The 910 only puts out I think I read about 200 ws. Yes you can do a lot with 200 ws but you can do a lot more having 500-1000 ws.
The SB-800 is 75 watt seconds, and SB-910 is the same (except minor reflector differences). The continuous lights before are 200 watts.
But there is a huge difference in watts (continuous lights) and watt seconds (flash).
Speaking equivalents, but if a 200 watt equivalent continuous light is used with a 1/100 second shutter speed, then the camera sees 200 watts x 1/100 second = 2 watt seconds. This is the cumulative energy expended in that time duration, all that can be useful behind the shutter.
If the shutter speed is 1 full second, it sees 200x1 = 200 watt seconds, which is a lot of light, for inanimate objects, but which is unacceptably slow for pictures of people (who move).
The bigger speedlights (SB-800, SB-910) are 75 watt seconds, regardless of shutter speed (because their duration is faster than the shutter). So, there is a big difference between 2 ws and 75 ws and 200 ws.
My SB-800 (at 24mm zoom at ISO 200) meters f/11 plus 4/10 stop in 45 inch white reflected umbrella at four feet from fabric (which is pretty close, the light stand is at about 2 feet). Turning full power down would be good for recycle speed and battery, but still, this is very usable for portraits. Even groups at 10 feet will be very near f/8 (same umbrella, ISO 200). Flash can do this at any shutter speed up to maximum sync speed, typically 1/200 second.
My Alienbees B400 is 160 watt seconds, which will meter f/16 there, one more stop. Twice the watt seconds (75 to 160) is one stop (same effect as doubling ISO). I normally use the B400 in a large 40" softbox, close, at f/8 or f/10, turned down to around 1/4 power or less. It is more than plenty indoors, but more would be needed outdoors, competing with the sun.
My Alienbees B800 is 320 watt seconds, and f/22 (one more stop).
Camera light meters can meter continuous lights, but cannot meter flash. So, we need to make other arrangements.
If using two speedlights for portraits, the Nikon Commander allows using TTL, which is self metering. There are pros and cons of this.
All studio flash is manual flash. Alienbees is a great choice - it is THE inexpensive choice that is still first rate gear and features.
If using manual flash for portraits, it will be very desirable (virtually essential) to also have a hand held incident flash meter, to set up the lights, the ratio between them. The more lights you have, the more necessary this is. This is about repeating your setup next time, with ease.
There is more info at the link below.
