12V Inverters: Running Mains Kit From a Car or Battery

A 12V inverter is the bridge between a car battery, a leisure battery or a LiFePO4 battery pack and the ordinary 230V plugs in your house. It takes the 12-volt direct current stored in the battery and turns it into the 230-volt alternating current that UK appliances expect, so during a power cut you can run a fridge, a lamp, a phone charger or a CPAP machine from a battery instead of the mains. It is silent, needs no fuel and can be used indoors, which is why many households prefer it to a petrol generator for short outages.

The catch is that an inverter is only as good as the battery behind it. Everything on this page therefore comes in two halves: what an inverter can do for you in a power cut, and how to keep the 12V battery that feeds it charged, healthy and safely stored so it is ready when the lights go out. The battery is the part that fails quietly, months before you need it, so most of the work of preparedness is battery care.

What a 12V inverter does in a power cut

When the mains fails, an inverter lets you keep a small number of essential appliances running from a battery you already own. The battery supplies 12V DC, the inverter converts it to 230V AC, and you plug your appliance into the inverter's socket just as you would into a wall socket. Portable inverter generators and backup units commonly offer 230V 13A sockets alongside 12V DC outlets and USB ports, which shows the shape of the arrangement: mains-type sockets for household kit, DC and USB for the small stuff¹.

An inverter is not a generator and it is not the whole house. Which? advises that for power-cut backup, most households will be well served by a temporary portable generator small enough to carry around and store away, though it will not power the whole home as normal¹. An inverter and battery arrangement sits below that in capacity: it suits a fridge, lighting, phone charging, a router or medical equipment for a bounded period, not kettles, showers or electric heating. Before choosing anything, work out what you actually need to keep running, because that decision drives everything else. Our page on deciding what to keep running when the mains fails walks through that choice, and watts, watt-hours and surge explained explains the units you will meet on every label.

Two appliance figures from Electrical Safety First's ratings guidance give a feel for the spread. A freezer draws around 200 watts, while a portable air conditioner can draw up to 2000 watts². The gap between those two numbers is the gap between what a modest battery-and-inverter set-up can carry and what it cannot.

If your essential list is longer or hungrier than a battery can feed, compare the options on portable power station or petrol generator for a power cut and portable generators: petrol, diesel, dual-fuel and inverter types. If you are considering wiring an inverter into the house rather than running extension leads from it, read connecting a generator or battery to your home's wiring first, because that work has safety rules of its own.

Keeping the battery ready: charge, float and check

The inverter is the visible half; the battery is the half that decides whether the arrangement works. A 12V lead-acid battery that is fully charged sits at roughly 12.8 volts, and a well-designed charger will hold it there rather than pushing it harder. Sterling Power's Pro Charge Ultra Lite 12V charger, for example, has an energy-saving mode that, in the maker's words, "saves energy by dropping the output voltage down to the voltage of a full battery (~12.8V)", ensuring the battery stays full while the charger acts as a power supply when a load is applied³. It can also be forced into float mode at around 13V, intermittently or indefinitely, for long-term holding³.

That matters because a battery left on a dumb charger, or left off charge entirely, slowly loses its edge. The practical routine for a household keeping a leisure battery for emergencies is:

1. Charge the battery fully after every use, and check it before winter each year.
2. Leave it on a charger with a float or maintenance mode if you have one, so it is held full rather than drifting down³.
3. If your charger has a power reduction setting, use it where your supply is limited: the Pro Charge Ultra Lite can be set to run at lower power outputs to complement a wider range of generators and low shore power connections³.
4. Reduce charge current where the battery is nearly full or the supply is weak; the same charger offers a half-current mode that cuts the charger's current by 50%³.
5. Check the terminals are clean and the connections tight whenever you inspect it.

A good charger also copes with imperfect mains. The Pro Charge Ultra Lite accepts a global input range of 80 to 270VAC at 40 to 70Hz, with only a minor power reduction when AC voltage drops below 100VAC³, so it will keep charging through the brownouts and voltage dips that can accompany a strained network. Our page on leisure and LiFePO4 batteries for emergency use goes into battery types in more detail.

