Electric Point of use HW Heater powered off of solar set-up

[Grimelowe]

Newbie questions...
I'm planning out my build, and I'd like to have a small HW source for bucket bath, handwashing, doing dishes etc.  

1. I understand that using electric for heat is not the most efficient, but is it doable?

I found this:
Bosch Tronic 3000 T 2.5-Gallon Electric Mini-Tank Water Heater
2.5-gallon point-of-use mini-tank fits under your sink to provide hot water right where you need it
Electric water heater is easy to maintain and has glass-lined tank for long service life
Plugs into a 120 volt outlet for independent installation or in-line with a large hot water source
1400 watts

2. What do you think?
3. I'm assuming the 1400W draw would only be occasionally...and when in use.  How do I convert this into Watt/hours for sizing my solar set-up?
 (Other big draws are 12V fridge, LED Lighting, and occasional blender and Phon/Computer charging...I was looking at something close to the Renology 400 Watt kit.)

-Grimey

 

[tx2sturgis]

I installed a 3 gallon point-of-use electric water heater in my old motorhome, and they do work, but the amount of hot water you get is not very much. I had installed a 900 watt element in mine, which took longer than the 1400 watt element, but was less drain on the motorhome systems when the AC or microwave was operating. 

3 gallons is just enough to take a very quick 'navy' style shower, but plenty for dishwashing a few pieces or other minor tasks. 3 gallons sounds like plenty, but as the cold water rushes in to refill the tank, it cools off very quickly, so figure roughly about 1 gallon of 'hot' water in the winter.

Mine took about 15-45 minutes to heat the water, depending on whether it was warm water sitting in the tank in the summer, or cold water in the winter. Generally speaking, during cold weather, I took showers less frequently. 

But if yours ran for 30 minutes a day:

1400 x 1/2 (of an hour)= 700 wh a day.

Figure inverter losses, making it ~900 wh a day.

That's a BUNCH!

Just FYI, if the water heater you buy can accept standard water heater elements, then you can buy 12v elements that just screw in, which will cut inverter losses, but will mean that it will not operate directly from 120v AC, if you were on shore power. 

With the other items drawing power like the fridge, I think you are looking at either a much bigger solar array, or conserving power and only using the tank heater a few times a week.

 

[akrvbob]

My microwave draws 1200 watts and that works out to 2 amps a minute. You need about 400 watts and 4 golf carts to run it along with a 2000 watt pure sine inverter.

Bear in mind that huge draws give your batteries a false reading. So if my batteries read. 13. Volts when the micro comes on, it will read 11.9. That works, but the longer it runs the further it will drop, the faster it will drop. Very soon the inverter will think your battery is low and cut out.

To run it for 20 minutes will take more batteries.

 

[jimindenver]

I use a electric Hott rod in my 6 gallon water heater. It is only 450w, plugs into the inverter and takes a few hours to get 6 gallons up to 150 F. With both it and the gas burner going I can take as long of a shower as I want with hook ups.

 

[tx2sturgis]

My microwave draws 1200 watts and that works out to 2 amps a minute.

I think ohms law took a left turn there somewhere. 

I believe you mean that a 1200 watt oven takes about 100 amps (more or less) of input current to run the inverter with that load, and if the oven actually ran for an hour, that would be 100 ah of drain from a battery bank. 

Divided by 60 minutes in an hour, you end with 1.66 amp-minutes...which is an unusual unit of measurement, but it actually could be handy when dealing with short duration loads like a microwave oven. Add inverter loss and I see where the figure comes from.

 

[John61CT]

2 amps per minute has no meaning.

> Divided by 60 minutes in an hour, you end with 1.66 amp-minutes

No, uses 1.66 AH per minute, is how that works.

Which is pretty close to 2AH / min, must be what Bob meant

 

[Weight]

Bob must have missed a decimal. I did notice my 2000 watt microwave pulled 130 DC amps running on high. Measured by the Victron BM.

 

[Weight]

Some solar systems are set up to dump the excess power into a water heateR rather than a heat sink. My guess is it too complicated for my simple van. I boil a couple of quarts of water using gas. That is poured into cooler water to have nice wash water. Those propane heated camping shower devices are better. On a past RV I had an insulated drum with a coil of copper tube circulating engine coolant to heat the potable water.

 

[Grimelowe]

So it seems like the concensus is to avoid this, and consider propane instead...  I appreciate all your feedback!

 

[tx2sturgis]

Rephrasing what I said, you can heat water with a modest solar PV array, thru a controller, batteries, and an inverter, but just don't expect dozens of gallons of abundant hot water every day.

And like HDR says: heating anything with electricity is inefficient, and certainly in this case, the overall system for doing it is expensive and complex, unless you already have an abundance of solar energy like Jimindenver has.

Solar power IS a renewable energy, whereas propane has to be bought on a regular basis.

Solar shower water bags are cheap and pretty darned efficient!

 

[jimindenver]

That is right, there is more than one way to skin a cat. Passive solar can be just as fast and much less expensive/ simpler to use and unlike a solar powered water heater, you can heat water on a stove and dump it into a solar shower bag when the sun isn't shining.

