http://greenpowerscience.com/SOLARHOTWATER.html The pump does not need to be the 100 watt version. There are lower wattage pumps on the market.
It is vital that the person who installs your solar power system is actually licensed to do so. An installer who is not licensed may install your solar panels incorrectly, or they may violate local regulations. In addition, a licensed installer can provide you with great information about these regulations that an unlicensed installer cannot.