Every EV comes with a cable that plugs into an ordinary wall socket. Your next guest has one in the boot — and if you don’t offer anything better, they’ll use it.
Last reviewed 6 July 2026 · ~7 minute read
The short version: the portable “granny charger” pulls close to a socket’s maximum rated current continuously for 8–12 hours — a load pattern ordinary sockets, old wiring and household extension leads were never meant for. Add a guest’s coiled extension lead running out a window into the rain, and you have the single biggest unmanaged risk of hosting EV drivers. It’s also invisible electricity you can’t bill. The fix is simple: a house rule, and a proper charger they’d rather use.
What a granny charger is — and why guests reach for it
Every electric car ships with (or can buy) a portable charging cable with an ordinary domestic plug on one end — a UK 3-pin, a US 110 V plug, an Australian 10 A plug. It’s nicknamed the “granny charger” because it’s what you use when visiting someone with no charging equipment: slow, but it works anywhere there’s a socket.
And that’s exactly the problem. From a guest’s point of view, your property has sockets everywhere — the garage, the porch, the socket just inside the window. If they arrive with a low battery and you offer no alternative or guidance, plugging in isn’t cheeky, it’s obvious. Most hosts never even know it happened.
Why it’s a genuine problem, not host paranoia
1. It’s a stress test for your wiring. A UK granny charger draws around 10 A (~2.3 kW) — close to a single socket’s continuous limit — for eight to twelve hours straight. A kettle pulls more, but for three minutes. Sockets on ageing circuits, worn contacts, or multi-way adaptors handle this badly: heat builds cumulatively, and scorched pins or melted faceplates are the classic result. Charities like Electrical Safety First publish specific guidance on this exact scenario.
2. Extension leads make it worse. The socket the guest can reach is rarely near the car, so out comes the household extension reel. Safety guidance is blunt here: never use a domestic multi-socket extension lead for EV charging; never daisy-chain leads; if a lead must be used at all it should be a single, heavy-duty, outdoor-rated (IP-rated) cable — fully uncoiled, because a coiled reel under continuous load traps heat and can catch fire. Some manufacturers (BMW among them) simply say don’t use extension cables at all. Your guest with the £12 indoor reel from the shed ticks every box on the don’t list.
3. Weather. The cable run usually goes through a cracked-open window or door to a car standing outside — connectors and reels sitting in rain or morning dew, carrying full load. Granny chargers themselves have weatherproof connectors; household extension leads don’t.
💡 The invisible bill: a granny charger delivers roughly 20–25 kWh overnight. At mid-2026 rates that’s about £6 / US$4 / A$7–8 per night of electricity — unmetered, unbilled, and silently absorbed into your overheads. An EV-driving family staying a week can quietly add the better part of £40 to your bill. (What you should be charging instead: our pricing guide.)
4. The insurance question. We won’t overclaim here — policies differ. But unattended overnight charging through domestic extension equipment, against both the lead manufacturer’s and the car manufacturer’s guidance, is exactly the kind of detail loss adjusters ask about after an electrical fire. At minimum, know your policy’s position on EV charging before your guests decide it for you.
What to actually do about it
- Say something. A line in your house rules and welcome pack: “Please don’t charge vehicles from household sockets or extension leads. A dedicated EV charger is available — details below.” Guests overwhelmingly comply with rules that come with an alternative.
- Provide the alternative. A dedicated 7 kW wallbox charges three times faster than a wall socket, on its own protected circuit, with proper weatherproofing — it removes the guest’s reason to improvise. UK landlords can currently get up to £500 per socket toward installation (holiday-only properties excluded — mixed-use may qualify).
- Meter or price it deliberately. Once there’s a proper charger, decide whether charging is included, flat-fee, or per-kWh — deliberately, not by default. Our pricing guide covers the numbers and the meter rules per country.
There’s no safe half-measure here: a wall socket can’t be managed, can’t be metered, and can’t be billed — so the electricity, the wiring risk and the liability all stay yours. The answer to guest EV charging is a dedicated charger, full stop; until one is installed, the house rule stands on its own.
The carrot that beats the stick
Guest Charge makes the proper charger the easy option: guests get a link, see the price for their stay up front, pay online, and charge at full speed — no socket roulette, no extension leads, no invisible electricity on your bill. You see every session and every payment.
Sources
- Electrical Safety First — EV charging safety guidance
- GOV.UK — EV chargepoint grant changes from 1 April 2026
General information, not electrical or insurance advice. For your specific wiring, ask a qualified electrician; for your cover, ask your insurer.