The same readiness habit applies to everything else with a battery in it. Official advice across the UK is consistent: keep mobile phones, laptops, tablets and power banks fully charged, so you will have use of battery power for a short time at least if there is a power cut⁶. For a planned power cut, charge fully any equipment with batteries in advance, and for unplanned cuts make sure any items not in use are charging or charged⁹.

Sulfation: what happens when a lead-acid battery sits flat

A lead-acid battery that sits discharged for a long period does not just go flat; it degrades. The chemistry inside forms hard lead sulphate crystals on the plates, a process called sulfation, which reduces the battery's capacity and can eventually make it unusable. This is the quiet failure mode of emergency batteries: bought with good intentions, left flat in a shed for two years, and dead on the night you need it.

The defence is not heroic effort but routine. Keep the battery charged, keep it on a maintenance charger if possible, and if your charger has an automatic desulphation mode, let it run. Sterling Power describes the Pro Charge Ultra Lite's automatic desulphation as "7-10 days cycle with anti-stratification program to keep batteries rejuvenated"³. In other words, the recovery process is slow, measured in days rather than hours, which is exactly why prevention beats cure: a battery that is never allowed to sit flat never needs the long rescue.

If you inherit or discover a battery that has sat flat, do not assume it is scrap, but do not assume it is fine either. Put it on a charger with a desulphation mode and give it the full cycle the maker describes³. If it will not come back to a full-charge voltage, or it will not hold charge over the following weeks, treat it as end-of-life and recycle it rather than keeping a heavy, useless object that still contains lead and acid. Never try to recover a battery that is physically damaged, cracked or leaking: that is a disposal job, covered later on this page.

Storing a spare battery: charged, cool and dry

Where and how you store the battery between power cuts decides whether it is worth anything when one arrives. The National Fire Chiefs Council's Charge Safe campaign is blunt about the conditions: "Store and charge batteries in a cool, dry place"⁴. That applies to the big backup battery and to the small ones alike. Very low temperatures carry their own warning: London Fire Brigade advises that very low temperatures can affect the internal structure of a battery and reduce its ability to hold a charge, and repeated exposure can lead to permanent capacity loss, internal damage and reduced battery life¹¹. A shed that swings between freezing winters and baking summers is a poor home for any battery you are counting on.

Water is the other enemy. Water entering a battery or its electrical components can cause short circuits, corrosion and battery failure¹¹, so a dry storage spot is not just about comfort. And for households with small children, storage also means height: the NHS advice on button batteries is to "Store spare and 'flat' batteries well out of reach too"¹², because a swallowed button battery is a medical emergency.

Your storage routine for the whole emergency kit, battery included, should let you find and use everything in the dark:

• Keep the battery charged and on a maintenance charger where possible³.
• Store it, and all spare batteries, in a cool, dry place⁴.
• Keep torches and batteries in an easy-to-find place, as the Scottish Fire and Rescue Service advises¹³; Anglesey Council similarly says to store a battery or wind-up torch somewhere you can access easily, so you can check the fuse box and move around the house safely¹⁴.
• Keep spare and flat batteries out of reach of children¹².
• Keep important telephone numbers written down somewhere accessible¹⁰.
• Check the kit before winter: the NHS winter emergencies advice is to keep a charged power bank, a torch and spare batteries at home¹⁰.

Charging safely: sockets, cables and heat

An inverter set-up involves charging, and charging is where battery incidents start. The fire and rescue services' advice is consistent and worth following to the letter. The Scottish Fire and Rescue Service says: "Avoid charging your batteries near flammable materials, do not overload power sockets or use unsuitable extension cords. Always use uncoiled extension leads that are properly rated for your devices"¹⁵. The National Fire Chiefs Council adds: avoid overloading sockets or extension leads, particularly when charging high-powered devices¹⁶. If you are running several chargers and an inverter from one area of the house during an outage, that is precisely the moment to check what each socket is carrying; our page on extension leads and overloading with backup kit covers the working-out.

Other rules from the same bodies:

• Do not leave batteries charging unattended overnight as a habit.
• Use the correct charger for the battery¹⁷. A charger with the wrong voltage or connector can damage the battery or worse.
• Do not modify the battery or electrical system⁴. Conversions and home-made adaptations are a recognised fire risk.
• Batteries can be damaged by dropping them, or if the bike or device they are in has been involved in a crash¹⁵. Treat a dropped battery with suspicion.
• Inspect charging leads for damage before use rather than assuming old cables are safe.

