Using solar water heating elements for hot water

Electricity is pretty inefficient at heating things up. The toaster takes a couple minutes to do its job. Takes a few seconds over a propane stove. I can make coffee water at least twice as fast with the propane stove vs the electric. For example.

Heating hot water sounds like a battery bank destroyer to me.

May be more info coming. None of us are the final word. Just sharing personal experience and what we think...

Zakarume wrote: »

Thanks for the Info.

Well, I've been heating water with a standard 240volt heating element 3600 watt and run off 120 which equates to about 900 watts, I've taken a couple luke warm showers and one pretty darn cold one in the last 6 months. Winter,I'll use the wood stove as an emergency shower water using a 'sun shower' type bag. Kind of roughing it, but it hasn't been too bad, I get 3 warm showers on a fully heated water tank. Having a shower head with a shut off valve helps, or you can install one behind the shower head. A low flow shower head also helps.

Takes quite a while to heat water at 900 watts, I think it runs around 3 1/2 hours in summer and 5 or more in winter. It's rare for me to heat water off the battery. It's usually an opportunity load, though I turn it on in the morning so it's drawing from the battery until the sun has a pretty good angle on my 4000 watt array. I need to set this up to run as an opportunity load from the Midnite charge controllers, but I've gotten into a rut of things working as is, Likely I'll change it this fall when wiring in the extra 1000 watt into the existing charge controllers.

Water heating elements are a pure resistance load, so you can use an A/C element and run DC through it, most of the non digital thermostats will work as well, there are some old threads here on the subject. You can hunt on the web and find the rough equivilant DC load from the A/C rating, the math is basically an inverse square half the voltage yields 1/4 the wattage.... I'm sure someone with the ability to understand how the math works out will let you know, but DC elements at 400 or maybe 600 watts at 24 volts will take a long time to heat even a 20 gallon tank, You might get a 10 or 12 gallon tank with the high insulating value from Ace hardware, and be able to heat enough water for a single shower on sunny days, perhaps put in parallel with your existing hot water heater.

I looked into this a half dozen years ago. My initial thoughts were that it just had to be good. Then after re-running the figures it looked less and less good. In the end I decided it was not worth the effort. Whether or not it might be useful for someone else may depend a lot of the size of the array, or rather how much excess is generated by said array. At home in the suburbs a roof top solar water heater does a super job at heating water. That heats water in a storage tank via a heat exchanger coil. When that is insufficient the net gas burner kicks in. In summer it hardly ever kicks in. I decided it was not worth the expense at the recreational use cabin so there we burn propane for hot water.

Hope this may help.



Many people think that a water heater would be a perfect device to dump power into it. Why not use the water heater to get rid of the extra power that a wind turbine or panel produces and at the same time heat your domestic water. Why not just hook your 12 volt or 24 volt to that 220 volt heat element to generate hot water.

Let's look at the possibility of using a water heater a little closer.
Let's also say that the heat element for that water heater is a 2000 watt heat element and it is operating at 220 volts.
Now we have some calculations to do.

We need the amps this heat elements will use first.
Watts divided by volts = amps. 2000W / 220V = 9.09 A amps.

Now we need the ohms this heat element has.
Volts of heat element divided by amps of heat element = ohms of heat element. 220V /9.09A= 24.20 ohms.

Now we can calculate the wattage that 2000 watt heat element can put out or dissipate at 12 volts or 24 volts input .
Output watt of the element = volt times volts divided by the resistance.
At 12 volts = 12V X 12V /24.20 ohms = 5.95 watts output from that 2000 watt 220 volt heat element.
At 24 volts = 24V x 24V /24.20 ohms = 23.80 watts output from that 2000 watt 220 volt heat element.

Lets re-check our numbers for that 2000 watt heat element and use the 220 volt it was intended for just to make sure that there was no mistake in calculating it the right way.
Same calculation again, output watt of the element = volt times volts divided by the resistance.
220V X 220V /24.20 ohms = 2000 watt output

Conclusion.
The output wattage on that 2000 watt heat element does not look promising at all if you use 12 volt or 24 volt as power source for that 2000 watt 220 volt heat element and you will get only a few watts out of it or will dissipate only a few watts.
Someone would be better off just to get a heat element with the proper voltage that is used for their system.
In other words, get a 12 volt or 24 volt heat element and install it in your tank if your system is a 12 volt or 24 volt system to maximize your power output.