Electric Water Heater and Batteries

[hummingway]

So far I've got 840 ah from 4 SRM-29 Interstate batteries. I'm ready to install a Pheonix Multiplus 12/2500/120 inverter/charger. This gives me a 120 amp charger along with a 2500 watt inverter. I'm uncertain whether I should wire the hot water heater seperately from the rest of the panel so that it feeds only from the shore panel or from the inverter and choose whether to run it or not by switching the breaker when needed. Would it be hard on the batteries if only run when on shore power (or generator) given the size of charger? Is there any chance that with a solar array of 440 watts that the hot water tank could be run for 30 minutes in a day (given a perfect day)?

I appreciate the help.

[hummingway]

By the way it's a Seaward 6 gallon 120v 1500 watt heater

[Windsun]

1500 watts comes out to 125 amps (plus any efficiency losses) at 12 volts, so your batteries would last about 2 hours max with the heater on.

[AnchorageGuy]

We have kept the hot water heater off the inverter and on shore power only. It is heated through the engine heat exchanger so if we really want hot water and we are not plugged in we run the engine for an hour. Most cruisers we know do this unless they have a generator. I am not sure about your question regarding harm to the batteries if it is connected to shore power. I don't see how that will matter. After a couple of decades of cruising with both solar and wind aboard, even though we did our calculations well, there were many times when we had to run the engine to help with both charging and to heat the water so we very seldom ran the engine just for hot water. In the tropics we never used the hot water heater. WG

[btrayfors]

Wiring the HW heater thru the Victron wouldn't do any harm to the batteries so long as you're plugged into shore power or if the generator is running. If the shore power or generator power should go out, though, you'd be pulling about 130 amps from the batteries while the heater was heating. Also, in the event that there wasn't enough shorepower available to run whatever loads are switched in, the auto-supplement feature of the Victron would kick in and begin drawing power from the batteries.

At present, I have a MultiPlus which provides 120V AC output to Line 1 -- one of two 30A lines on my boat. Unfortunately, my hot water heater is wired into line 1 as it was originally on the boat. The problem is that my generator -- a NextGen 3.5KW model -- has a pretty lousy AC waveform unless it is loaded a bit. If the hot water heater were wired to the other line, I could use it to smooth out the waveform so the Victron would be happy. But it's not, and the Victron won't deal with the unloaded output from the NextGen, even though it's configured to "accept anything" in the way of AC power. The only way I can get it to settle down is to switch in loads on Line 2 (which does not pass thru the Victron), like an AC or electric heater. When I do this, immediately the waveform improves enough for the Victron to like it, so it begins passing AC downstream to Line 1.

The problem with the NextGen is pretty common, I think. And, it persists even though I have a special voltage regulator module installed on the NextGen.

So, in your stead I think I'd opt for wiring the hot water heater separately, i.e., NOT thru the Victron. That way, it could be used to smooth the output of the generator in case your MultiPlus doesn't like it :-)

Hope this makes sense.

Bill

[hummingway]

It does thanks.

[goboatingnow]

your setup is well capable of heating water via the invertor, if fact there is no point having 2500watts of invertor power if you cant use it.

The issue is can you recharge your batteries in a reasonable amount of time.

[hummingway]

That's what I keep thinking. I'm trying to decide on a solar setup but also a generator. Use the generator as required but if the end users aren't on the inverter then you can't utilize "free" sources as they are available. I'm guessing that my POV requires more monitoring but I'm OK with that.

[johnar]

Hello,

I first have a question I need to ask so I do not seam as stupid as I really am.

Question: when rating watts or ah as in this treard,

First off you state your battery bank is 840 ah, in my research I have found that most wet cell battery makers recommend that you do not draw below 50% so you would have 420AH useable ???

Now here is where I get lost the heater is 1500 watts @ 120volt = 12.5 amps, is that 12.5 amps per minute or per hour????

The reason I ask is if it is per minute that would be 750 amps in one hour.

I know I either have my math wrong or misunderstand something because I have an electric over onboard that draws 1500 watts and the other night my wife baked a chicken in the oven 35 mins but the draw on the battery bank was very low it was the first time we used it off the inverter so I kept a very close eye on the drain. I was very shocked when I seen a 20% drop after the oven use and the microwave. When we decided to change from LPG to Electric I thought we would have to cook off the genset.

Now onto what I have researched, solar power has come way down lately in price and so has wind, you can now find solar pannels from $0.68 to $4.00 per watt depending on grade you want. As for wind depending on where you look you can find it cheap in some unlikely places, Homedepot sells 400 watt Air X units for $585 and 900 watt units for just over $1,000 on the site they say they are land base units but if you go to the Air X site and look up the modle number you will see the marine unit is the same with the only differance is the nose cone.

