Easy question isn't it?
Well, actually it is not. But if you do not want to read a long blogpost about this topic, then simply take the following rule of thumb. Multiply the length times the width of the room. Then, multiply that number times 1.5 which gives you the amount of wattage you need to light the room properly for general illumination.
A room is 12 ft. X 16 ft. (12 x 16 = 192). Then multiply 192 x 1.5 which is 288 watts. So go for a 300 watt solution, which can be five 60 what bulbs or eight 40w bulbs.
We advise to spread the light over at least three lights points minumum. So instead of one chandelier in the middle of the room emitting 300 watt. We suggest a mix of recessed spots, chandeliers or pendants, floor and table lamps.
So what if you use energy efficient light sources like CFLI or LED?
First of all, great choice! Energy bulbs are much more efficient when it comes to energy use (electricity) and transform it into light rather than heat. Also they last a lot longer compared to incandescent. A little bit more pricey on the shelf, but much cheaper over the lifetime!
Back to the question now, CFLI or LED bulbs often have the incandescent equivalent on the packaging which enables you to use the above rule of thumb. In case it does not, you have to look at the lumen value. Lumen represent the actual amount of ambient light coming from a lamp rather then the energy put into the bulb (which is watt). If we simply look at the lumen, an incandescent bulbs emits, 14.5 lumen per watt for a 60w bulb, We simple have to multiply our rule of thumb by 14.5 and be done with it. In most cases you will be good and have a clear lighting atmosphere in your room.
In our example above we need 288 x 14.5 = 4176 lumen for a 12ft by 16 ft room.
Great, we are all set? Yes, you are but if you want to optimize your lighting even further, read on. Using lumens gives us an great opportunity to adjust the light output to the purpose of the room. I guess it is no surprise if we tell you that you need more light in your kitchen then in your bedroom.
Based on the IESNA Lighting Handbook use the following rule of thumbs.
Tables and Raised Surfaces: 30 Lumens per Square Foot
Desks and Task Lighting: 50 Lumens per Square Foot
Going back to our example and in case your 12ft. by 16ft. room is a kitchen where you will be mostly doing tasks. Here you will need 12 x 16 x 50lm = 9600 lumen to light it up. On the other hand, in case this is your hallway, 12 x 16 x 20 = 3840 lumen should be sufficient.
As you probably already have noticed, you can use this technique to adapt lighting within a room. For example take your living room and suppose it is 250 ft. As a base lighting, you need 250 x 20 = 5000 lumen to light it up. However, you might want to read while sitting in your couch for which you need additional lighting. Therefor, we advise to add a floor lamp, table lamps or dedicated recessed spots around the seating area to increase the light output to (in case the couch is 4sq ft) 4ft x 50 = 200lm.
One last remark, when you buy light bulbs, please keep in mind that shades will lower the lumen output. Also dark walls and furniture as well as high ceiling will require you to have more light.