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Running 1kw Immersion Heater Irom Inverter


Motters79

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Hey Canal Peeps

 

I've just installed 500w of solar panels on my boat with a 40 amp mttp controller. I have 3 110ah domestic batteries and a 1500w inverter.

 

Can I run my 1kw immersion heater from this setup during peak solar power generation hours for say an hour without taking my batteries too low?

 

A fellow boater insists he does with no problems. He does have 5 batteries though but on 150w of solar panel.

 

I have powered it from the inverter for about 15 mins and it certainly drew some current and got hot though the battery voltage dropped to about 12.4 in that time. The charge controller was putting in 28 amps which is the most I've seen going in but we'll within its capacity, but is it enough to put back I not the batteries with out discharging them too deeply?

 

My electrical know how is very limited as you can probably tell!

 

Any knowledgeableadvice would be welcome. 

 

Cheers

 

Matt

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Hi Matt,

 

A rough rule of thumb is that a 1Kw load on a 12v boat will be drawing 100a.  If angled at 90' to the sun, 500w of pannels might just about get to the 40a that the mttp controller you mention is rated at.  So, it looks like you'd be drawing 60a (100a - 40a) from the batteries, and if you have 150ah of effective capacity (though it's likely to be lower), you might get about 2.5 hours from the system before the batteries have been drawn down to 50% state of charge.  This is probably a best case scenario though!

 

Something that I've done is to replace the 1Kw immersion heater with a 500w one (e.g. a vetus one).  Mine draws 44a using a 2.5kw sterling inverter so is fine for my 60a mppt controller to keep up with on a sunny day.  If you switched to the 500w immersion heater with your current setup, you'd be drawing a tiny 4a from the batteries so could generate hot water with a tiny impact on your batteries. 

 

The hot water will take twice as long to warm up with the smaller immersion heater, but if it's hardly drawing from your battery bank then it being on for a good few hours shouldn't be a problem.  Once the tank is hot the summer evenings would give plenty of time for the solar to put back the slight discharge the batteries experienced when you were making hot water. 

 

A further option is that you can then turn the immersion heater on and off depending on how sunny it is with something like this: link

Edited by mattcyp
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Let’s do a couple of sums.

Your load including inverter losses will be approx 1080watts which at 12.5V is a current of  87amps.

Your solar panels - assuming uk summer mid day sun and flat to the rough and you are in the south of the uk - will be perhaps 400w of output which is 32amps.

demand is 87 A and solar is 32Amps the ‘shortfall’ of 52Amps to come from the batteries.

 

So after 1 hour you will have used about 50ah.  After 2 hours about 100amp hours.  

 

If if you did this between 11am till 1pm and you don’t use much power after 1pm there is probably time to recharge your batteries before the sun goes down.

 

This also assumes your batteries were reasonably full by 11am.   The danger with this method is that if it clouds over at 1pm there is no chance of recharging that day.

 

added - no so different from the thoughts above

Edited by Chewbacka
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The big questions are :

 

1) is the sun going to shine and give you max panel output for sufficient time ? (I'd suggest that max output is for minutes only, and as an average across a number of hours you will only achieve 50% of the rated output)

 

2) Will you remember to switch off the inverter when the sun 'goes in'. I didn't and drained the batteries in very short order.

 

It takes more power to heat the water using a 500w heater than it does a 1000w heater (not only twice as long, but the temperature losses will be at the same level, but for twice as long)

Imagine the heat losses are equivalent to 250 Wh, using a 1000w heater you are actually achieving 750Wh, using a 500w heater you are actually achieving 250Wh.

 

Having done it and left the immersion heater 'on' ( I have a 1300Ah battery bank) I would say never again, don't do it.

Edited by Alan de Enfield
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I think I’d only want to do it if I had an automatic system to shut off the immersion heater at a predetermined (and fairly high) state of charge. You can do this sort of thing with Mastervolt kit, but not on a tight budget!

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1 minute ago, Alan de Enfield said:

 

Having done it and left the immersion heater 'on' ( I have a 1300Ah battery bank) I would say never again, don't do it.

My 500W of solar rarely gives over 30A even in the best sun and then deduct the other stuff running ie fridge, chargers, tv so unlikely you will have 30A to offset the draw from the immersion.

The calcs above show it can be done but you may be drawing 60-80A from the batteries. Think then about lifetime of the batteries.

