Electrical Draw Calculations

[PNWorBUST72]

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!

[Defjr333]

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.
Doug

[Twigg]

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.

[PNWorBUST72]

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?

[joeblack5]

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

[Twigg]

Originally Posted by PNWorBUST72

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.

[joeblack5]

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

[pbeggs]

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

[Defjr333]

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.
Doug

[PNWorBUST72]

Originally Posted by joeblack5

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

[joeblack5]

Yeah, I know, it is confusing.. watts is a unit for Power,,, watthour a unit for energy.
You can see in your power table that they list the fridges in Watthour because they are operational 24 hours a day, other items like an iron in watts because it depends on YOUR usage ( hours or minutes) how much energy they use.

The sites are good.

good luck J

[joeblack5]

Actually the conversion site messes up the terminology:
In the top example line the power is 10Wh should read the energy is 10 Wh.
in the second one the current is wrong Ah would be mentioned as capacity.

Later J

[JDOnTheGo]

Originally Posted by PNWorBUST72

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

A Kill-A-Watt meter is a fairly useful tool for determine the power that a particular item draws. This assumes you have the item and are using it - not always the case at the design stage.

Since there has been some discussion about the definition of amp/amp-hours, it might also be worth using state of charge (SOC) instead of 'drained'. I know... I know... picky, picky, picky...

I wrote a couple posts that might help with some of your questions.
Energy Audit - Watt For? - JdFinley.com
Is solar power for you? - JdFinley.com

Their are some scientific rules of thumb for sizing the solar array based on battery size but we (RV type use) generally end up installing the maximum size array that we can fit or afford. Depending on your use and where you are located, having battery capacity to last a couple days (before reaching 50% SOC) is a plan to account for cloudy days when solar power is limited.

[JDOnTheGo]

Originally Posted by PNWorBUST72

NuWave Induction Plate 15 2hr 30

I just did some measuring of my NuWave cooktop and posted them on my build thread at:
http://www.skoolie.net/forums/f32/mi...tml#post244304

Not sure this will help with your planning but I think it will show you amps number low - at least if it's the same NuWave that I have. One burner on a setting of "5" consumes about 900 watts (80 amps @ 12.5 volts). Good thing it is fast!!

[PNWorBUST72]

So if I go high and say that the NuWave uses 1500 watts@120v = 12.5 Ah - rounded high to 15 ah. Assuming the math on that conversion website is correct. Then since I estimated that I would use the plate for 2hrs, I estimated 30ah for that plate.

Is that math wrong?

[JDOnTheGo]

Originally Posted by PNWorBUST72

So if I go high and say that the NuWave uses 1500 watts@120v = 12.5 Ah - rounded high to 15 ah. Assuming the math on that conversion website is correct. Then since I estimated that I would use the plate for 2hrs, I estimated 30ah for that plate.

Is that math wrong?

Yes, sorry... bad math.

Watt is unit of power. There is no such thing as x watts at y voltage. The voltage is already factored in.

Amp-hour (Ah) is a measure of power over a period of time).

For this discussion, 1500 watts can be arrived at via AC (shore power/generator) or DC (battery via inverter). 1500 watts / 12.5 VDC = 120 amps (said as 120 amps at 12.5VDC). OR it is 1500 watts / 120 VAC = 12.5 amps (said as 12.5 amps at 120 VAC).

At this point, you can begin factoring in the time you are consuming the power (to get to Ah).

And you thought all the multiplication/division stuff in school was for someone else!!

BTW; these are not precise calculations. The inverter introduces losses that should be factored in, your battery bank may be more or less than 12.5 volts (hopefully never less) and 120 VAC power is often closer to 110 VAC.

[PNWorBUST72]

First, thanks for trying to get me to understand. Sorry if I am being difficult here.

So lets look at this appliance:
Energy Plus - CE Electric Wall Heater | Cadet Heat

In the details its listed as 1000 watts @120V OR 8.33Amps - thats the exact math I used of the NuWave, etc.

