Planning light and sockets by room

Illuminance is measured in lux (lx) — lumens per square metre. Different zones need different levels: living room about 200 lx, bedroom 100 lx, kitchen 300 lx with 500 lx at the desk or worktop, bathroom 200 lx, hallway 75–100 lx, sales floor 300 lx (displays are lit separately and brighter).

To estimate the total flux, multiply the area by the needed illuminance. A 20 m² room at 200 lx needs 4000 lumens on the work plane — and, allowing for losses, about 6000 lm from the fittings, gathered from several of them, not one.

Don't chase a single bright chandelier: several sources give even light without harsh shadows and hotspots. Complement the general light with local light — over a table, mirror or work zone.

Sockets are almost always under-provided, and adding them after a renovation is hard. Guides: kitchen — 6–10 points (appliances plus the work zone over the counter), living room — 5–7 (TV area, sofa, spare), bedroom — 4–6 (a pair each side of the bed plus work points), bathroom — 1–2 protected, home office — 6+.

Count by zone, not "the room" as a whole: where appliances stand, where people sit, where phones charge. Every active zone gets its own pair of sockets.

Build in a margin: one or two spare sockets per room cost almost nothing during the works and remove the extension-lead problem later.

The site has calculators that do this estimate for you — and more precisely than the rules above. “Room illuminance” counts fittings from the room's size, the ceiling height and the fitting's flux; “Sockets and switches” walks through the rooms question by question and builds an exact list of points.

It's a starting point, not a hard rule: for your own scenario — lots of appliances, a home office, a workshop — the numbers can go up.

Where their figures come from is taken apart in the course “How the Electrica calculator works”.