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Old 01-09-2018, 04:24 PM   #1
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Electrical Draw Calculations

So I am trying to figure out what size battery bank AND what kind of charging power I am gonna need for this usage:

Item Amp Hours Usage Per Day Daily Draw on Coach
Batteries
NuWave Induction Plate 15 2hr 30
15cu-ish Fridge/Freezer - AC or LP? 15 24hr 360
single serving coffee maker 15 5m 1.25
Small blender / Ninja 7.5 5m 0.625
32in Samsung Smart LED 0.5 5hr 2.5
Toaster 10 15m 2.5
397 Amp Hours

So if I have a coach side bank with 400AH, I would power this usage, and its not including alot of the smaller stuff, for one day before the batteries are 100% drained? I understand we also dont want to get below 50% drain if I want to make the batteries last.

Is my logic right or am I missing something here? How does solar size play into this limited equation?

Thanks again!
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Old 01-09-2018, 04:32 PM   #2
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On refrigerators, the max current draw is when the compressor kicks in. So you do not need to calculate the full amount. In 1 hour, my fridge's compressor will kick on 2-8 times an hour(depending if hot out). So if its winter, and the compressor only kicks on once an hour, for 5 mins before cutting off, then you only calculate that amperage for 5 mins x 24 hours. A LOT less than you thought! BUT a larger battery bank is not a bad thing. Just gives you more reserve.
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Old 01-09-2018, 04:35 PM   #3
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Standard rule of thumb is to use a 30% duty cycle for the fridge ... so it should use one third of the rated power, and only use half the amp hours of the battery bank. SO if you have 400, then 200 are available for use before you risk damage to the batteries over the longer term.
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Old 01-09-2018, 05:15 PM   #4
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I took the 1500 watts divided by 120v to get 12.5ah, rounded up to 15 and then times 24hrs for 360ah, I prefer to be too high then too low here.

So your saying it is probably more like 100-110ah?

How do I determine my recharge rates in order to make this thing self sufficient on solar or genny?
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Old 01-09-2018, 05:28 PM   #5
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I doubt a ridge takes 1500 watts.
Our large fridge / freezer is about 1.5 kwh/day for a 12 volt battery system that would 1500 watthour/12 volts=125 Ah (No inverter losses)

later j
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Old 01-09-2018, 05:29 PM   #6
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Quote:
Originally Posted by PNWorBUST72 View Post
I took the 1500 watts divided by 120v to get 12.5ah, rounded up to 15 and then times 24hrs for 360ah, I prefer to be too high then too low here.

So your saying it is probably more like 100-110ah?

How do I determine my recharge rates in order to make this thing self sufficient on solar or genny?
You way over-estimated the power consumption of the fridge, and as that's the biggest draw you ended up with a much bigger usage than it really would be.

Charging is another matter. Maximum charge rate depends on the batteries. At the 10% of the capacity rate, lead/acid batteries that size will happily take 40 amps for the bulk charge, but might need a charger capable of delivering that current at 14.8 volts to get to full charge. Be very careful what you buy. Most chargers sold for RVs cannot deliver 14.8 volts and will never fully charge the batteries, even though the meter will tell you thay are at 100%.

If you have different chemistry batteries, different rules apply.

To put 200 amphours back into your 50% charged batteries will take a good few hours so you are going to have to work out the solar required and have a decent safety margin. Most of the small generators cannot adequately charge these batteries from their 12V outputs, but can from 120V so I'd do it differently.

I'd get an inverter/charger with an auto-transfer switch. You can feed that with both generator and/or solar. The inverter will then charge the batteries, and the good ones do it properly from shore-power, generator or solar.
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Old 01-09-2018, 05:34 PM   #7
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Sorry, waht i am actually saying your calculation formula is incorrect.

1500 watt and 120 volt = 12 amp.... not amphour.
Ther are many ways to do it but the easiest is to go to watthours directly and from there go to your battery voltage andthen ah.

watthours / battery voltage= Ah.

later J
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Old 01-09-2018, 05:47 PM   #8
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if the fridge is one of those Propane/120v camper units the draw is constant (ammonia fridge not Freon/compressor based) i had a brother run one on my electricity it added like 130 bucks to my bill (basically doubled it) it may be more cost effective to run it on propane vs the cost to accommodate the 120 volt heater that runs constantly to replace the small propane flame
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Old 01-09-2018, 08:22 PM   #9
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All good info here so far as I can see. I would add that for your solar be sure to use a true MPPPT charger controller. Higher end ones let you change your output voltage to match your battery type and voltage.
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Old 01-09-2018, 08:57 PM   #10
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Quote:
Originally Posted by joeblack5 View Post
Sorry, waht i am actually saying your calculation formula is incorrect.

1500 watt and 120 volt = 12 amp.... not amphour.
Ther are many ways to do it but the easiest is to go to watthours directly and from there go to your battery voltage andthen ah.

watthours / battery voltage= Ah.

later J
Ok, I was using this site to convert it, I guess I assume that the "watt" rating you find on appliances was a watt per hour reading? - https://convert-formula.com/ah-wh

And this list to get the watts - https://www.wholesalesolar.com/solar...gy/power-table
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