I was fooling around on another board and started thinking about lighting costs. I am looking at adding a studio strobe or two. I came up with a metric that I have not seen much: Cost per watt second. Below numbers based upon Paul C. Buff lights and his listed prices. Makes the B1600 look like a steal and the Einstein close behind.
If we needed to accumulate many watt seconds, this cost parameter surely would be a concern. There are other parameters more reasonable however.
One way to look at it is that if we (likely) only need B400 power levels, then the larger lights are simply a more expensive way to provide it... with added downside that they are less stable and more white balance shift (more red color) when turned way down to be able to use them (with likely exception of the Einstein).
There are of course uses for higher power, normally always involving more extreme distances (basketball court, or on stage, etc), but studio portrait lights are used up close (close as possible), and don't need so much. I use a B400 at 1/8 power in a Large softbox for main light (ISO 200 at f/10). A B400 fill light might be 1/2 power when placed behind the camera.
A worst case? Used for groups, a B400 at full power in a white reflecting umbrella will do f/8 at ten feet (ISO 200). How much more power do we need indoors?
If we do not buy enough power, we can always increase ISO, open aperture, or place the lights closer. A 75 watt second SB-800 speedlight is normally mighty effective (except for slow recycle speed and possible modifiers).
If we buy too much power, we have harder limits. We can only turn them down so far, and can only lower ISO so far. Greater distance decreases the light quality (softness). Lower power settings decrease stability and color balance.
The D7000 was a surprising exception, but most of the Nikon cameras now are ISO 200. This already doubles the effective power of the light (over what ISO 100 needs). That is to say:
ISO 100 320 watt seconds, and ISO 200 160 watt seconds,
are exactly the same situation (same aperture at same distance, if at same fractional power level setting). ISO is an important factor. So, many of us already need only half of the power some others need. I started with B800s, but at ISO 200, I had to always turn them way down, main light at 1/16 power, and found that to be a lot to always deal with. So I added B400s which I now use (still have to turn them down), and now I relegate the B800s only to background and hair light use (can use grids to knock their power down, and color is less important there).
So there seems another usable theory that a B400 is all the power living room portraits will ever need (ISO 200). Then each larger light simply costs more than necessary, and Einstein possibly excepted, only provides the disadvantage of having to turn it down more (only causing less stability and redder color balance). If you do have occasional use to overpower the sun outdoors, or to do ceiling bounce in the gym, etc, then yes, you do need a larger light, but it ought to be a second light, not the same light you want to always deal with to use for living room portraits.
Sometimes less is more.
