How to make a solar water heater

Passive solar heater is any water container exposed to sun. 
Try a batch solar heater free by putting water bottle in sun. 

Here is one example: a bare tank is placed in plywood box that faces sun > buy old 40 gallon electric tank from plumber and remove outer shell > remove sediment, flush tank with two pints diluted hydrogen peroxide Read more, replace anode rod Read how > tank and plywood are painted black to absorb heat; piece of clear glass is placed over box; box is insulated >> put drain hole in bottom of box to release condensation > 40 gallon tank + water + plywood weighs 460 lbs >> put expansion tank on incoming cold water line > > temps are too high for plastic pipe > electrically ground tank and copper pipes using #6 copper wire > insulate pipes > design project so cleaning and maintenance are easy.

End of sunny day, you have 40 gallons hot water > or more if you put 2-40's in series >>> hot water cools quickly but lasts until early evening > put insulation over glass when sun goes down > 115V element put on switch can keep water warmer longer and prevent freezing in winter > shower uses 5-9 gallons of hot water > bath uses 12-15 gallons hot > once you start using hot water, cold water enters tank so take all baths-showers in succession > for morning hot water, put 115V 2.5 gallon or 6 gallon under sink > connect under-sink heater to light switch or timer > reducing consumption and planning are key.
How to install under-sink electric water heater

Planning makes solar into everyday money & energy saver. 

For more ideas: See long list of DIY solar water heat ideas.

High heat applications like solar require expansion tank on incoming cold water line