A rechargeable light bulb carries its own lithium-ion battery, so it keeps giving light when the mains does not. Some are emergency bulbs that sit in an ordinary ceiling or lamp fitting and take over during a power cut; others charge from a USB cable and turn any shade, lantern or tent hook into a cordless light.
Both look like a familiar GLS bulb, but they solve different problems, and the specifications worth comparing are not the same. Working out which of the two you need saves most of the disappointment people report with these bulbs.
The emergency type is a standard-shaped LED bulb with a bayonet B22 or Edison screw E27 cap, made for 220–240 V. While the fitting is live, it lights the room like any other LED and tops up its battery in the background. When the supply drops, a small circuit in the cap detects it and switches to battery power within a fraction of a second. The light dims rather than going out: battery mode deliberately runs at a lower output so the charge lasts.
The cordless type never touches the mains. It has an integrated LED, a USB-C or micro USB port and usually a hook, a loop of cord or a magnet so it can hang inside a lampshade. Many come with a remote control for on, off, dimming and a timer, and some have a touch-sensitive top.
| Specification | Emergency (mains) bulb | USB-charged cordless bulb |
|---|---|---|
| Cap | B22 or E27, occasionally E14 | none or dummy cap, hook or magnet |
| Output on mains | around 450–900 lm, similar to an old 40–75 W bulb | not applicable |
| Output on battery | typically 250–300 lm | from about 20 lm for mood light to a few hundred |
| Battery | Li-ion 3.7 V, roughly 1200–2000 mAh | Li-ion, roughly 400–4500 mAh |
| Runtime without power | usually 2–4.5 hours | about 4–20 hours depending on brightness |
| Full charge | several hours lit on mains, up to a day on some models | about 2.5–10 hours via USB |
| Protection | mostly IP20, dry rooms only | IP44 or IP54 versions for covered outdoor spots |
Two practical notes. The battery adds length, so an emergency bulb is often 135–155 mm long against roughly 110 mm for a standard A60 – measure the shade or harp before buying. And quoted runtimes are normally measured at the lowest setting; at full brightness expect noticeably less.
Hallways and landings are the obvious spot for an emergency bulb, because that is where you need light to reach the consumer unit or get the children downstairs when the power goes. Kitchens are another, given that a cut often happens mid-cooking. Rural homes on overhead lines, where winter storms bring the supply down more often, get the most out of them.
Cordless bulbs suit the opposite situation: a pendant shade over a table with no ceiling point, a lantern on a garden table, a shed without wiring, a caravan awning or a tent. A model with a timer is handy in a window or a child's room, where it can switch itself off after a few hours.
Neither replaces the tested emergency lighting that fire safety law requires in workplaces and buildings open to the public, designed to BS 5266. These are domestic convenience products.
The most common complaint is a bulb that stayed dark during a power cut. In most designs the automatic switch-over only works when the wall switch is on; if the light was already off, the bulb usually has to be lit by hand, either with a button on the cap or by touching the contacts once it is out of the lamp holder. Try it at home before relying on it.
Keep them off dimmer circuits, as the charging electronics do not cope with a chopped supply. Avoid tightly enclosed, hot fittings, because heat shortens lithium-ion life. Do not charge one in a freezing shed: Li-ion cells should not be charged below 0 °C. For storage, leave the battery about half full and top it up every few months.
The LED will normally outlive the battery, which tends to fade after a few hundred charge cycles and is rarely replaceable. When it is done, the whole bulb goes to an electrical recycling point, not the household bin.
For what they are designed for, yes. An emergency bulb gives you room-filling light during a cut without hunting for a torch, and a cordless bulb makes a lamp work where there is no socket. They are not a substitute for permanent bright lighting on battery alone.
An emergency bulb fits any lamp with the matching cap, provided the shade is long enough and the lamp is not on a dimmer. A cordless bulb does not use the lamp holder at all, so it simply hangs or sits inside the shade.
On a single charge, typically two to four and a half hours for an emergency bulb and up to around 20 hours for a cordless one on its lowest setting. Over its life, the battery is the limit: usually a few years of normal use.
Yes, mostly among the cordless, USB-charged types. The remote typically switches the bulb on and off, steps through brightness levels and sets a timer.
Yes, though less often than E27 or B22. The smaller body holds a smaller cell, so the battery runtime tends to be shorter.
Barely. A 9 W bulb uses 0.009 kWh per hour on mains, the same as an ordinary LED of that wattage; recharging adds only a little on top.
For other caps and bulb types, see light bulbs. If what you really want is a whole cordless fitting over a dining table, have a look at battery-powered pendant lights.
