I wanted to figure out a way to save money on my water heating costs and be more green in the process.

I live in FL and my electric w/h is in an un-conditioned garage. 

I have the opportunity to get a slightly used additional 80 gallon w/h for $100.  I was thinking that I would use it as a pre-heater for the municipal water coming into the house by just routing the water through it first (no electrical connections).   Since most of the year the outside daytime air temp is in the 90's and low temp is about 80 and it's usually much hotter inside the garage,   I figure inside the garage is probably averaging about 90 - 100 degrees.

The only question I have with this senario is that the insulation on the water heater,  in this case,  is going to be working against me.  But I not sure how to calculate how much.  The 50 degree water will enter from the street and sit in the pre-heater until called upon by the main w/h.  But with the insulation on the pre-heater tank,  will it be able to gain enough heat to make it worthwhile?

A second  option would be to skip the 80 gallon tank,  get a long length of Pex pipe instead (say 300')  place it in the attic of the garage and route the water through it using it as a pre-heater.    300' of 3/4" Pex I think holds about 4 gallons,  and 300' of 1" pex holds about 9 gallons.  That could preheat most water coming in,  at least in the summer time.

Both senarios are decidedly low tech but also cheap with short and easy paybacks.

I looked into solar water heaters and they start at about $1500 and that's not in the budget,  unless I build one myself.

Any suggestions?  Feedback?

Personally I would go with the least expensive method which would be to take a metal or plastic potable tank black absorbs best and put it in line before enetering the hot water heater. As long as the tank is exposed to the sun the water should get to around 120 now when it gets to your conventional WH the thermostat set at 110 or so should not kick in

I thought of that but I don't think those tanks are rated to handle municipal water pressure. Those, I believe, are for gravity fed systems.

Yes the tank needs to be pressure rated sealed tank

I suspect that your water comes in at ~70F, not 50F. You would have to remove the insulation from the water pre-heater to make it work and the attic or in the sun (especially with a glass cover) sounds warmer than the garage. The potential is there to cut your water heating bill in half.

If it's 50F water in a tank in a 50F (average temp) garage as it might be in the depths of a FL winter, the insulation neither hurts nor helps. But if it's a 90F garage with 70F summertime water it's hurting you- you get hardly any tempering advantage.

300' of 1" PEX in a 100F garage attic would likely be your best bet, but it's annual performance would be well-under a batch solar hot water heater. It would only provide significant output in summer, whereas batch solar would deliver something even in January.

A 100' loop of PEX embedded in sunny driveway/walkway pavement with 1" of EPS between the pavement & the dirt can also be a real winner in FL 3/4 of the year if you're an afternoon-bather rather than morning bather. (Only for frost-free areas though- not all of FL would apply.)

Personally, I'd prefer for the cheaper approach. As much as possible, I'd love to save a large amount of money. That way, I could still spend them for other important stuff.