Converting kWh/24hr to amps

[Dave_P]

I was looking at some 240v freezers online. Some were shown with this as their energy consumption.

 

What I'm confused over is this. To find out how many amps are taken from my batteries I divide the watts by volts (and multiply by 1000 since its kilowatts given). My question is do I divide by 240 since its a 240v freezer, or by 12 since my batteries are 12v? I'm using an inverter, obviously.

[Paul C]

P = V I

 

You have to also allow for inverter (in)efficiency and quiescent current.

[Paul C]

Try this, its still in development but the maths is right:

 

Below is a calculator to convert annual power consumption figures, typically quoted for appliances such as 230V fridges, freezers etc, into an Ah/day figure. It also calculates the current draw (while the appliance is "on", according to its duty cycle) and the calculated minimum inverter size, based on a peak/startup current draw.

Instructions are very simple, enter the "Annual consumption (kWh/annum)" figure, typically this is in the range 60-500 or so; then press "Calculate". It is also possible to alter the constants from the suggested values below

 

http://www.canalworld.net/other/annualpower.html

[WotEver]

Divide by 10. That allows for inefficiencies. It's also erring on the safe side.

500W @ 230V is still 500W @ 12.V, it's only the current that's 20 times as large on 12V (taking inefficiencies into account).

Tony

 

edited 'cos I can't divide 200 by 10.

Edited February 20, 2016 by WotEver

[John V]

Be aware that published figures seem to vary considerably from readings taken in actual operation

 

http://www.canalworld.net/forums/index.php?showtopic=82616&view=findpost&p=1762572

[WotEver]

Be aware that published figures seem to vary considerably from readings taken in actual operation

 

http://www.canalworld.net/forums/index.php?showtopic=82616&view=findpost&p=1762572

When you've measured it over a year, putting warm goods into it, opening the doors, sun beating down on the boat then let's see your figures

[Paul C]

Be aware that published figures seem to vary considerably from readings taken in actual operation

 

http://www.canalworld.net/forums/index.php?showtopic=82616&view=findpost&p=1762572

 

Published figures can only be used as a "first guess" in a power audit anyway. It would be a good idea to do a power audit initially (in designing/planning a boat electrical system) but it would be just as good to regularly check and revise that power audit based on real/measured data. No way of measuring? --> No way of managing the situation.

[John V]

If my kwhr didn't reset to zero everytime there is a blip in the supply, I would be pleased to do so. However as the boat is probably not much different in temperature summer/winter as you seem to indicate....I doubt it.... temp indoors at moment 24.1 degrees.

As it normally feels pleasantly cool when you come below in the summer, I doubt that much higher. Add to that, the freezer draws it's cooling air directly from the bilges, which should maintain a high efficiancy

[Jess--]

I was about to add that the figures you are being given will only get you an average usage, actual power required at some time may be far higher.

 

 

best example I have seen of this was on a new fairground ride we got, which was specified as requiring 3 phase @ 5a (avg), in actual use it presented an initial demand for just over 50A while it started the ride moving but once the ride was moving and had momentum it took next to nothing.

[Dave_P]

Does anyone have an answer to my original question? Do I divide by 12 or 240? My guess is it's 240. Otherwise mains fridges and freezers would be hopelessly inefficient on a boat.

[David Mack]

Does anyone have an answer to my original question? Do I divide by 12 or 240? My guess is it's 240. Otherwise mains fridges and freezers would be hopelessly inefficient on a boat.

Dividing by 240 weill give you the current the freezer draws from the inverter. Dividing by 12 (or 10 to allow for inverter losses) will give you the current the inverter draws from the batteries.

[John V]

by 12 if you are 12v.....as Wotever says dividing by 10 will probably give you a figure allowing for losses.

 

amps are watts divided by the supply voltage ....in your case 12. The fact the 12v goes through an inverter to 240v is irrelevant for your batteries. You would only divide by 240 if you wanted to know the current drawn FROM the inverter at 240v

 

 

eta.....must learn to type faster

Edited February 20, 2016 by John V

[Loafer]

by 12 if you are 12v.....as Wotever says dividing by 10 will probably give you a figure allowing for losses.

 

amps are watts divided by the supply voltage ....in your case 12. The fact the 12v goes through an inverter to 240v is irrelevant for your batteries. You would only divide by 240 if you wanted to know the current drawn FROM the inverter at 240v

 

 

eta.....must learn to type faster

 

It continually amuses me how there's always a desperate rush to get the answer posted! I'm rarely in there first, and often pleased because I turned out to be wrong!

[Paul C]

Does anyone have an answer to my original question? Do I divide by 12 or 240? My guess is it's 240. Otherwise mains fridges and freezers would be hopelessly inefficient on a boat.

 

You're oversimplifying it, you MUST take into account duty cycle and inverter quiescent current too (to get anywhere near real-world useful figures). For example if your duty cycle was low (ie 20% or so) but the inverter quiescent current high, you'd consume far more than simply by multiplying by a factor.

 

Look at the maths in the source code of the Javascript to see the calculations done. Its not complicated to follow.

[sjc]

Divide by 10. That allows for inefficiencies. It's also erring on the safe side.

 

500W @ 230V is still 500W @ 12.V, it's only the current that's 10 times as large on 12V (taking inefficiencies into account).

 

Tony

 

This is somewhat misleading, the current at 12v is 20 times that at 240v.

