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Old 08-16-2022, 07:52 AM   #1
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figuring amp hours used. Check my math?

Using my kill-o-watt meter yesterday in 17 hours my computer system used 0.64k watt hours. (640 watt hours right?) In a 12 volt system is this 53.3 amp hours?


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Old 08-16-2022, 07:57 AM   #2
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Your math/physics is correct.

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Old 08-16-2022, 08:07 AM   #3
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Originally Posted by unyalli View Post
Using my kill-o-watt meter yesterday in 17 hours my computer system used 0.64k watt hours. (640 watt hours right?) In a 12 volt system is this 53.3 amp hours?


- Jeff
Watts = Volts X Amps = amp hours = Ah

0.640k Ah = 640 Ah

640 watt hours at 120 volts is still 640 watt hours at 12 volts.

amps = (V X A) / V

640 Ah / 120 volts = 5.33 amps

640 Ah / 12 volts = 53.3 amps
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Old 08-16-2022, 08:29 AM   #4
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Your math is correct. Keep in mind that if you apply this to the real world, it will take more than the 53.3 DC amps to get the 640W because of inverter losses.
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Old 08-16-2022, 08:52 AM   #5
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So once my new fiver arrives and I install my off grid power system, to survive a few rainy days, just my computer streaming video will need (rounded up) 200ah lithium. I'm thinking I need to start with 400ah.


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Old 08-18-2022, 02:30 PM   #6
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Cheaper to buy a tablet, which has no hard drive. It will use approx 6watts to stream video. When we boondock, and want to preserve batteries, we stream on our iPad, then recharge it with a powerstation. Almost never use our computer while boondocking. It uses ten times more power than the iPad.
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Old 08-18-2022, 02:40 PM   #7
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Quote:
Originally Posted by Marine359 View Post
Cheaper to buy a tablet, which has no hard drive. It will use approx 6watts to stream video. When we boondock, and want to preserve batteries, we stream on our iPad, then recharge it with a powerstation. Almost never use our computer while boondocking. It uses ten times more power than the iPad.
Good to know about the iPad...thanks
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Old 08-18-2022, 03:55 PM   #8
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Quote:
Originally Posted by Persistent View Post
Watts = Volts X Amps = amp hours = Ah

0.640k Ah = 640 Ah

640 watt hours at 120 volts is still 640 watt hours at 12 volts.

amps = (V X A) / V

640 Ah / 120 volts = 5.33 amps

640 Ah / 12 volts = 53.3 amps
Some minor details here about the units for others that read this thread

Watts = Volts X Amps but that does not equal amp hours

watt hours = volts X amp hours however

640Wh from his meter is 5.33Ah at 120V
640Wh from his meter is 53.3Ah at 12V (which is what the OP posted and is correct)

If you are going to assume a uniform amperage across that entire 17 hours of recording, then you can drop the "hours" from the units and say that it's 640W total thus at 12V it's 53.3 amps total consumption. It still does not give you instantaneous amp requirements but it'll help build out a solar array given an expected solar day or battery capacity to recharge or simply power the system "directly".

But this is also important if you are trying to determine wire and fuse sizing.

Example could be a fridge. They have minimal continuous draw but when the compressor cycles on the amps jump. If you measured the total watt hours for a day and then divided by 24 hours you get a lower amp rating than if you consider the 20% duty cycle the compressor is actually running and your system would be undersized.
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Old 08-19-2022, 10:31 AM   #9
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I hope you get batteries with blue tooth or some sort of other metrology. That way you can see what your usage actually is. If not a batter monitor would be well worth the investment. Without at least one of those you'll be pretty much blind about your usage or SOC. LiFePO4 have such flat voltage curves it doesn't help much.

Even with the inverter turned off our RV pulls 3A 24 hours a day for various things I've never managed to track down. I suspect the little status lights one all the switches, the propane monitor, maybe the solar charge controllers sink some current, the tank monitors and displays just to name a few. 3 A 24 hours a day is 72AH or over 3 days that would be 216AH. That's a big chunk of your 320 or so available AH. And that is the parasites, add lights, fans, propane fridge, a coffee pot and you'll never make three days even without the laptop.

As for the laptop itself one way to stretch your energy might be a 12v power supply. You could leave your inverter off and save the loses in it. A 12v power supply may itself be more efficient. Not sure if those are generally available anymore or worth the price.
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Old 08-19-2022, 10:53 AM   #10
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Quote:
Originally Posted by Marine359 View Post
Cheaper to buy a tablet, which has no hard drive. It will use approx 6watts to stream video. When we boondock, and want to preserve batteries, we stream on our iPad, then recharge it with a powerstation. Almost never use our computer while boondocking. It uses ten times more power than the iPad.
Another option would be to replace the mechanical hard drive in the laptop with a solid state drive. I did that when my mechanical HD failed, and the improvement was very noticeable in power usage, heat generation, and speed.
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Old 08-19-2022, 10:56 AM   #11
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Yep cruizered aware of all that but thanks. In anticipation of the dealer calling I all ready have Victron 100/30 smartsolar, smartshunt with temp sensor, cerbo gx, 400 amp class t fuse, etc...


Looking at amperetime lithium 200ah with 200a BMS x 2 in parallel for under $1600.


Not concerned with missing low temp shut off as smartshunt with temp will take care of this.


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Old 08-19-2022, 11:22 AM   #12
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How much solar are going with?
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Old 08-19-2022, 11:30 AM   #13
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Rig comes with 380w and pwm. Will re-wire this to vic mppt. Looking at another 420w to second vic.
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Old 08-19-2022, 11:56 AM   #14
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'If' you go with 400ah lfp and thinking about 800w solar some of the worries of usage will not be a problem unless you have a couple days of poor solar production. Are you going with a full rig inveter/ charger?

We only have 500ah lfp and 1,280w solar with 3,000w inverter/ charger. We boondock fulltime and live like were on hookups...microwave, laptops, phones, tablets, humidifier, tv, ice maker, air conditioner. Our last power pole hookup was 11/3/21 for a night when moving from the north to south and no boondocking was near our stop. Charging is done by solar 99% of the time, generator is used if a hold over charge is needed.
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