A lot of poeple on here have stories of batteries lasting less than 12 months. Most of the problems are down to sulphation caused by persistant under charging - but I dont think we know enough about failure modes. My gut feel says that to get your batteries to last 4 years plus, you need to get them back to fully charged regularly (3 to 4 times per week????) BUT also not to expose them to rapid discharges. On our lumpy water boat 10 years ago, our batteries lasted 5 years and were not always fully charged regularly but they only ever saw 80A discharge current for 20-60 secs or so when the anchor windlass was operated. We only had a small inverter so never really exposed the batteries to more than 10A discharge ...if that.

Back to my current NB, I see discharge rates of 100A when the Nesspresso machine is on, or Mrs Bobs hair drier (a bit less than 100A)- both for only a minute or so at a time,  but I certainly wouldnt put the batteries under the strain of discharging at 60-80A for half an hour or an hour just to heat water. If you do then I guess you will get plate shedding (or whatever...... @WotEver will be along shortly to expand on that!) or other bad things and that will affect battery life. It will certainly add to the 'cycles' count.

I would never try this even on a really sunny day and I have 660Ahr of capacity. I would run the engine or run the ebersparky thingy to heat the water. Must be cheaper in the long run.

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12 minutes ago, Alan de Enfield said:

Having done it and left the immersion heater 'on' ( I have a 1300Ah battery bank) I would say never again, don't do it.

I think you make a good case for trying it but I recognise the risk of leaving it on is a big one, especially with my brain ? however, my element has a stamdard plug on the end and so I could easily use a timer between it and the inverter so it would automatically turn element off after given period.

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3 minutes ago, Motters79 said:

I think you make a good case for trying it but I recognise the risk of leaving it on is a big one, especially with my brain ? however, my element has a stamdard plug on the end and so I could easily use a timer between it and the inverter so it would automatically turn element off after given period.

You are entirely free to do as you please, but just take note of the previous few posts - drawing 50 / 60 / 80 amps from you battery over a sustained period is definitely 'not good' for your batteries.

With your 3 batteries you have an effective 'usable storage' of 150 Ah, (assuming they are brand new or have been recharged to 99.999% every day to avoid sulphation)

Don't forget any other loads (fridge, freezer, pumps, TV, phone charger etc etc) will all be making further demands on the batteries.

 

It would be cheaper to run your engine for an hour per day (1 litre of diesel - say £1) for 3 months than it would be to replace your batteries.

 

 

 

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11 minutes ago, Motters79 said:

I think you make a good case for trying it but I recognise the risk of leaving it on is a big one, especially with my brain ? however, my element has a stamdard plug on the end and so I could easily use a timer between it and the inverter so it would automatically turn element off after given period.

Just another thought. If you assume the current in varies a bit with clouds and you are running other stuff then running the immersion for an hour could use 80Ahr net which is not far off 30% of your battery bank, which then will take a lot of hours to replace. Assume your batteries are good for 300 cycles (to 50% DoD) and cost is £300 then you are not that far of £0.60p per hour used in battery cost - and maybe £1 per hour if you cant get back to full charge.

Cheaper to use your eberspaky thingy.

 

ETA. Post crossed with Alan's. Agree on cost implications.

Edited by Dr Bob
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1 hour ago, Alan de Enfield said:

 

 

It would be cheaper to run your engine for an hour per day (1 litre of diesel - say £1) for 3 months than it would be to replace your batteries.

 

 

 

This is the best advice so far!! Heating water thro leccy when not on hook up is not a clever idea. We are in the UK not Spain. If you have a modern keel cooled engine on a narrowboat for instance then as Alan says running for one hour and ive today paid 73 p per litre so say a quid will throw lots of charge in to your batteries and give a tank of hot water. Solar is fab but has its limitations even in summer.

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27 minutes ago, mrsmelly said:

This is the best advice so far!! Heating water thro leccy when not on hook up is not a clever idea. We are in the UK not Spain. If you have a modern keel cooled engine on a narrowboat for instance then as Alan says running for one hour and ive today paid 73 p per litre so say a quid will throw lots of charge in to your batteries and give a tank of hot water. Solar is fab but has its limitations even in summer.

And, would it be reasonable to assume, that the boat will be moving occasionally anyway (even if in London, they need to get water etc) so 2 or 3 hours engine running and 'hot water for free'.

