Plug in emergency lights are sold widely for domestic use: a unit that sits in a socket, charges from it, and lights when the power fails. They are useful objects and they are not emergency lighting in the sense the law and the standards mean, and buying them for a workplace is a false economy that is discovered at the first inspection.
What they are
A small luminaire with an internal battery and a plug, charging whenever the socket is live and lighting when it is not. Some double as torches. In a home they mean the stairs are not pitch dark during a power cut, which is a genuine benefit and is not the same thing as a designed emergency lighting installation.
Why they cannot form part of a compliant installation
Three reasons. They are at socket height, so they light the skirting rather than the escape route, and the standards specify illuminance on the route. They are not fixed, so nothing guarantees the unit is still in the socket that the design assumed, or in the building at all. And they depend on a socket circuit that somebody may switch off, so the changeover does not reliably track the failure of the lighting the route depends on.
What a compliant installation looks like instead
Fixed luminaires positioned according to the design standard: at exits, at stairs, at changes of direction and level, at intersections, outside the final exit, at fire equipment and call points, with the specified illuminance on the escape route and a rated duration. Tested monthly and annually and recorded per luminaire.
Where a plug in unit is genuinely useful
In a home, where there is no requirement and any light is better than none. As a supplementary comfort in an office that already has compliant emergency lighting. As a temporary measure during a failure while a proper fitting is repaired, provided the failure is recorded and the repair happens. None of these is a substitute for the installation.
Questions people ask about plug in emergency light
Can plug in lights be used in a small shop?
Not as the emergency lighting. The requirement is for adequate illumination of the escape route in case the normal lighting fails, and a socket mounted unit does not deliver it.
Are they tested like fixed fittings?
They have their own test buttons and no obligation attaches to them, because they are not part of a compliant installation. Anything that is part of one is on the monthly and annual regime.
Do they help during a power cut at home?
Yes, and that is what they are for. The objection is to using them where a designed installation is required.