. That covers a garage, a shed, a conservatory, a cellar, a porch and a workshop.

The fire and rescue services extend the same rule to the places people shelter in. North Wales Fire and Rescue Service says never to use barbecues or portable fuel-burning equipment indoors or in enclosed spaces⁴. The London Fire Brigade says never to take portable or disposable barbecues, grills or charcoal fuel burners into your home or any confined space such as a tent, houseboat or caravan⁵. A generator belongs in the same category: it burns fuel and gives off exhaust.

The Northern Ireland Executive's storm advice for farmers and livestock owners is blunt about standby generators: never use them indoors, as the fumes from the engine can be lethal⁸. That applies to a permanently installed standby unit as much as to a portable one. If a generator is fixed to a building, its exhaust must still discharge outdoors, away from windows and vents.

Why carbon monoxide kills without warning

Carbon monoxide gives no warning. You cannot see it, taste it or smell it, but it can kill quickly and without warning, as the Mayor of London's office puts it in gas safety guidance for tenants⁹. The Health and Safety Executive says the same: carbon monoxide levels can quickly kill without warning¹. There is no smell of burning, no visible smoke and no obvious sign that anything is wrong until someone becomes unwell.

The risk peaks in winter. Government guidance notes that many carbon monoxide deaths take place between November and February, caused by faulty fossil fuel and wood burning appliances leaking the gas¹⁰. A generator run in a cold garage during a winter power cut sits squarely in that window. The gas builds up in the still, cold air and seeps into the house through the connecting door, gaps and vents.

The fire and rescue services treat an alarm as essential in any room with a fuel-burning appliance. Northern Ireland Fire & Rescue Service says carbon monoxide is invisible and odourless, so having an alarm in rooms with fuel-burning appliances is essential¹². Devon and Somerset Fire and Rescue Service says alarms should be fitted in all rooms containing solid fuel appliances such as open fires or wood burners, and any gas appliances such as central heating systems, boilers, gas fires and paraffin heaters¹³.

An alarm is a backstop, not a substitute for siting the generator properly. It warns you once gas is present, which may be too late if the generator is in the room next door. Fit the alarm, and still keep the generator outside.

Siting a generator outdoors: air flow and distance from doors and windows

Outdoors is the starting point, not the whole answer. The Health and Safety Executive advises locating petrol generators outside in well ventilated spaces so that fumes disperse and cannot gather or drift into building openings¹. Three things decide whether a spot is safe: airflow, distance from openings, and which way the exhaust points. Position the generator on firm, level ground away from combustible materials, standing water and areas where exhaust gases could accumulate¹⁴.

• Airflow: choose open ground, not a corner between a wall and a fence where fumes can pool.
• Openings: keep the exhaust away from doors, windows, air bricks and vents, so fumes cannot drift inside¹.
• Direction: point the exhaust away from the building, not towards it.
• Weather cover: never bring the generator inside to keep it dry. If it must be sheltered, use an open-sided structure that does not trap fumes, and never a closed shed or garage.
• Fuel: always decant fuel in the open air, not inside a garage or shed, and do not smoke near where petrol is stored¹⁵.

The same logic applies after a flood, when people are drying out a house and reach for a generator. Runnymede Borough Council's flood aftercare advice is to never run a petrol or diesel-powered generator indoors, because the exhaust gases contain carbon monoxide, which can kill³. Which? gives the same instruction in its flood recovery guidance: never use petrol or diesel-powered generators indoors¹⁶. Wiltshire and Swindon Prepared repeat it: do not use petrol or diesel generators indoors because the exhaust gases contain carbon monoxide, which can poison if in high concentrations in the air¹⁷.

If you are weighing up a generator against a battery unit, our comparison of a portable power station or petrol generator for a power cut sets out what each can and cannot do. For the full picture on fumes, fuel and electric shock, see generator safety.

