I would assume that you have read this already but just in case, here it is anyway.
http://www.hsus.org/about_us/awards/the_hsus_honored_with.htmlConservation is the first place to start in my opinion. Reduce what is used and reuse if possible. Use of non toxic renewables of any kind is good.
This magazine goes way back to the early 70's and the Eco movement and back to the land movement. Back before "going green" was a profitable business. There is tons of good howto info in the archives.
http://www.motherearthnews.com/I took the liberty of doing a search using "taking care of cats and dogs" and this was at the top of their list.
http://www.motherearthnews.com/Energy-Matters/Green-Your-Life.aspx#14 might be a bit of a, "kick in the head" for you though.
If PV is a serious option for you its always best to have your specific site evaluated by a professional.
http://www.solarbuzz.com/companylistings/unitedstates.htmIf you want to kick around some numbers yourself, start with the shelters monthly electric bill. For example: if the shelter consumes 1,000 kilowatthours per month then divide that by 30 days and that would be 33.33 kWh's per day. If the shelter is in an area that sees a daily number of hours, of equivalent full rated power from a PV module, averaged out over a year of 6 hours, then it would take a PV array of no less than 5.5 kilowatts in size.
http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/Its self explanatory.
Others ideas.
http://sierraclub.typepad.com/greenlife/2009/05/pet-projects-the-greening-of-animal-shelters.htmlSupposedly this next place is solar powered but I could not find any pics of that.
http://www.sanjoseanimals.com/ Surprisingly, San Jose sees an average of 5 to 6 hours of equivalent full rated power from a PV module with a "Flat Plate Tilted South at Latitude."