When you see a seller with strobe power packs, sometimes they do not have the correct power ratings listed. For example, one seller has Federal SPS2P Power supplies listed at 55 watts. That's incorrect, as they are 31.2 watts total, and that's divided between the two outlets for 15.6 watts per bulb.  Lots of difference in strobe intensity between 15.6 watts per bulb and 27.5 watts!

In general, when you purchase a strobe power supply, the output is split between the total number of strobe tubes or bulbs installed. That means a 90 watt power supply with 6 outlets will provide 15 watts per outlet. However, if you only use 4 strobe tubes, the output is 22.5 watts per outlet, or a 50 percent brighter flash effect. There are some power supplies that allow higher power on two out of six outlets (Federal SPS6J I believe is one) but then you need to be sure the strobe bulbs are capable of accepting the power supplied to them. 20 watt bulbs will not take 30 watts for long and you'll have an impaired system fast.

I don't think many understand the "joules" rating either. Joules is a measure of brightness assigned to the strobe as a measure of strobe energy. A 13 Joule flash is brighter than an 11 Joule flash, etc. Then you have candlepower, and not many are rated in candlepower. A strobe that puts out a relatively high joule energy will have a high candlepower. Many good strobes have 1 to 2 million candlepower equivalency. But, the dome or lens makes a great difference here too. A strobe with a domed Fresnel lens will provide lots more punch than the standard pop-in strobe installed in a tail light. And, clear strobes are much brighter than colored ones.

One more thought is that some strobe packs put out more usable light than others due to flash pattern. The 20 watt Tomar Neobe pack driving two Tomar Series 37 heads is pretty bright, and that's at only 10 watts per head. This appears due to the almost continuous flash they call the "Neobe" effect. Plus the heads have reflectors built in. Yet, 10 watts with most other packs provides a lot less light, and put that 10 watts to a hide away strobe behind a red tail light lens and it's virtually invisible during the day, and only marginal at night. But, in any case, remember that strobe inserts and inset grille lights are considered secondary light sources and not primary warning devices. If you consider that most people won't see you coming, and drive accordingly, you'll likely survive EVO's.

Consider purchasing a larger than needed power supply for your present use so you can get the most power for your strobes and plan for expansion. As above, just be sure not to over drive the strobe tubes with too much power. I use a 6 outlet 90 watt power supply for the front cornering lights and tail light strobes, which gives me 22.5 watts per strobe tube, and provides brilliant flashes that can be seen in bright sunlight as well as at night. The other two outlets on the power supply are unused and will not be used, as if they were, it would diminish the power to the existing strobes and reduce their visibility. A separate strobe pack drives my deck lights which are higher, easier to see, and not behind red tail light lenses, so in my case 16.5 watts each for those deck lights is sufficient for plenty of visibility. I might have been better purchasing a 180 watt power supply with 8 outlets and using all 8 outlets with two extra strobes on the deck or grille, which would have provided 22.5 watts to each one. But I run LED's in the grille, so I didn't want the strobes there and didn't want to push 30 watts into the cornering and tail light tubes either. You have to choose between cost and your ultimate vehicle setup and visibility goal.  

Finally, beware of "cheapie" power supplies and strobe tubes or strobe heads. Look for power supplies and heads that are professional grade, if you are going to be using them for responding to emergency situations. This doesn't mean that you can't find a good bargain on eBay on Whelen, Federal, Code 3, Sho-Me, Galls or other high quality units, but beware of the cheap imports that will leave you literally "in the dark" when your life, or that of others, is at stake. A unit that quits because it's poorly constructed is worse than no unit at all, because you've come to depend upon it. If you had no lights at all, at least you'd know that nobody could see you! I have tested low grade power supplies and had them quit flashing in a few hours, and even a few minutes. Burned out strobe tubes pose the same problem. Sure, any tube can quit at any time, but not after just a few minutes or hours normally! Return privileges and warrantees on junk mean nothing in the middle of a rainy or snowy night when you're sitting with no emergency lights. My caution follows:

After you jump, it's too late to wish you bought the better parachute!  Same with emergency lighting!

I hope this provides some help. Be safe out there.