Can you guys give me a reason why tankless electric water heaters would be such a bad idea? I'm just not seeing it and would rather seem like an idiot to you guys and ask this question here than to put it out to the military and have it not work.
As has been noted, the big problem with electric tankless water heaters is their _huge_ power demand. Many hot water users do not have continuous demand, so by using a storage (tank) type water heater your electrical demand is closer to the average power consumption.
Say that you need 1200 GPH for a half hour at a time, every 4 hours. If you have a 600 gallon tank, then you can spend 4 hours heating it up, rather than having to heat it up in 1/2 hour. You use the same number of kWh, but at 1/8 the kW. Smaller service, smaller wires, etc.
However, if the requirement is 1200 GPH of water on a continuous basis, 24x7, then the above benefit goes away. If you need 1200 GPH, every hour, then you need enough thermal power available to satisfy that requirement.
To meet that requirement you need about 233 kW of thermal power, which means 233 kW of electricity if you are using resistance heating.
You started this thread asking about demand factors, which is another way of comparing the average to the peak consumption. If the requirement is 1200 GPH, then your demand factor is 100%. If you were told that you need to supply 1200 GPH 25% of the time, then your demand factor would be 25%, but... With electricity the 'time constant' is very short; even if you _know_ that average consumption is only 400 GPH, if you design for this and by chance all 1200 GPH gets used, you will have breakers tripping in 10s of seconds to scant minutes.
This is were a tank comes in; by sizing your tank you can design the 'time constant' of your system. If your actual usage is only 400 GPH _on average_ but your peak consumption is 1200 GPH, then a tank large enough to run for the longest expected 1200 GPH period would let you cut down on the peak electrical power consumption.
As far as the choice between electric resistance heating or burning oil to provide the thermal energy, the question is 'where is the electricity coming from'?
If you are burning oil to run a generator to make the electricity, then keep in mind that the efficiency of that generator is somewhere around 40%, meaning to get your 233 kW of electrical power you are putting about 580 kW worth of fuel into the generator (in terms of heat output). So burning fuel directly to heat your water means you use quite a bit less of the stuff.
You can, in fact turn this equation on its head. Use a 200 kW diesel genset. Heat your water with the waste heat coming off the genset. Export the electricity to other users. Probably a hell of a lot more in the way of design hassle, and well off tangent of your original question!
-Jon
