converting shore kwh to battery amp hrs

[Alan de Enfield]

 

Yes but where does the "per week" come from - if weekly readings, then fair enough, but he's effectively made a new unit up, and somewhere (seems the consensus, unless he's doing something abnormally energy intensive) made an error. I am suggesting the "100" bit is correct but the "per week" might not be, that's all.

 

Fair comment.

I am working on an assumption (which may well be faulty) that OP has taken two meter readings (or the marina have provided his bill) and he has worked out his usage.

 

Maybe better if we cease the speculation and wait for the OP to come back with why his figures are what they say they are.

[Blazeaway]

Hi folks

I really appreciate all the replies, and will take heed of the comments when i get back on board.

For now though I am on a 'meter maid' that shows ive used 100 kWh in 10 days.

Immersion is on 24/7 as is fridge, freezer and inverter charger.

Domestic is 4 trojans.

I wont be using the immersion when out on the cut as will be moving everday exploring the whole network.

[Dave Payne]

Would suggest getting a timer for the immersion....

[Jambo]

Do you really need the immersion heater on 24/7 at the mooring? It means that you're losing heat from your tank at the maximum possible rate all the time. Much better to have it come on as little as possible (e.g. a few hours in the morning and a few hours in the evening or even less if you can get away with it).

 

What kind of fridge and freezer? If they're 230V do you know what their energy rating and annual consumption are?

 

4 x T105s (?) will give you 225Ah usable capacity which is a quarter of your demand on shore power. Losing the immersion heater will go a long way to getting there but I suspect the freezer is going to be too much. I'd suggest starting cruising without your freezer switched on and spending a week cruising at the sort of rate you expect to continue and monitor your battery SoC closely. You could then try switching it on and seeing what kind of extra impact it has and then work out whether it's feasible.

[Dave Payne]

Put your freezer on a timer also....

[blackrose]

Or to put it another way, you're gonna have drastically reduce your power consumption!

 

Yes, a lot of newbies moored in marinas on shore power don't seem to realise that once you disconnect that blue cable everything changes and it's all about finding ways to reduce one's power consumption. You can't just carry on as if you were still on grid.

[David Mack]

Hi folks

I really appreciate all the replies, and will take heed of the comments when i get back on board.

For now though I am on a 'meter maid' that shows ive used 100 kWh in 10 days.

Immersion is on 24/7 as is fridge, freezer and inverter charger.

Domestic is 4 trojans.

I wont be using the immersion when out on the cut as will be moving everday exploring the whole network.

 

I still find it difficult to believe that you are using significantly more electricity than a reasonable sized house. Are you using electricity to heat your space, as well as water? Otherwise, your figures are just not credible.

 

I should check your meter! Looks to me like you are being overcharged for electricity use.

[Dave Payne]

I would say with the immersion on 24/7 the figures could be correct?

[John V]

I would say with the immersion on 24/7 the figures could be correct?

 

if it has got any sort of insulation a hot water tank will heat up to its thermostat temperature (depending on the size of the element/size of tank) in typically an hour max. after that time the element will only draw power when the temperature of the water drops. If you are not drawing off hot water then the immersion will use very little more power if on 24/7 than if it is on a timer running a couple of hours in the morning and a couple of hours in the evening.

[Dave Payne]

Guess we need the OP to confirm then if his immersion has crap/no insulation or he is having lots of lovely hot showers!!

[Jambo]

If you are not drawing off hot water then the immersion will use very little more power if on 24/7 than if it is on a timer running a couple of hours in the morning and a couple of hours in the evening.

 

Only if it's very well insulated - I would argue that boat tanks aren't particularly great on the insulation front from what I've seen.

Domestic hot water tanks typically have 50mm of PUR insulation. Pretty sure boat tanks have about half that. Also the temperature set point is often a lot higher than a domestic tank due to the available storage volume being much lower. Both of these together means that standing losses will be quite high.

[ditchcrawler]

To put 100kWh/week into context, I think this is a typical energy usage for a 4 bedroom house with electric heating and cooking. A 3 bedroom house with gas central heating and cooking typically uses 40kWh/week.

 

I wonder if the "per week" is in fact a "per year" or something?

That is about what we use at home with oil heating the hot water

The immersion heater if 3Kw for 2 hours a day would account for nearly half of it, That still leaves a big lump he will need to trim down. It may of course also include his washing machine, tumble dryer and space heating through the winter.. He may have answered this but I lost the middle of the thread.

[Blazeaway]

Hi folks, very inteŕesting replies, thank you.

 

more info

We are averaging 10 kwh per day with immersion on washing machine 2 x per week with drier occassiobally maybe on aveage 1 per week. No excessive shower use. Tank is insulated but not very good, I will see if I can source an over jacket to fit to teduce heat losses.

[Alan de Enfield]

Our immersion (1Kw) is via one of those 'plug in mechanical timers' (where you click in buttons along the perimeter for every 1/4 hour you want it to come on, Simply plug in and rotate the 'timer' to the correct time.

