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BjarneK · 08-02-2017, 15:58

In another thread, I noticed that some people are contemplating using an inverter to run their water heater on their boats. This is a nice way to use excess power from solar panels. The problem, of course, is that it uses a lots of power leading to high drain on the batteries, large currents, and the need for a decent sized inverter.

To solve that problem, I run my 220 V water heater using a dedicated 110 V inverter. Since the power used grows with the square of the voltage for a resistive load (P = U*I = U*U/R), the heating element only uses a quarter of the power at 110 V as it does at 220 V.

I am aware of a long and painful thread about this subject here: http://www.cruisersforum.com/forums/...ly-143207.html

With this post, I just wanted to let people know how this actually works in real life.

My water heater has a 700 W heating element at 220 V. This comes out to 175 W at 110 V. I use a 110 V Victron Phoenix 12/375 inverter which is rated at 260 W continuous load at 40 degrees C, so has plenty of power. When running, it uses about 17 A at 12 V.

I run the water heater whenever I have excess power available or when I expect this to become the case during the day. This is enough for (short) hot showers for my wife and I as well as hot water for dish washing. This is using leftover power from 600 W of flat mounted solar panels in the tropics, so lots of sun and fairly warm water in the water tanks. But further north, the days are longer in the summer, so I expect this setup to be useful there as well.

The water of course heats slower at 110 than 220 V, but the heating element is still able to heat the water to the point where the thermostat turns off the heater. This takes many hours, so I have only run it this long once for testing.

For 110 V boats, I would think that it may be possible to buy a 220 V heating element and create a similar setup to mine.

You can install a remote switch for the Victron inverter, so it is easy to switch on and off. An added benefit is that I now have a backup inverter if needed for charging computers, cameras, etc. (most chargers accept both 110 and 220 V).

Paul L · 08-02-2017, 16:16

What is the advantage of using 175 watts for 3 hours versus using 700 watts for .75 hours? They both end up using 575watt-hr.

senormechanico · 08-02-2017, 16:23

It's all the same except longer times, smaller inverter and wiring.
We use our 1500 watt sine wave inverter to make hot water in the summer when on vacation.
The hot water tank element is 1,000 watts, and we replaced the insulation with a spiral wrap of about 2 inches of Aerogel.
20 minutes a day keeps us in steaming hot water for dishes and showers.
440 watts of solar.

BjarneK · 08-02-2017, 16:24

Quoting from the first paragraph of my post:

Quote:

high drain on the batteries, large currents, and the need for a decent sized inverter

Also, some water heaters probably use somewhat more than 700 W at 220 V making these issues even more relevant. My own main inverter is 800 W and would not appreciate 700 W continuous.

offpist · 08-02-2017, 16:31

Its always more efficient to use excess energy instead of using energy from the batteri and then to charge battery again. And with lage energy the Peukert plays in..

We have LIFEPO4, so I dont mind to use 230V water heater at full power (about 1000W) on days when we can afford to waste power.

Paul L · 08-02-2017, 16:57

Quote:

Originally Posted by offpist

Its always more efficient to use excess energy instead of using energy from the batteri and then to charge battery again. And with lage energy the Peukert plays in..

We have LIFEPO4, so I dont mind to use 230V water heater at full power (about 1000W) on days when we can afford to waste power.

There's inefficiency in the inverter too. Best would be to have a secondary 12v heating element and run the excess solar directly to it. There was a company in Fla that used to sell these.

rwidman · 08-02-2017, 17:14

It takes the same amount of electrical energy to heat the same amount of water to the same temperature.

Wouldn't it be more efficient to heat the water directly from the sun without the conversion to and from electricity?

Cheechako · 08-02-2017, 17:17

Quote:

Originally Posted by Paul L

What is the advantage of using 175 watts for 3 hours versus using 700 watts for .75 hours? They both end up using 575watt-hr.

Load on the invertor, load on the wiring for example....

dlymn · 08-02-2017, 17:24

Running a system simple radiant heater designed for 240v on 110v works but very slowly. Much smarter to use a dedicated 240v inverter. A modified sine wave uint is cheap and should do OK. Don't forget that you will waste a little energy operating the inverter.

Hobie_ind · 08-02-2017, 18:04

Heating a bunch of water up with electricity just in case you use some of it later never made any sense to me, particularly if you're spending the bulk of your time in the tropics. I can see it when it's a byproduct of running the engine from coolant heating but not electrically on a boat.

When the time came to replace our hot water heater, I was seriously considering replacing it with a 220v 'on-demand' unit. Then for the occasions when we needed hot water, simply start the generator and have a shower while giving the batteries some extra charge and make some water. Plus I could have used the space in the engine room where the hot water tank was for a tool box and vice.

When we lived in Australia our house had one and we loved it. Unlimited hot water and zero cost when you weren't using it. I spoke to a few Kiwis who had done the same and they were quite happy with their choice. As it was, we sold the boat before the final decision was necessary. Next boat I guess!

senormechanico · 08-02-2017, 19:26

Quote:

Originally Posted by rwidman

It takes the same amount of electrical energy to heat the same amount of water to the same temperature.

Wouldn't it be more efficient to heat the water directly from the sun without the conversion to and from electricity?

We actually did that on our previous boat.
I made a 2'x4' (approx) hot water panel with copper tubing covered with plexiglass and integrated it in the center of our stern arch solar panel array.
A food grade march pump recirculated the water from the output and input connections on our hot water tank after going through a non return valve.
The pump was controlled by a couple of thermisters and a differential controller.
One thermistor was on the tank and the other was on the array.
The system's pump ran only when the panel was hotter than the tank and it was silent.
We always had hot water for showers and dishes.
The only down side of the system was that I needed to periodically burp the air at the panel on the arch.

For that operation, I installed a small needle valve at the aft end of the array.
People looked at us with very strange looks as they saw water shoot our of our apparently electrical solar panel array.

On our current boat, I just use the inverter.
LiFePo4 cells work great with this system.
It's lighter overall, simpler and no burping is necessary.

Stumble · 08-02-2017, 19:27

Quote:

Originally Posted by Paul L

There's inefficiency in the inverter too. Best would be to have a secondary 12v heating element and run the excess solar directly to it. There was a company in Fla that used to sell these.

Seaward still makes them, but they are pretty small. 3gallon 300w units.

valhalla360 · 08-02-2017, 21:21

Fundamental problem with your analysis. To heat the same amount of water to the same temperature uses the same amount of power. Changing the voltage doesn't solve that problem.

Most people using solar to heat water put in a voltage switch so when the batteries reach a high state of charge, it diverts the excess power to the water heater (or you can do it manually if you like to watch your battery state). At this point, however many watts your solar panels put out is how many you can put into heating water. For best efficiency, it would probably be better to use a DC heating element and power it directly off the panels DC output.

Heating water off the batteries is just silly.

PS: As others have said, in the tropics (or even just summer in most areas), you don't need a lot of hot water and direct solar heating works well. We have solar shower and on a hot sunny day, the water is almost to hot.

**StuM** · 08-02-2017, 22:24

Quote:

Originally Posted by rwidman

It takes the same amount of electrical energy to heat the same amount of water to the same temperature.

Wouldn't it be more efficient to heat the water directly from the sun without the conversion to and from electricity?

\
Yes, if you can arrange enough sun exposure to the water.

But the point is to use surplus energy available from the sun hitting the solar panels, not to try to capture more energy by taking up additional space for solar capture.