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2 hours ago, mattcyp said:

A rough rule of thumb is that a 1Kw load on a 12v boat will be drawing 100a.  If angled at 90' to the sun, 500w of pannels might just about get to the 40a that the mttp controller you mention is rated at.  So, it looks like you'd be drawing 60a (100a - 40a) from the batteries, and if you have 150ah of effective capacity (though it's likely to be lower), you might get about 2.5 hours from the system before the batteries have been drawn down to 50% state of charge.  This is probably a best case scenario though!

If your pulling 60A from  a 3x 110AH (at C20) battery bank thats 20A per battery so much closer to C3 or C5 discharge rates so capacity will not be 150AH.

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Maintaining battery health in a simple boat installation is difficult enough as the briefest of searches on here will reveal. To me, the answer you need is a simple one: if you have to ask this question, you don't have sufficient knowledge to do it successfully. :)

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Thanks for all the constructive comments! 

 

As suspected the disadvantages seem to outway the perceived benefits. To be honest it never crossed my mind till a fellow boater suggested it. Unfortunately he's not around to question further.

 

I think I won't be doing it. I've just bought the batteries and I want them to last a few years. The solar panels are working a treat, so much so that I no longer need to run the engine daily for power thus I've been feeling a bit sore about running it just for hot water, but its my only source, other than boiling a kettle on the hob or lighting the stove (and cooking myself) which I have been doing as my wood is plentiful and free. 

 

I'm thinking seriously about fitting an Eberspatcher for hot water and putting some rads in too. 

 

I was rubbing my hands together at the prospect of free hot water from my panels but I have to be realistic and maybe its a leap too far!

 

Cheers guys

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9 hours ago, Alan de Enfield said:

And, would it be reasonable to assume, that the boat will be moving occasionally anyway (even if in London, they need to get water etc) so 2 or 3 hours engine running and 'hot water for free'.

Don’t let the tax man hear you say that.

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13 hours ago, Motters79 said:

Thanks for all the constructive comments! 

 

As suspected the disadvantages seem to outway the perceived benefits. To be honest it never crossed my mind till a fellow boater suggested it. Unfortunately he's not around to question further.

 

I think I won't be doing it. I've just bought the batteries and I want them to last a few years. The solar panels are working a treat, so much so that I no longer need to run the engine daily for power thus I've been feeling a bit sore about running it just for hot water, but its my only source, other than boiling a kettle on the hob or lighting the stove (and cooking myself) which I have been doing as my wood is plentiful and free. 

 

I'm thinking seriously about fitting an Eberspatcher for hot water and putting some rads in too. 

 

I was rubbing my hands together at the prospect of free hot water from my panels but I have to be realistic and maybe its a leap too far!

 

Cheers guys

Doing it in fine weather very occasionally is probably fine, but doing it regularly is going to not end well.  You can if your alternator is big enough turn on the immersion when running the engine at a fast idle (say 1200rpm) but again forgetting to turn off the immersion when you stop the engine is only going to end in tears.........

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40 minutes ago, Chewbacka said:

Doing it in fine weather very occasionally is probably fine, .....

Not really. It will reduce battery life which will increase the cost per day of the bank - and it probably works out cheaper to do it other ways as said above.

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47 minutes ago, Chewbacka said:

You can if your alternator is big enough turn on the immersion when running the engine at a fast idle (say 1200rpm)...

But if the engine is running you’re going to get hot water anyway unless it’s an air-cooled Lister. (And if it is, then it’s unlikely to have a big alternator...)

 

Edited by WotEver
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22 minutes ago, WotEver said:

But if the engine is running you’re going to get hot water anyway unless it’s an air-cooled Lister. (And if it is, then it’s unlikely to have a big alternator...)

 

Yes, but you would only need to run for (say as an example) one hour rather than 2, and it puts a bit of load on the engine which is no bad thing.  But sooner or later ‘I’ would forget to turn off the heater, which is why I very rarely do it.

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32 minutes ago, Dr Bob said:

Not really. It will reduce battery life which will increase the cost per day of the bank - and it probably works out cheaper to do it other ways as said above.

I meant just once or twice a year if there is no real alternative, for example moored next to someone’s house which could mean blowing fumes into their open windows.  

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8 minutes ago, Chewbacka said:

I meant just once or twice a year if there is no real alternative, for example moored next to someone’s house which could mean blowing fumes into their open windows.  

No one forced them to live that close to the canal ? besides how grim is your engine :o

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