I have read a number of treads that talk about "you get what you pay for" when you buy non Marine items they fail alot faster and I do agree to a point. This brings me to Gensets.

A friend had a 3,400 watt Genset installed in his boat, his cost was around $10,000. I spent hours doing research and have found A deisel genset new 6,500 watt remote start,water cooled, sinewave commercial grade for $850. No it is not a Kabota and was made in China but comes with a 3 year warrany.

I am sorry I did get a little off the track of the hot water heater but I felt that it did relate in power use and converion.

Dutch

[hummingway]

I believe it's per hour but because I want to use the inverter it is calculated at 12v not 120. Of course it won't be 12v exactly and 1500 watts is maximum power (probably during startup is my guess) but the draw is more likely 125 amps at maximum. My understanding is that amp hours is really for slow draw and may not mean anything at the high rate that the hot water heater imposes but it is the hours until the battery dies. My batteries being North American in origin are rated at C20 - based on 20 hour draw down. 840/20=42. I can draw 42 amps for 20 hours before they die. Not sure how this applies to this situation though :-(

[bobfnbw]

I rewired with a victon 12/3000/125. Using a 50 amp 125/250 shore power cord, it is split into 2 120v panels. One for inverter use, one for high loads. Even though I installed a isotemp water heater with a 750 watt load, I chose to use the heavy load side for it, meaning not thru the inverter. I felt that the while the inverter would handle it fine, I didn't want to do it that way. The heater is also wired thru the engine, and will keep water hot for several days.
Of course the high draw your water heater requires is only to heat it up to temp, once there the load will drop significantly I would think, except when you use hot water. So you could heat it up at dock side, then only have the heater keep it to temp... but you will use a lot of amps.
Consider changing to a lower draw unit like the isotemp or the or the volvo at 800 watts. They have a better insulation than most as well, so your water stays warm longer.
Bob

[SouthernComfort]

That water heater is the same as on my boat. The heat from running the auxilary engine for 30 minutes makes a nice shower. This is the design of the unit. Why bubba the system for what you need when you can simply start the ol' deisel up and take a shower?

[Zanshin]

I have the Victron unit and left my hot water heater to run through the inverter while I moved the watermaker and AC to bypass the inverter. In retrospect I should have done that with the water heater, since it draws over 150amps from the bank when active; I noticed this because I inadvertantly left the heater breaker on once and also left the Victron in 'inverter' mode overnight as well.
So far I've always heated sufficient water via the main engine or, if at anchor, flipped the water heater on with the genset running when the batteries are no longer being charged at high rates so that I've got a bit of load on the generator. I find that the hot water lasts a good 24 hours as well before going lukewarm.

[Windsun]

Originally Posted by johnar

First off you state your battery bank is 840 ah, in my research I have found that most wet cell battery makers recommend that you do not draw below 50% so you would have 420AH useable ???

That is approximately correct yes.

Now here is where I get lost the heater is 1500 watts @ 120volt = 12.5 amps, is that 12.5 amps per minute or per hour????

Dutch

There is no such thing as "amps per hour" or amps per anything. Amps is defined as so many electrons flowing per second, so saying amps per hour is like saying "miles per hour per hour". And 12 amps at 120 volts comes out to around 130 amps at 12 volts.

See this Home Power Magazine article for more of an explanation.

[btrayfors]

Dutch and hummingway. Amps is the amount or quantity of electricity moving in a circuit. It is only really meaningful when the voltage is known. Then, you can figure out the total power at any given moment, e.g., amps x volts = watts.

To figure how much power is consumed over time, you need to add the time factor. So, 150 amps drawn thru a 12-volt circuit will be 150 x 12 = 1800 watts. Over a period of an hour, this will be 1800 watt-hours. Or, divide by 12 to get 1800/12 = 150 amp-hours @ 12 volts.

If this were a 120-volt circuit instead of a 12-volt circuit, the total power (watts) would be the same, i.e., 1800 watt-hours. But the amp-hours would be only a tenth as much, i.e., 15 amp-hours.

When calculating total load (draw) on the batteries, you also have to account for inefficiencies in the voltage conversion from 12V DC to 120VAC. Victrons are pretty good in this regard, usually 90% or better.

BTW, a heater element is a pure resistive load. If it draws, say, 1500 watts it will draw that 1500 watts so long as it's on. It's 1500 watts or nothing. Just like a space heater. But, when the water is already hot and it cycles on and off to maintain the set temperature, the total power consumption (watt-hours) over time will be 1500 x actual on time in minutes/60.

Bill