 

[Weight]

Bob must have missed a decimal. I did notice my 2000 watt microwave pulled 130 DC amps running on high. Measured by the Victron BM.

I didn't allow for the solar the day I noted these amps. During a cloudy raining day the microwave needed 148 amps for the 5 minutes needed to cook vegetables.

 

[akrvbob]

2 amps per minute has no meaning.

> Divided by 60 minutes in an hour, you end with 1.66 amp-minutes

No, uses 1.66 AH per minute, is how that works.

Which is pretty close to 2AH / min, must be what Bob meant

No, I added 10% for inverter waste and the rest is for Peukert Effect. The math for Peurket is well beyond me so I made a wild guess. But it must be included.

 

[akrvbob]

I didn't allow for the solar the day I noted these amps. During a cloudy raining day the microwave needed 148 amps for the 5 minutes needed to cook vegetables.

My micro draws 1200 watts, divided by 12 volt that's 100 amps per hour. I can't imagine how any 5 minute burn of a microwave can draw 148 amps DC.

 

[tx2sturgis]

No, uses 1.66 AH per minute, is how that works.

Which is pretty close to 2AH / min, must be what Bob meant

Yep, my mistake in typing the result. So many threads, so little time...

 

[akrvbob]

Bob must have missed a decimal. I did notice my 2000 watt microwave pulled 130 DC amps running on high. Measured by the Victron BM.

That's per hour. How many amps does it pull if the micro is only used for 30 minutes? Half of that. How many Amps does it run if only used for 1 minute? 1/60th of that.

 

[tx2sturgis]

That's per hour. 

No, if the figure for a 2000w oven is 130 amps, being shown on an ammeter for example, that's 130 amps current flow. No time figure, just 130 amps. Not per hour, or per any unit of time, UNLESS you are talking about energy being drawn from a source like a battery over time, then the figure, in this case, is 130 amps TIMES one hour, (which is ah or AH) not PER one hour.

PER means 'divided by' but we are not dividing, we are multiplying.

130 amps (being used by or drawn into a load) TIMES one hour. So thats 130 AH in one hour. 

Sounds redundant, I know, but that's how it works. 

Ah or ah or AH means amps TIMES one hour, not amps PER one hour. 

Unfortunately, its VERY common to see it printed that way on websites. I've seen it stated that way on the truckfridge website for example.  A certain fridge uses '20 amps per hour'. It's still not correct. 

What the webmaster means is that the fridge pulls 20 ah per day...(20 amps TIMES one hour, PER day).

Then it's correct.

 

[John61CT]

No, if the figure for a 2000w oven is 130 amps, being shown on an ammeter for example, that's 130 amps current flow. No time figure, just 130 amps. Not per hour, or per any unit of time, UNLESS you are talking about energy being drawn from a source like a battery over time, then the figure, in this case, is 130 amps TIMES one hour, (which is ah or AH) not PER one hour.

PER means 'divided by' but we are not dividing, we are multiplying.

130 amps (being used by or drawn into a load) TIMES one hour. So thats 130 AH in one hour. 

Sounds redundant, I know, but that's how it works. 

Ah or ah or AH means amps TIMES one hour, not amps PER one hour. 

Unfortunately, its VERY common to see it printed that way on websites. I've seen it stated that way on the truckfridge website for example.  A certain fridge uses '20 amps per hour'. It's still not correct. 

What the webmaster means is that the fridge pulls 20 ah per day...(20 amps TIMES one hour, PER day).

Then it's correct.

Just to nitpick.

You're absolutely right, cannot says "amps per" any time unit.

Only AH or watt-hours can be per hour, minute, or day.

And 20 AH isn't really "times" but time duration.

Like 20A over an hour, for an hour.

Or 5A for 4 hours, 1A over 20 hours.

Think AH are gallons, Amps is the flow rate at a given moment, oz per sec.

 

[tx2sturgis]

Well that's slicing frogs hairs...and it depends on whether you are talking about an electrical quantity of current, (which I was) or capacity in hours from a battery.

20 amps for one hour duration is the same thing for our purposes as 20 amps times one hour. I was trying to help the readers understand the difference, spelling it out in common person-to-person language. 

In fact the very definition (on wikipedia) of amp-hour or Ah is:

"An ampere hour or amp hour (symbol Ah; also denoted A·h or A h) is a unit of electric charge, having dimensions of electric current multiplied by time"

If you are strictly referring to battery drain, then yes, the formula is more correct when thought of as the current flow FOR one hour of time. 

 

[Weight]

slicing frog's hair? We-all is good at that. My microwave drew 148 amps to power the microwave. It drew 148 amps continuous for 5 minutes. My wire and fuses are sized so the microwave can draw that many amps plus some. At any one moment.
As 5 minutes equals 1/12 of one hour. 1/12 is 0.0833 decimal. 148 amps times 0.0833 hours is 12.33 amp hours. If I planed to use 13 amps for my microwave the fuse would open. No fuse? The wire's insulation would melt and burn.