Watch for the warning signs during and after charging. If a battery starts swelling, overheats badly, smokes or catches fire, get out, stay out and call 999. Our pages on warning signs a lithium battery is dangerous and what to do if a lithium battery catches fire cover this in detail.

Recycling and disposing of old batteries

Every battery eventually reaches end of life, and how you dispose of it matters more than most people expect. Batteries put in household waste or normal recycling can be punctured or crushed in bin lorries and at waste and recycling centres, where they can cause fires. The Scottish Fire and Rescue Service's guidance is explicit: "If you need to dispose of a damaged or end of life battery, don't dispose of it in your household waste or normal recycling. These batteries, when punctured or crushed can cause fires in bin lorries, recycling and waste centres"⁵. The National Fire Chiefs Council's guidance says the same¹⁹.

The right route is recycling. Anything with a plug, battery or cable can be reused or recycled, as the North Yorkshire Fire and Rescue Service's recycling campaign puts it²⁰. For householders, that campaign advises using a recycling locator to find your nearest electrical recycling point, often found in supermarkets and shops, or checking your council's website to see if they collect batteries for recycling²¹. For larger batteries, the maker may offer a recycling service; the fire services' guidance for e-bike and e-scooter batteries is to check with the manufacturer, and alternatively with your local authority for suitable battery recycling arrangements in your area⁵.

A damaged battery needs more care than a merely spent one. Do not put it in the bin, do not put it in with other batteries for recycling without telling the collection point it is damaged, and keep it away from anything flammable in the meantime. Our page on how to dispose of a damaged lithium battery sets out the steps.

During the cut: using the inverter well

When the power actually goes, the inverter earns its keep, but a few habits make the battery last. Run only what you decided in advance was essential [see /backup-power/priorities/]. Keep the fridge and freezer doors closed as much as possible: Food Standards Scotland advises that during a power cut it should be safe to store food in the fridge as long as the temperature does not rise to 8°C, and that you should keep fridge and freezer doors closed as much as possible²³. Northern Powergrid gives the same advice to limit opening the fridge or freezer so food stays cold for longer²⁴. If the freezer is on your inverter's list, that 200-watt draw² is what you are feeding.

Keep a charged power bank to hand even if it only charges a phone, as Wiltshire and Swindon Prepared advises, to help you stay in contact with loved ones in an emergency or power cut²⁵. Report the cut by calling 105, the free power cut helpline²⁶. If you rely on powered medical equipment, tell your electricity network; the NHS advice for families of technology-dependent children during power shortages is to charge fully any equipment with batteries for a planned cut and to keep unused items charging otherwise⁹, and network companies run support schemes for those who need them, which our page on powering CPAP, oxygen and other medical devices from a battery covers. Northern Powergrid also advises that if you rely on heated tanks or aquariums, you should consider a battery-powered backup or insulation methods²⁴.

It is worth knowing that organised, nationwide emergency power cuts are rare and are treated as a last resort. Northern Powergrid states that emergency power cuts, known in the industry as rota load disconnection, "would only be used as a last resort if there was ever a major national energy shortage that could not be dealt with by a range of other measures first"²⁷. The outages you are realistically preparing for are weather-driven local cuts, which is why the battery-and-inverter approach, silent and indoor-safe, fits them well.

Where to get help

Help during a power cut starts with knowing who to call before you need them. Store the power cut helpline number, 105, on your phone²⁶, and keep important telephone numbers written down somewhere accessible in case your phone dies¹⁰. The full picture of what to do, who to contact and what to expect is on our power cuts: a household guide page, and the kit to have ready is listed on the power cut backup power checklist.

For people who need electricity for health reasons, contact your electricity supplier and network operator in advance and ask to join their priority support register; the NHS letter to families of technology-dependent children during the 2022 power shortage planning shows the kind of arrangements networks make for those households⁹. For everyone else, the practical help in a cut is 105 for information, and 999 only for danger to life, including a battery or electrical fire.