Just like a wall light indoors, with three extras: choose a light with the IP rating that suits the spot, keep the seal intact where the cable comes in, and work on a dry day. Circuit off, measure, old light off, connect to live, neutral and earth, and mount the new light so that no water can run behind the cover.
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First the spot, then the light
The most important choice comes before you undo anything: how wet does the spot get? A light with IP44 is resistant to splashing water. It belongs under eaves, in a porch or under a canopy. If the light hangs on a bare wall where the rain hits it head-on, choose IP65: dust-tight and resistant to a water jet. How far you get with IP44 is in Can an IP44 light hang outdoors?
Two examples from the range. The Zaro has IP44 and is meant for a sheltered spot. The Kubo has IP65 and can go right in the rain, on a garden wall or a carport without an overhang.
Check what is behind the old light
With an outdoor light the cable usually comes out through the wall, sometimes in a back box, sometimes loose through a hole in the brickwork. Two things to watch when you take the old light off:
- Is the cable still sound? Outdoor cable that has been in the sun for years can turn brittle. If the outer sheath cracks when you bend it, have the cable replaced before you hang a new light.
- Is the hole sealed? Where the cable comes out of the wall, it should be sealed. An open hole lets water run into the wall, along the cable to the inside.
The replacement step by step
Choose a dry day. Manufacturers state it literally in their manuals: do not install in the rain.
Circuit off and measure. Outdoor lights are often on a different circuit than you think, for example together with the shed. So always measure. See How do you check that the power is off?
Old light off and take a photo of the connection.
Mark out the new mounting plate. For new holes use a cable detector, because cables can run in an outside wall too. Use plugs for masonry.
Cable through the light's cable entry. That is a rubber grommet or gland in the back plate. Do not make a bigger hole than necessary; that rubber edge is what keeps the water out.
Connect. Brown or black to live, blue to neutral, green-yellow to earth. A metal class I outdoor light must be earthed.
Close the cover with the seal in place. The IP rating only applies if the light is closed according to the manual. A forgotten rubber ring or a half-tightened screw turns an IP65 light into a light that fills up with water.
Circuit on and test.
Sensor or not
At a front door or a path to the shed, a light with a motion sensor is handy: it only comes on when someone is there. The Senso has a built-in sensor, shines up and down and has IP54, suitable for a covered spot. How much light you actually need outdoors, and why more is not better, is in How many lumens do you need for outdoor lighting?
All models suitable for outdoors are among the wall lights; the IP rating is always in the title.
What IP rating does an outdoor light need?
Under eaves or a canopy IP44 is enough. If the light hangs on a wall where the rain hits it directly, choose IP65.
Does an outdoor light need to be earthed?
A metal light with an earth terminal does. Only a light with the symbol of a square inside a square is double insulated and needs no earth.
Why does my outdoor light fill up with water?
Usually the seal is not right: a missing rubber ring, too big a hole in the cable entry or a cover that is not fully closed. A light with too low an IP rating for the spot also fills up.