Pre-heating your water is a good idea, but not cheap..I'm using 500w of PV to add some BTUs to my hot water..
http://ecorenovator.org/forum/solar-...eat-water.html
http://ecorenovator.org/forum/projec...r-project.html
It might work pretty well on an old hot water heater that has good insulation.
My system is leaky, sending heat up the flue, which you don't have..
Some cheap thin-film panels could work for you at higher voltages,
so you may not need to invest in 12v elements..