Symptoms of carbon monoxide poisoning and what to do at once

Carbon monoxide poisoning is easy to miss because its early symptoms look like common illnesses. The Royal Society for the Prevention of Accidents says carbon monoxide symptoms can be mistaken for flu, food poisoning, viral infections and tiredness⁷. Headache, nausea, dizziness and feeling generally unwell are the kind of thing most people put down to a bug or a long day.

The NHS advises that you should not leave vehicles or other engines on in enclosed spaces such as a garage¹⁸. If you suspect carbon monoxide, act at once:

1. Get everyone out into fresh air, including pets.
2. Open doors and windows on your way out if you can do so safely.
3. Turn off the generator if it is safe to reach it, but do not go back inside for it.
4. Call the gas emergency line on 0800 111 999 if you smell gas or suspect a carbon monoxide leak⁶. The National Gas Emergency Service also offers a textphone line on 0800 371 787¹⁹.
5. Seek medical help if anyone has symptoms, and tell the clinician you suspect carbon monoxide.

Petrol vapour carries its own risk. The fire and rescue services warn that petrol vapour can cause irritation of the eyes, nose and throat, and in confined spaces can cause dizziness and unconsciousness¹⁵. That is another reason fuel is decanted in the open air and generators run outside.

Who is most at risk

Carbon monoxide does not discriminate, but some people are more vulnerable to fire and fumes at home. The government's fire safety campaign identifies older adults, those who live alone, people who smoke and drink at home, and people with disabilities, physical mobility issues or mental health conditions as at higher risk from home fire²⁰. If any of those describe your household, the case for keeping the generator outside and fitting an alarm is even stronger.

Age matters too. Across Devon and Somerset, the fire and rescue service says the group most at risk of fire is the age group aged 65 and over, with around three-quarters of this group's domestic fires started by cooking²¹. Older residents living alone may also be slower to notice symptoms or to get out.

Children and pregnant women deserve particular care. The safest rule is that nobody should be in a building where a generator is running, and that includes children and anyone who is pregnant. If you have a health condition, are pregnant or have young children, treat any headache, nausea or dizziness during a power cut as a possible warning and get everyone into fresh air.

If you have a neighbour who lives alone, is older or has a disability, check on them during a power cut. Fire and rescue service guidance identifies older adults, people who live alone, and people with disabilities, physical mobility issues or mental health conditions as at higher risk from home fire²⁰. A quick knock to make sure they are not running a generator in the garage is worth the walk.

Generators, caravans and tents

Tents and caravans are the most dangerous places of all for a fuel-burning device, because they are small and seal tightly. Fire and rescue service prevention advice is never to take portable or disposable barbecues, grills or charcoal fuel burners into your home or any confined spaces such as a tent, houseboat or caravan⁴. A generator is a fuel-burning device and falls under the same rule.

Caravan and mobile home guidance adds that you should make sure heaters work properly²². If you are using a caravan during a power cut, keep any generator outside, well away from the door and any vents, and never bring it in to keep it dry or to stop it being stolen overnight.

Getting advice before you connect anything

A generator that simply powers an extension lead outdoors is one thing. A generator connected to your home's wiring is another, and it needs professional advice. Official guidance says it takes research by you and the advice and expertise of a qualified electrician for a generator to help you²³. Connecting a generator to the house can back-feed the network and endanger the people working to restore your supply.

Your electricity network will not supply you with a generator. NIE Networks, which runs the network in Northern Ireland, states that it does not fit generators in individual homes or businesses, and fits them at the local substation where possible²⁴. Network operators restore supply from their side; a generator for your home is your own responsibility.

For short cuts, a generator may not be worth the cost at all. Official guidance notes that for the few hours you may be without power, torches and lanterns are a suitable alternative²³. Our guide to deciding what to keep running when the mains fails helps you work out what actually matters, and the power cut backup power checklist lists what to have ready.

If you do go ahead, read connecting a generator or battery to your home's wiring before you buy anything, and ask a qualified electrician to look at your plans.