Well, those things use MASSIVE amounts of electricity when they are on.
if you are planning a 12V system, and one of those water heaters (I'm assuming 4KW draw when running. . .)that is 333AMPS at 12V into an inverter. . .
Even if your system was 48V, you'd still need 83AMPS.
And these figures don't take into account losses in conversion. . .
So, you have a Tankless water heater that uses 333 amps and we'll say 220 amp-hour 6V batteries on a 24volt system.
Those batteries can do 220AH, but you need 333 so to start with you need 8 batteries. Then you never want to drain them below 80% charge so you have to multiply the batteries by 5. that is 40 batteries. So, now you take a shower. Say it was 15 minutes long. You pulled roughly 1/3 of your usable battery capacity, about 120 amp-hours.
Now, get your solar panels out. suppose thay are 150 watt panels at 24Volts.
150 Watts at 24V is 6.29 amps.
Using the sun-hours for my area, 5 hours, times 6.29 amps, is 31 amp-hours.
You used 120 amp-hours on one shower. Now suppose you take one shower a day.
120 amp hours for that shower, divided by the 31 amp-hours from the panels, you need 4 panels just to take that one shower per day. And this depends on sun, FULL SUN every day. And this does not take into account losses associated with charging and discharging the batteries and in the inverter.
So, lets do some other math here. . .
- KW inverter (guessing this here and the price) $3000
- 150W solar panels at $5 per watt is $3000
Batteries, 40 of them and we're going to be cheap here and use Wal-Mart golf car batteries at $50 each is $2000.
Add this up and then buy the wiring and all other associated stuff and it is going to be in the neighborhood of $8000-$9000 to equip yourself to run that tankless, on demand water heater on solar energy.
Spend about $1000 and make a solar thermal water heater with a tank and be done with it. You could ptobably build a great one for less than $1000. . .
I am sure my numbers here are off, but this hypothetical. Those who are more into the math thing can probably get better numbers, but these are probably usable numbers. . .
JMHO - YMMV
Doug