I thought it would be fair to compare their power when used with a modifier. The modifier would mitigate the differences in coverage. In any case, I believe it's a more practical comparison.
In my test, I compared a Nikon SB-800 speedlight, a Quantum X2 battery-powered strobe (with PXC capacitor and a total power of 400 watt seconds), and an Alien Bee B1600 monolight (rated at 640 watt seconds). I used two kinds of modifiers: an 18-inch beauty dish and a 48-inch octagonal softbox.
Hit the jump to see the results!
Testing protocol: For the speedlight and the Quantum, I used a Cowboy Studio Bracket T (reviewed here) with a flat speedring (reviewed here), which allowed me to use the same modifiers with all three lights. I measured the output using the flashmeter function of a Paul C. Buff CyberCommander at 10 feet (I set the flashmeter to ISO 100, 1/250 shutter speed). These measurements were taken outdoors at night to reduce bounce effects and ambient light.
SB-800 at 24mm: f/2.8 + 8/10
SB-800 at 14mm: f/4.0 + 0/10
Quantum X2 w/ PXC: f/5.6 + 3/10
Alien Bee B1600: f/8.0 + 7/10
SB-800 at 24mm: f/5.6 + 8/10
SB-800 at 14mm: f/8.0 + 0/10
Quantum X2 w/ PXC: f/11 + 9/10
Alien Bee B1600: f/16 + 4/10
The X2 output is between three to four times as powerful as that of an SB-800. With an octagonal softbox, the X2 is approximately 1.5 stops more powerful (or about 3 times as powerful as an SB-800). With a beauty dish, the X2 is approximately 2 stops more powerful (or 4 times as powerful as an SB-800).
The B1600's output is between six to seven times as powerful as that of an SB-800. When used with an octagonal softbox, the B1600 is about 2.7 stops more powerful than an SB-800 (about 7 times as powerful as an SB-800). When used with a beauty dish, the B1600 is about 2.5 stops more powerful than an SB-800 (about 6 times as powerful as an SB-800).