 

Ours comes on 6am-8am, for morning washes, showers and breakfast 'pots and dishes', then comes on in the Evening 6pm-8pm for lunch time and evening pots and dishes + anything else.

 

Kettle is 1Kw for when 'plugged in' and we have a gas one for cruising.

Microwave is 600w for use either on land-line or inverter as its only on for 'minutes'

George Forman grill is 750 watts but only ever on for 20 minutes

Toaster is 900w for land line, or gas grill for cruising.

 

All lights now LED 3watt or 5 watt (spots for reading)

Batteries 6 x 230Ah

Alternators 2 x 70 amp

Solar 170w low-light panel

 

Cruise most days.

Can happily last more than 4 days without running engines

[Dave Payne]

Our immersion (1Kw) is via one of those 'plug in mechanical timers' (where you click in buttons along the perimeter for every 1/4 hour you want it to come on, Simply plug in and rotate the 'timer' to the correct time.

 

Ours comes on 6am-8am, for morning washes, showers and breakfast 'pots and dishes', then comes on in the Evening 6pm-8pm for lunch time and evening pots and dishes + anything else.

 

Kettle is 1Kw for when 'plugged in' and we have a gas one for cruising.

Microwave is 600w for use either on land-line or inverter as its only on for 'minutes'

George Forman grill is 750 watts but only ever on for 20 minutes

Toaster is 900w for land line, or gas grill for cruising.

 

All lights now LED 3watt or 5 watt (spots for reading)

Batteries 6 x 230Ah

Alternators 2 x 70 amp

Solar 170w low-light panel

 

Cruise most days.

Can happily last more than 4 days without running engines

 

 

Want to swap boats?

[Paul C]

I too, think that the culprit is the immersion element not being on a timer. The steel shell of a boat is thinner and lets more heat in, than the thick walls of a house; in addition to previous posters' suggestions of jacket thickness.

[David Mack]

Is your calorifier located inside the cabin, where the surrounding air will be reasonably warm, and the heat it loses will at least warm your surroundings? Or is it located in a cold dank space under a cruiser stern deck, where it will lose heat much faster, and you won't get any benefit from it?

[Dave_P]

I would say with the immersion on 24/7 the figures could be correct?

When I'm on shore power, I leave my immersion on 24/7 and use a tiny fraction of that states here.

[Blazeaway]

Well I have now done some expermenting and have found that 80% of the power consumption is the Immersion heater.

Also I believe much of that power is because the tank is also heated by the engine and on checking the engine bay I found the cold engine is being heated by the hot water tank. I assume that the engine coil in the hot water tank is being heated and by convection the hot water is circulating around the engine.

Looks like there are two pipes on the engine block, an out and return that attach to the coil in the tank.

 

So how do I stop the convection induced circulation which is wasting loads of heat to the engine bay ?

[BEngo]

Your belief will be correct. Stop heating the engine and you will save a lot of power!

 

Fit a non-return valve in the hotter of the two pipes from the caloryflower to the engine, as close as possible to the caloryflower. Alternatively fit a full-flow lever ball valve but you will have to remember to open it at the start of cruising and shut it when you stop the engine.

 

N

[Dave_P]

Sounds possible but your consumption still sounds very high. I would imagine that even if your immersion was constantly heating, it still wouldn't account for your usage.

[Dave Payne]

Do you need it turned on that much, i tend to fire up by boiler for 30 minutes when i get home in the evening, full tank for washing up and stuff, then 30 minutes in the morning for a shower.

[Paul C]

I'd say its plausible, given the calorifier was effectively heating the engine via thermosyphon effect so its going to be very inefficient/uninsulated as a system. The figures mean that the immersion element would have been on for most of the time.

 

I'd say:

 

1. Fit a one way valve to prevent this happening

2. "Double up" the insulation around the calorifier

3. Put the immersion heater on a timer, eg 1 1/2 hours in the early morning; 1 hour in the afternoon/evening if needs be. If there's going to be a greater need for hot water, override the timer to turn it on (but then turn it back to timer).

[Jambo]

Well I have now done some expermenting and have found that 80% of the power consumption is the Immersion heater.

Also I believe much of that power is because the tank is also heated by the engine and on checking the engine bay I found the cold engine is being heated by the hot water tank. I assume that the engine coil in the hot water tank is being heated and by convection the hot water is circulating around the engine.

Looks like there are two pipes on the engine block, an out and return that attach to the coil in the tank.

 

So how do I stop the convection induced circulation which is wasting loads of heat to the engine bay ?

 

Aha! Great that you've found the cause. I wonder how many other marina-based boaters have this issue?

[nb Innisfree]

 

Quite possible immersion thermostat isn't being satisfied, result with 1kw immersion = 24 kwh per day

Edited March 22, 2016 by nb Innisfree