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Old 26-05-2020, 12:46   #1
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Amp hours to kWh (for a Tesla guy)

I'm a Tesla guy, so I think in kWh - but in sailboats everything is in Amp hours. From what I read a 40-45 foot cat will use around 200-300 amp hours per day. What would be the equivalent of this in kWh?

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Old 26-05-2020, 13:04   #2
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Re: Amp hours to kWh (for a Tesla guy)

Divide kWh by Voltage to get Amp hours.


Your example typically assumes 12v so 2400kWh - 3600kWh
But this is meaning less in boat terms as the 200-300Amps will be the total amps used over 24 hours.



The real problems arise because 12 volts isn't always 12 it can be as low as 10.8 (more amps, wires get hot and motors burn out) or as high as 13.8 (less amps but charging must be on).
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Old 26-05-2020, 13:05   #3
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Re: Amp hours to kWh (for a Tesla guy)

Watts= amps X Volts so 300a at 12v = 3600w or 3.6kw roughly
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Old 26-05-2020, 13:11   #4
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Re: Amp hours to kWh (for a Tesla guy)

Don't Tesla guys need to know the relationship between watts, amps, and volts? It's batteries and electricity, after all!
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Old 26-05-2020, 13:34   #5
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Re: Amp hours to kWh (for a Tesla guy)

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Originally Posted by Tupaia View Post
Your example typically assumes 12v so 2400kWh - 3600kWh
But this is meaning less in boat terms as the 200-300Amps will be the total amps used over 24 hours.
I'd say it means precisely the same thing

Both Ah and kWh are fishpig units, similar to saying Hawaii is about 14 "knot-days" from SF Bay. I do get that it makes the thinking and planning simpler than using Joules.
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Old 26-05-2020, 15:18   #6
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Re: Amp hours to kWh (for a Tesla guy)

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similar to saying Hawaii is about 14 "knot-days" from SF Bay.

A nice way of describing it.
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Old 26-05-2020, 15:33   #7
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Re: Amp hours to kWh (for a Tesla guy)

Quote:
Originally Posted by Tupaia View Post
Divide kWh by Voltage to get Amp hours.
Your example typically assumes 12v so 2400kWh - 3600kWh
But this is meaning less in boat terms as the 200-300Amps will be the total amps used over 24 hours.

Everything stated there is wrong!


Divide kWh by voltage and then multiply by 1000 to get Amp hours.
Your example typically assumes 12V so 2.4kWh - 3.6 kWh
But this is meaning less in boat terms as the 200-300Amps hours will be the total amps hours used over 24 hours.
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Old 26-05-2020, 15:40   #8
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Re: Amp hours to kWh (for a Tesla guy)

Quote:
Originally Posted by requiem View Post
I'd say it means precisely the same thing

Both Ah and kWh are fishpig units, similar to saying Hawaii is about 14 "knot-days" from SF Bay. I do get that it makes the thinking and planning simpler than using Joules.

Also completely wrong.


Ah and kWh are perfectly valid units, not in the least like "knot -days"
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Old 26-05-2020, 16:49   #9
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Re: Amp hours to kWh (for a Tesla guy)

Ok, Ah and kWh aren't quite the same since a voltage must be specified to convert between them, but watts, being a measure of power, are defined as Joules / second. Thus, watt-hours are literally hours * Joules / second.

But, it is a largely meaningless detail, probably best used to illustrate the distinction between energy and power. I suspect one reason people often have a hard time keeping amps and amp-hours straight is because there's an unfortunately habit of dropping the "hours", whether in conversation or in product specs. Thus, my choice of something nonsensical but illustrative such as "knot-days", being an analogous measure of days * sea miles / hour. (Although I should correct the number to ~85, since 85 knots for a day would get you there, not 14).
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Old 27-05-2020, 01:31   #10
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Re: Amp hours to kWh (for a Tesla guy)

So if I run my 12v electric motor that draws 83.3amps for an hour. The watts consumed would be 12 * 83.3 = 1000 Watts = 1Kw (1KVA), as it was over an hour that would be 1 KWh.


The amps consumed are 83.3 amps which is 83.3amp hours. So a 12V battery to power this (simply speaking) to run the battery down to say 50% would need a (83.3 * 2) amp hour battery = 166AH. I know there are a bunch of inefficiencies to deal with in reality, but are the math correct?
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Old 27-05-2020, 01:51   #11
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Re: Amp hours to kWh (for a Tesla guy)

^^ close enough
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Old 27-05-2020, 04:46   #12
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Re: Amp hours to kWh (for a Tesla guy)

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So if I run my 12v electric motor that draws 83.3amps for an hour. The watts consumed would be 12 * 83.3 = 1000 Watts = 1Kw (1KVA), as it was over an hour that would be 1 KWh.


The amps consumed are 83.3 amps which is 83.3amp hours. So a 12V battery to power this (simply speaking) to run the battery down to say 50% would need a (83.3 * 2) amp hour battery = 166AH. I know there are a bunch of inefficiencies to deal with in reality, but are the math correct?
Your use of the word "consumed" leaves me a little twitchy for an instantaneous measurements (amps or watts) but otherwise, yes, you have the math correct.

As you say, there are simplifications and inefficiencies, so you might bump up to a 180AH battery bank.

For example, with Lead Acid batteries, the actual voltage is rarely exactly 12V. At 10% charge, it's actually around 11.5V vs 100% is around 12.7V
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Old 27-05-2020, 05:21   #13
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Re: Amp hours to kWh (for a Tesla guy)

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Originally Posted by valhalla360 View Post
Your use of the word "consumed" leaves me a little twitchy for an instantaneous measurements (amps or watts)

Me too


A more appropriate word would be "draws".
Misapplication of that one word is the cause of much of the confusion between Amps and Amp hours.
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Old 27-05-2020, 05:49   #14
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Re: Amp hours to kWh (for a Tesla guy)

Ampere hours is in some ways a meaningless term. It is a piece/ part of an electrical unit that refers to energy. What makes it useful is that we all/ most assume it in reference to a 12 volt system. If your battery bank is 600 AH and mine is 300 AH then a pretty safe bet is yours has twice the energy as mine. Kind of a gentlemen's convenience.

When we include the actual voltage (Volts X Amps X time) now we have a real measure of energy (sometimes referring to storage capacity, sometimes referring to usage).

Because a Tesla has such a large energy storage battery we refer to kilowatt hours whereas for us boat types watt hours is more appropriate. Both indicating energy just differing by a factor of 1000.


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Old 27-05-2020, 06:06   #15
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Re: Amp hours to kWh (for a Tesla guy)

I wish we could all go to Wh or KWh since there are beginning to be more 24v boats.

I was watching a Delos video and Bryan kept talking about how many Ah they'd installed and I kept thinking "Wow I expected it to be nearly twice that much". Turns out they use a 24v system. It was twice that much!
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