Related to posting on generator heat recovery, everything is relative and value/return is constantly changing.

A unique situation arose after hurricane Wilma here in FL at the end of October. Immediately after the storm an early cold front moved through. Lacking the ability to make hot water (for days, weeks for some) because of no electricity replaced the usually grumblings of not having air conditioning.

I had recently purchased a 15KW generator. It started with my next door neighbor then grew rapidly til I was a popular guy in the neighborhood. They would bring me gas to my front door if I would please give them some hot water. Ten days after the storm I had 45 gallons of gas in my garage which was about 25 gallons more then I bought before the storm struck.

This got me thinking about making a water heater-heat recovery around the exhaust pipes, particularly when you see the exhaust pipe and first half of the muffler glowing red at night.

Generators are about 5 to 25% efficient depending on size and % loading. About two thirds of the fuel BTU capability is going to wasted heat.

After the hurricane, gasoline is gold. People would wait in lines for 2-3 hrs to get the ration limit of 5 or 10 gallons.

I have a whole house battery-inverter system and used the generator about three hours a day, at about 90% rated load to recharge the batteries and 'recharge' the hot water heater. I am yielding almost twice the KWh's per gallon of gas over running a generator alone because I can run the generator near max rated load. I could almost half the gas usage again if I replaced the electric hot water generation with generator heat recovery.

I am also looking for info on lead-acid recharge efficiency. I know recharging efficiency is in the high 90's up to about 85% recharge but after the hurricane I detected a slow degradation, day after day I believe due to stopping recharge when efficiency dropped. The 85% point seemed to drop a couple of % each day. I was stopping the recharge when the bulk current dropped to 70% (2nd stage of charger) of the 20% A-H charge current limit that I set.

After seven days I got close to my self imposed rule of drawing no more then 40% capacity from batteries (60% remaining capacity) before recharging and bit the bullet and did a top off recharge. The batteries reverted back to the original characteristics.

I have no idea where I read it now, but I read about a setup that had the exhaust from a genset plumbed to a regular natural gas/propane 40 gallon water heater. The exhaust simply hit the bottom of the tank and vented straight up and out, heating the water as it passed. He had plumbed this tank in front of his "regular" propane water heater. He said he cut his propane water heater usage by about 90%. It sounds straight forward enough, but I am not sure how you could physically put this together, maybe in an attached garage with the vent out the roof? I wonder if the natural convention was all he had or even if in a warmer climate you could just set this up outside.

I know in the marine world it is common to use the engine radiator coolant to pass through the water heater to heat water for the boat. So if your genset was water cooled that might be another option.

Check out Polar Power, they have some DC generators that can provide heat and cooling and run on multiple fuels.
http://www.polarpowerinc.com/products/generators/cogenset.htm

As for battery efficiency, I have heard that the Concorde AGM batteries are achieving ~95% efficiency.
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What RCinFLA is talking about is as batterys get above 80% SOC, they don't accept a charge at the same amp/hr rate ( they decline to 0 the closer to 100% SOC )

So when running off ANY charging source, if you can't get the batterys to take the full charge available, its wasted and reduces the charge efficiency

To say any battery is 95%, only means Ah in vs Ah out, but one has little control of the Ah source, if it's solar, its what the panels are putting out. For a GenSet, its burning fule regardless of the load.

One's system can easily be below 50% system efficency due to this and its often overlooked

Something that CAN be done re Solar Guppy's comment is to use a diversion controller to switch excess energy into a domestic hot water "preheater". Controllers such as the Trace C60 or Morningstar TS series can be used or a solid-state relay driven from an MX60 charge controller can also do the job.

I don't think it's unusual to have batteries hit the "absorb" voltage right around noontime, which is generally when max solar input is available. As Solar Guppy says, constant voltage charging means that current input is limited which means that available energy isn't being used.

You have to design carefully and understand NEC requirements but it can and does work. Our 1920 nominal watt system supplies 45 amp and 25 amp DC water heaters, the former via a TS-60 controller and the latter via a Crydom relay driven by the MX60. The MX is simply part of the "amp generating" equipment but can still act in stand-alone mode should a fuse blow or something happen to the DC heaters. The MX never leaves MPPT mode any more.

We have two tanks (90 gals total) in our preheat system and I feel good knowing that every single watt of PV is being used to my families' benefit.