Trying to work out how many watts I'll need to light rooms in the new pad. Have Googled, but it is surprisingly hard to find a dummy's guide to the subject.
Can anyone get me started? Simply to answer questions like: "My lounge is 20 x 10 feet, ceiling 10 feet, how many watts in total will I need for a "normally-lit"[1] environment?
Feel free to enlighten me[2] on how lux / lumines come into the picture.
[1] Yes, I know... [2] Boom boom.
say area is 21 sq m.
To illuminate to 100 lux x 21 sq m you need 2100 lumen
A typical filament lamp has 15 lm per W, so you'd need 140 W filament light. Typical T12 tube with magnetic ballast has 60 lm per W, so you'd need
35 W fluoro light.This ignores things like reflectance of surfaces etc.
Owain
It assumes the light bulbs are all bare, and the ceiling and walls are perfectly reflecting. It's unlikely any of that is true, or even anywhere near true.
For comparison, just took some measurements in an 11' sq room with
8' ceiling, so roughly half the size of yours. Lighting is 3 x 11W CFL centrally, in a fairly efficient fitting (lamps are above a translucent domed glass, which hangs a few inches below the ceiling). In a living room, you probably want to take readings at about 2' height, but to compensate for my lower ceiling, I did it at floor level. Ceiling is brilliant white matt. Walls are an off-white colour (which dramatically reduces reflectance of surface). For any colour which isn't almost white, assume no reflectance at all.Centrally under the light, I get 88 lux. Moving out to a wall, it drops to 55 lux, and towards a corner, 40 lux. I find this satisfactory for a living room. 55 lux is going to be a bit low to read a paperback for a long time, but you don't tend to do that right at the edge of the room, and that's the sort of thing for which you can add task lighting.
Extrapolating that to your room, I would guess you would need about
60W of CFL (spread if possible), either bare or in high efficiency fittings. You need to scale up to compensate for the loss in whatever fittings you choose, and this can be very significant in some cases; it's not uncommon to find fittings which lose over 75% of the light before it gets anywhere useful, and in that case you would need 4 times the power rating to achieve your desired light output.In my office area, my desk is at 400 lux, which is a fairly typical office workspace lighting level. The light is a twin 35W T5 office fitting with high efficiency reflectors suspend over my head, about
4' higher than the desk surface, which if I disable its auto-level-adjust, will generate about 620 lux at full output.For my electronics workbench, I have 1900 lux, generated by a twin
58W T8 office fitting (again high efficiency reflectors), 3' over the bench.Note that both the linear fluorescent fittings have high efficiency electronic control gear, and this always runs tubes at a maximum of around 85% of tube rating, so the total circuit watts is usually under 90% of the total tube rating. This is an EU requirement on fluorescent lamp ballast manufacturers. In the case of T8 lamps, you still get the full light output, because light output is measured using old magnetic ballasts, and the tubes are more efficient on electronic ballasts.
Just checked and that's wrong - it's 3 x 14W CFL
Make that a total of 80W instead, due to the error above.
My rule of thumb is 1W of light for every square foot of floor in a normally-sized room (ie, 9ft/10ft ceiling).
Eg, my living room, 12ftx12ft = 144ft = 150W light.
JGH
I think it depends on what you call lighted. Some of the pretty cheap cfls are pretty crap to see with but come out well on light meters againsts watts consumed. The human eye is not like a light meter! Brian
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