Hello @Alan, @Tim_S & @Joey_van_der_Gaag,
Did you know you can also power these and deliver the required voltages to operate the PoE Function from a 12VDC or 24VDC Source? We have successfully used a high quality DC-DC converter to achieve this with Solar powered installations, though it would work just as well with mobile locations such motor vehicles and water based vessels.
We have been sourcing DC-DC converters from Powerbox Australia, you can see some of the selection at http://www.powerbox.com.au/dc-dc-converters/dc-dc-converter-eta-pbih.html, they ship internationally, and although the units are on the high side (in $) they have been designed for performance.
Happy to Help,