kdbgoat said:

Calculations may be a bit more complex than that. A couple of things to consider is what is the desired temperature of the water when heated, and what is the temperature of the incoming water. Then figure the rate of consumption of the hot water. Then get the BTUs needed to heat the incoming water to the desired temperature at the rate of consumption. Figure out how many watts are needed to provide the required BTUs.

here are some quick calcs to show you why you need to use propane not batteries..

I'll do this for the typical six gallon dsi that is in a lot of rv's

for this we need the water temperature, amount and final temperature.

so:

1. water in is 60 F

2. Amount is 6 gallons

3. Final temp is 125 F

so we need to raise 6 gallons by 65 degrees

energy required:

to heat one pound of water 1 degree F takes 1 BTU ( British thermal unit )

1 gallon is approx 8.3 pounds

we get 8.3 pounds x 65 degrees * 6 gallons = 3237 Btu assuming no losses

now it just so happens that 1 kW/hr = 3413 Btu so we will use 1 kW/hr as it's close

( that's 1000 Watts applied for 1 hour )

your heater is stated to be 1650 Watts.

so if we apply that energy to the water it will reach the desired temperature in 1000/1650 * 60 minutes = 36 minutes..

you are using an inverter and they are typically 90 % efficient.

and you have a 24 Volt source, so you need to draw ( 1650 * 1.1 ) / 24 Volts = 75 amps from the battery.

great you say.. well there is another factor here and it's the Peukert factor.

Peukert's law, presented by the German scientist Wilhelm Peukert in 1897, expresses approximately the change in capacity of rechargeable lead?acid batteries at different rates of discharge. As the rate of discharge increases, the battery's available capacity decreases.

so your 100 A/h battery is not 100 A/hr when you discharge it at a very high current..

i did the calculation and a 100 A/hr battery with a 75 amp load has an effective capacity of 64 A/hr

you need to draw 75 amps for 36 minutes.. so if you could do this without the massive voltage drop tripping your inverter then

your battery will be seriously mad at you !

next, deep cycle batteries will only tolerate a discharge of C/4 at best.. so for 100 a/h your looking at 25 amps.. !

not enough.. so I did a reverse calculation to establish the capacity you would actually need to achieve the goal..

basically you need 75 amps * 4 plus a Peukert correction and you get: 465 amp/hrs at 24 V..

with this size battery you will stand a chance of some success..

so you see it's a tall order to heat water this way.

now, propane has an energy of 91500 btu's per gallon.. much better.. !

this is approximately equivalent to 23 kW/hrs of electricity..

so i would strongly suggest you heat with propane.. !