The first factor is simply "fudge" in the datasheet numbers. This isn't really meant to deceive, but is more about giving conservative, worst-case values so that nobody can rightfully complain that their lighting is drawing more power than the manufacturer promised. Let's call it the cover-your-butt factor. There's a certain amount of manufacturing variance in every component (e.g. 5% resistors), and rounding up to a figure like 60 mA for a pixel makes sure that every case is represented...it's also simpler in print and for doing mental estimations for many-pixeled installations.
Second is the resistance of the wire joining the LEDs. The longer the wire, the greater the resistance...which, by Ohm's law, is inversely proportional to current. If you apply power to a single end of a long run of LEDs, those toward the opposite end get progressively dimmer as they're starved of current. If you measure 1 and 2 meter lengths powered in this manner, you'll see that the numbers don't add up -- the 2 meter strand is using less than 2x the current, because of this resistance. This is why I always strongly recommend power taps every meter for the strip-type LEDs, and every 25 pixels for the strand type.
Third, the datasheet numbers represent the maximum case -- for an RGB LED, that's white at 100% full intensity. But on average you'll probably be running them much less than this. For a game of "snake" on an LED matrix, you might have fewer than 10% of the pixels actually lit...or if displaying animation, like the rainbow swirl from the LED belt kit, most of the LEDs are at an intermediate brightness level, they're not all going full blast all the time. The relationship is pretty much linear, or close enough...an LED PWMing at a 50% duty cycle will use about 50% the power of a fully-lit one.
Finally, a phenomenon I can only describe as "mojo," wherein the power draw of a color pixel is slightly less than the expected sum of the components (even after factoring out the power use of the driver chip). For example, measuring current for 100% red and 100% blue, then for 100% red+blue (magenta)...the latter comes out slightly less. Why? I couldn't tell you, but it's there. This is more pronounced when a strand is inadequately powered.
Measured figures:
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WS2801 Pixels
A = 50 pixels, powered from one end (average taken from 2 separate strands)
B = 50 pixels, powered from both ends (2 strand average again)
Configuration A B
----------------------- ---- ----
Off (driver chips only) 33 33 mA @ 5VDC
100% R 947 948
100% G 943 944
100% B 944 946
R + G 1804 1861
G + B 1788 1858
R + B 1820 1863
R+G+B 2342 2778
A couple of observations: notice the higher consumption on strand "B," the one powered from both ends. This is a good thing, it means more pixels are running at their intended full color. Note also that the "mojo" problem goes away when the strand is fully powered (B). Moral of the story? 25 pixels to a power tap...it's not just a good idea, it's...well, okay, it is just a good idea!
From the above, we can estimate a better average current use for each WS2801 pixel at 0.66 mA (driver chip) + 18.3 mA per color component (R, G, B) * duty cycle (0-100%). For a properly-powered white pixel at full brightness, that's 55.6 mA, or about 1.4 Amps for a fully-lit 25-pixel strand.
And here are some figures for LPD8806 strips:
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LPD8806 Pixels
A = 32 pixels (1 meter), powered from one end
B = 32 pixels (1 meter), powered from both ends
C = 160 pixels (5 meters), powered from one end (don't do this)
D = 160 pixels (5 meters), powered from both ends (don't do this either)
Configuration A B C D
----------------------- ---- ---- ---- ----
Off (driver chips only) 4 4 19 19 mA @ 5VDC
100% R 572 583 1700 2038
100% G 563 575 1649 1993
100% B 562 573 1645 1988
R + G 1031 1062 2340 3021
G + B 1022 1052 2284 2976
R + B 1026 1056 2330 3020
R+G+B 1412 1470 2701 3037
Notes: the 5 meter cases are shown here mostly to illustrate that even powering both ends of a long strand is not adequate...you really, really want a power tap per meter, period...but I didn't want to cut into my pristine 5m strand until it's being installed, so regard the rightmost columns above as a lesson in What Not To Do (it should be drawing about 7 amps for proper color & brightness, but is so starved of power that we measured less than half that). Using the more reasonable "A" column figures (1m max to power), we can estimate current use per LPD8806-based pixel at 0.125 mA (driver chip) + 14.67 mA per color component (R, G, B) times duty cycle (0 to 100%). For a properly-powered white pixel at full brightness, that's 44.1 mA...so figure about 1.4 Amps per meter, fully-lit.
That's a lot of annoying math, especially when you get into colors and animation. We have a big WS2801 LED tutorial/kit in the pipeline, and this includes a Processing library which, among other things, can do these calculations automatically...pass it an RGB image, and poof, it gives back the estimated current required for the whole thing...also charge (mAh) over time for animation. Neat stuff. Keep your eyes peeled!
