It is important to know that about the two main types of photo-voltaic solar panels. The less expensive types are the poly-crystalline panels, but they also are less effective than the mono-crystalline panels. You should invest in a very efficient solar energy system if that is something you can afford.
http://greenpowerscience.com/SOLARHOTWATER.html The pump does not need to be the 100 watt version. There are lower wattage pumps on the market.
Your solar energy system needs to be exposed to the sun at all times. Most homeowners have their solar panels installed on their roof but you could also have some panels installed on your front lawn. Do not hesitate to cut a few trees to avoid shadows and have a professional help you determine the best place for your panels.