 

Example to estimate average continuous drain :-

Rating : 365KWh per year ( often given for white good etc )

Where : 24 * 364 = 8760 hours per year

And : 365Kw = 365000W

Divide rating in Watts by Nbr hours per year :-

Power : 365000W / 8760hours = 41.7W

Now divide by appropriate voltage to get current drain :-

@12v : 41.7 / 12v = 3.5A

@240v : 41.7 / 240v = 0.174A

Notice the current at 12v is 20 times that at 240v.

[Dave_P]

 

You're oversimplifying it, you MUST take into account duty cycle and inverter quiescent current too (to get anywhere near real-world useful figures). For example if your duty cycle was low (ie 20% or so) but the inverter quiescent current high, you'd consume far more than simply by multiplying by a factor.

 

Look at the maths in the source code of the Javascript to see the calculations done. Its not complicated to follow.

In English?

[John V]

 

This is somewhat misleading, the current at 12v is 20 times that at 240v.

 

Example to estimate average continuous drain :-

Rating : 365KWh per year ( often given for white good etc )

Where : 24 * 364 = 8760 hours per year

And : 365Kw = 365000W

Divide rating in Watts by Nbr hours per year :-

Power : 365000W / 8760hours = 41.7W

Now divide by appropriate voltage to get current drain :-

@12v : 41.7 / 12v = 3.5A

@240v : 41.7 / 240v = 0.174A

Notice the current at 12v is 20 times that at 240v.

 

He didn't say it was 10 times more than 240 v what he said was you should divide the wattage by 12 but dividing it by 10 would give a figure that allowed for losses. Not multiply the 240v current by 10

[Alan de Enfield]

500w will be 50Ah from your battery

 

5kw will be 500Ah from your battery

 

Divide watts by 10 to get amps (Ah0 from your battery)

[Paul C]

In English?

 

I think you're trying to oversimplify it. I suppose a simple "factor" could be come up with - but its assumptions are too broad to be useful (IMHO).

 

Worth understanding the following terms:

 

duty cycle

quiescent current - ie the current that the inverter draws, when its switched on but the device which is plugged into it is off. IMHO this is MASSIVELY important - for efficient fridge/freezers, half the power could be used for powering an inefficient inverter!!

 

The rest is just boring old maths and numbers, which computers are good at doing and humans aren't.

[Loafer]

500w will be 50Ah from your battery

 

5kw will be 500Ah from your battery

 

Divide watts by 10 to get amps (Ah0 from your battery)

 

Er, not quite Alan.

 

500W will be 50 Amps. Instantaneous reading!

[WotEver]

 

This is somewhat misleading, the current at 12v is 20 times that at 240v.

 

Yes, 20 times. I'll edit my post.

500w will be 50Ah from your battery

 

5kw will be 500Ah from your battery

 

Divide watts by 10 to get amps (Ah0 from your battery)

 

No...

 

500Wh will be 50Ah from your battery...

 

500W will just be a current draw of 50A.

 

Tony

Does anyone have an answer to my original question? Do I divide by 12 or 240? My guess is it's 240. Otherwise mains fridges and freezers would be hopelessly inefficient on a boat.

 

The answer was given several times prior to this post.

 

Tony

In post #1 you asked how "To find out how many amps are taken from my batteries"

 

In post #4 I answered "Divide by 10".

[Alan de Enfield]

 

No...

 

500Wh will be 50Ah from your battery...

 

500W will just be a current draw of 50A.

 

Tony

 

 

I do know that (really I do) its just my Dyslexic fingers refuse to believe that I do and keep changing things.

I am getting them (slowly) to accept that what I think actually gets transcribed onto the screen but we still have occasional relapses and this was one such occasion.

[WotEver]

 

 

I do know that (really I do) its just my Dyslexic fingers refuse to believe that I do and keep changing things.

I am getting them (slowly) to accept that what I think actually gets transcribed onto the screen but we still have occasional relapses and this was one such occasion.

 

I know you know, honestly

 

I just didn't want anyone that doesn't know being confused by your post.

 

As for error between brain and screen - it happens to me all the time.

 

I once joked to a gun dealer that my shotgun must be faulty because I kept missing the clays. He replied that "It's a technical problem called 'the nut behind the trigger' "

 

Tony

[Loafer]

 

 

I do know that (really I do) its just my Dyslexic fingers refuse to believe that I do and keep changing things.

I am getting them (slowly) to accept that what I think actually gets transcribed onto the screen but we still have occasional relapses and this was one such occasion.

 

I knew you knew that too, Alan, but I couldn't let it go unmentioned. Other less electrically knowledgeable may have seen it as backing up the anti-correct-units brigade!

[Paul C]

 

I think you're trying to oversimplify it. I suppose a simple "factor" could be come up with - but its assumptions are too broad to be useful (IMHO).

 

Worth understanding the following terms:

 

duty cycle

quiescent current - ie the current that the inverter draws, when its switched on but the device which is plugged into it is off. IMHO this is MASSIVELY important - for efficient fridge/freezers, half the power could be used for powering an inefficient inverter!!

 

The rest is just boring old maths and numbers, which computers are good at doing and humans aren't.

 

 

 

Annual consumption: 200kWh/annum

Quiescent current: 0.5A

12V system Ah/day: 62.7

 

Annual consumption: 200kWh/annum

Quiescent current: 2.0A

12V system Ah/day: 98.7