Powering a soekris is really simple. just remember a few simple rules.
The power rating specified is the maximum number. This should be read like the gas mileage in car ads: "You'll never see it higher than this". How much power your particular configuration will use depends what cards you put in it and how much work your CPU does. I have seen as little as 5W from idle boxes with no cards.
If you supply 5 Volt, it must be fully regulated and the spec is probably +/- 10% or so. You can use the DC jack on the front with a 5 Volt supply if you jumper the two middle pins on the internal power connector. (net45xx: J9, net4801: J10).
If you use the higher voltage range, the supply does not have to be fully regulated, but experience seems to indicate that "merely recified" is not enough. If in doubt: experiment.
The power circuitry on the soekris cards is very robust as long as you supply through the front side DC jack and have not shorted on the internal power connector.
It is perfectly feasible to supply a soekris from batteries, but use rechargeables if you are not a dot-com millionaire.
If you want battery-backup, it is much more efficient to run your soekris directly from the rechargeable battery, rather than take the detour through an UPS. Using an UPS means you draw power from the batteries through the DC/AC converter in the UPS (loosing 10-50% in the process) and then through a power-supply for the soekris (loosing 5-25% in the process).
I run a number of my soekris boxes from lead-acid batteries, and that works like a charm. You can either try to hunt down a "battery-backed power supply", these are usually used in fire/burglar alarms, or you can try to roll your own. See for some inspiration
If you live in less sunshine challenged parts of the world than I do, it is certainly feasible to run a soekris with a solar panel and a lead-acid accumulator.
Power over ethernet: TBD.