Guide · 11
Granny charger (3-pin plug) charging: how long it takes and what it costs
Slow, but often enough for low daily mileage, as long as the socket and cable are up to it.
Last updated:
A "granny charger" is a portable charging cable that plugs into a normal 13-amp household socket, with a control box part-way along the cable. Some cars come with one; for others it is an extra. It is the slowest way to charge, and safety bodies recommend a dedicated charge point for regular use. But for some drivers it does the job.
How fast is a 3-pin plug?
Most portable cables charge at about 2.3 kW, which is about 10 amps at 230 volts. Pod, a charger company, says a 3-pin plug at 2.3 kW gives up to 8 miles of range per hour. A 7 kW wallbox is about three times faster.
The Energy Saving Trust gives a handy rule of thumb: the charging power in kW is roughly the number of miles you get for every 20 minutes of charging. At 2.3 kW, that is about 7 miles an hour.
How long does a 3-pin charge take?
Time = energy you need ÷ charging power. We allow 10% for losses, so the battery gets about 2.07 kWh for each hour plugged in. Your car may differ.
| Charge needed | Energy into the battery | Time at 2.3 kW |
|---|---|---|
| A 30-mile day (at 3.5 miles per kWh) | 8.6 kWh | about 4 hours |
| 40 kWh battery, 20% to 80% | 24 kWh | about 11½ hours |
| 60 kWh battery, 20% to 80% | 36 kWh | about 17½ hours |
| 60 kWh battery, empty to full | 60 kWh | about 29 hours |
In practice, the useful question is how many miles you can add overnight:
- 12 hours plugged in: about 25 kWh, or roughly 87 miles at 3.5 miles per kWh.
- A 7-hour off-peak window (EDF GoElectric, 11pm to 6am): about 14.5 kWh, or roughly 50 miles.
- A 5-hour off-peak window (Octopus Go, 00:30 to 05:30): about 10.4 kWh, or roughly 36 miles.
So if you drive 30 miles a day or less and can plug in every night, a 3-pin plug can keep up, even within a short off-peak window. If you drive much more, or need a big top-up after a long trip, it cannot.
What it costs
A kWh costs the same whether it comes through a 3-pin plug or a wallbox. The difference is how many cheap kWh you can fit into an off-peak window.
At the Ofgem price cap average of 26.32p per kWh (1 October to 31 December 2026, Direct Debit), a car doing 3.5 miles per kWh costs about 8.4p a mile, including 10% losses. On EDF GoElectric at 6.66p per kWh it is about 2.1p a mile (rate checked 9 October 2026). Losses vary by car and charging speed, so treat these as estimates.
- Topping up a 30-mile day: about 9.5 kWh from the socket. That is about £2.51 at the price cap, or 63p at 6.66p.
- 20% to 80% on a 60 kWh battery: about 40 kWh from the socket. That is about £10.53 at the price cap, or £2.66 at 6.66p.
If your off-peak window is too short for what you need, the rest is billed at the day rate. With a 3-pin plug that happens sooner than with a 7 kW charger, which can add about 31 kWh in a 5-hour window. Our EV tariff guide lists the windows for each tariff.
Some smart tariffs, such as Intelligent Octopus Go, only work with a compatible car or charger. Check the supplier's eligibility checker before you rely on one with a portable cable.
Using a 3-pin charger safely
Electrical Safety First, a UK electrical safety charity, says the safest way to charge at home is a dedicated charge point installed by a registered electrician. It warns that the long, heavy load of EV charging is likely to exceed what a standard socket and standard wiring are designed for. Pod also recommends a 3-pin plug only as an occasional back-up. If you do use a 3-pin cable, Electrical Safety First's advice includes:
- Have your home wiring checked before charging from a 13-amp socket.
- Consider a socket made for EV charging. These are marked BS1363/EV and should be on a dedicated circuit.
- Use a cable from the car maker or a reputable retailer, and check it regularly for damage.
- Never use a household multi-socket extension lead, and never plug extension leads into each other. If an extension is unavoidable, use only one that is suitable for outdoor use.
- Use a cable protector where a cable crosses a path, to reduce the risk of tripping.
When a wallbox makes more sense
A wallbox is worth considering if you regularly need more than the 35 to 50 miles a 3-pin plug adds in a 5 to 7-hour off-peak window, have a large battery, or share one car between several drivers. Our guide to home charger installation costs covers prices and the grants that remain.
Frequently asked questions
How many miles does a granny charger add per hour?
About 7 to 8 miles per hour at 2.3 kW, depending on how efficient your car is. Pod quotes up to 8 miles per hour.
How long does a granny charger take to fully charge an EV?
About a day or more for a mid-size battery. A 60 kWh battery takes about 29 hours from empty at 2.3 kW once charging losses are included.
Is charging with a 3-pin plug cheaper than with a wallbox?
No. The price per kWh is the same. A 3-pin plug saves the cost of a wallbox, but it adds fewer cheap off-peak kWh each night.
Can I use an extension lead with a granny charger?
Electrical Safety First says never use a household multi-socket extension lead, and never join leads together. If you must use one, it should be a single lead suitable for outdoor use. Follow your cable maker's instructions too.
Related guides
- Home charging basics: 3-pin vs wallbox, and what your home needs.
- How much does a home EV charger cost to install?
- How much does it cost to charge an EV at home?
Sources (all checked 9 October 2026)
- Energy Saving Trust: Charging electric vehicles (last updated 23 April 2026)
- Pod: Charging an electric car at home (updated 6 October 2026)
- Electrical Safety First: Electrical safety advice for electric vehicles
- Electrical Safety First: Driven to danger (advice for EV owners on extension leads)
- Ofgem: Changes to energy price cap between 1 October and 31 December 2026
- EDF: GoElectric tariff
- Octopus Energy: Intelligent Octopus Go and Octopus Go