I have had the fortune to deal with solar water heaters over the years. I didn't install them myself, they came with the houses that I had purchased. The first house, the system had an 80 gallon tank with 1 single 4' x 8' roof top collector panel. It supplied almost all of my hot water. The tank of course eventually rusted out and had to be replaced. I opted to put in a purely electric water heater. At the time, I was single and noticed maybe $20-$30 per month increase in my electric bill.
The second house had a 120 gallon water tank and twin 4' x 8' collector panels. It had a problem with the control box which I was able to repair by fixing the traces on the circuit board that had erroded away. Well, the tank on this one rusted through after 3 years. The cost of a replacement "solar" water heater tank was around $800-$1000. Now I was married and had a kid. If it went up $20-$30 before what would it go up to if I went only electric? Electricity is also more expensive now too. I decided this time to keep using solar but I didn't want to spend for the expensive solar water heater tank. Besides, what is different? It just holds hot water right? I ended up looking around at standard electric water heaters. I decided 80 gallons would be enough (since 120 gallons was impossible to find anyway) and looked around for one I thought I could adapt somehow to solar. I bought one and converted it. It worked out very nicely.
The conversion process
Look for a tank that has at least one extra removeable 3/4" plug on the top. Don't confuse the anode rod plugs with an extra plug. The one I got has two. I only used one though. Make sure there is no check valve or restriction on the cold water intake side. If there is, remove it. Mine had a rubber flapper. I removed it with pliers. If you don't remove it, the water pump may have trouble pulling water back from the tank. Take 6' of 1/2" copper pipe, solder a copper 3/4" thread to 1/2 pipe adapter and solder the adapter so that the bottom of the pipe sticking into the tank will end up about 12" from the bottom. Screw this assembly into the tank using teflon tape on the threads. cut off the remaing 1/2" pipe sticking out - leave about 4" above the adapter. Solder a ball shutoff valve to the that. Add some small pieces of pipe and elbows. Add a anti-siphon check valve if you don't have the electric shutoff type and some more pipe to run to the hot output side of the solar panel. Add a tee to the cold water intake and adapt it down to the intake of your water pump going to the solar collector panels. The other option is to use another spare plug and do the same thing but with a 1/2" pipe that is only 6" from the bottom of the tank. This way, you don't have to remove the check valve built into the tank - if it has one. Once everything is soldered. Apply water pressure and check for leaks. Insulate all your pipes. Don't use that cheap poly stuff, it melts within a week when exposed to the high temperatures of the solar heated water. Now when the panel is hot enough, the motor will turn on and draw water from the intake side and the hot water will go through the check valve and into the water tank.
Pictures and diagram to come.