Two questions - Is that I am taking those numbers and calling them "hours" that wrong? Is the volt side of this the volts of they system you are using, shore 120v AC or the device itself?

https://www.nuwavecooktop.com/common...5/features.asp

[JDOnTheGo]

Originally Posted by PNWorBUST72

First, thanks for trying to get me to understand. Sorry if I am being difficult here.

Not at all. This is a very common discussion in the RV world. None of us are used to thinking about having TWO electrical systems in our house or caring about what it means when we flip a switch.

I was basing the whole discussion on your desire to figure out what you need for battery bank capacity. Therefore, I understand that you are running this device from you battery bank/inverter.

Originally Posted by PNWorBUST72

So lets look at this appliance:
Energy Plus - CE Electric Wall Heater | Cadet Heat

In the details its listed as 1000 watts @120V OR 8.33Amps - thats the exact math I used of the NuWave, etc.

I understand that many devices list it that way but it is not technically accurate. Nonetheless, it is correct that this 120 VAC appliance consumes 1000 watts which breaks down to 8.33 amps at 120 VAC.

However; you will be running that device from your battery bank and inverter. Assuming your battery bank is a (generic) 12V bank, it's resting voltage at 100% SOC will be around 12.6/12/7 volts (DC) but let's just call it 12.5 volts. The inverter is going to supply 8.33 amps at 120VAC to the device. To provide that, it has to pull 1000 watts (plus conversion losses) from the battery bank. That becomes 80 amps (1000 / 12.5 VDC) coming out of the battery (plus conversion losses).

Originally Posted by PNWorBUST72

Two questions - Is that I am taking those numbers and calling them "hours" that wrong? Is the volt side of this the volts of they system you are using, shore 120v AC or the device itself?

Yes. A watt is a unit of power at a point in time. A watt-hour is unit of power over a period of time (one hour). If you ran this device for one hour, you would use 2000 watt-hours or 160 Ah (using our above calculation).

I don't understand the second question - try me again.

[PNWorBUST72]

If the 1000 watt device is run for one hour it uses 1000 watts or is rated at 1000 watt-hours?

Hence, I run it for two and its now using 2000 watt hours.

If I take the watt hours 1000, and the voltage 120VAC, using that website converter, I am using 8.3 Amp hours no?

Now lets say I have 4 6v golf cart batteries in series/parallel that gives me a total 12v@ 480ah - https://www.amazon.com/Trojan-T-125-...cart+batteries

Since I am trying to figure out my load for sizing my battery bank, I take my 480ah battery bank, divided by 50% maybe 60% so to make sure I dont draw them down too much, so say 240ah.

I take 240ah divided by 8.3ah to learn that running that SINGLE device, and not taking into account any lose or system inefficiency, I could run that device for 28-29hrs before hitting my 50% mark?

Damn I got a headache already...

[JDOnTheGo]

Originally Posted by PNWorBUST72

If the 1000 watt device is run for one hour it uses 1000 watts or is rated at 1000 watt-hours?

Almost the latter, it consumes 1000 Wh (watt-hours).

Originally Posted by PNWorBUST72

Hence, I run it for two and its now using 2000 watt hours.

Correct.

Originally Posted by PNWorBUST72

If I take the watt hours 1000, and the voltage 120VAC, using that website converter, I am using 8.3 Amp hours no?

Almost. You have the element of time included when you say 'watt hours'. I think you mean just watts, in this context. You are using 8.3 amp at 120 VAC. If you run it for one hour, you are using 8.3 Ah (amp hours) at 120 VAC.

Originally Posted by PNWorBUST72

Now lets say I have 4 6v golf cart batteries in series/parallel that gives me a total 12v@ 480ah - https://www.amazon.com/Trojan-T-125-...cart+batteries

Since I am trying to figure out my load for sizing my battery bank, I take my 480ah battery bank, divided by 50% maybe 60% so to make sure I dont draw them down too much, so say 240ah.

I take 240ah divided by 8.3ah to learn that running that SINGLE device, and not taking into account any lose or system inefficiency, I could run that device for 28-29hrs before hitting my 50% mark?

No, sorry. You are missing the fact that source of the power is now coming from a 12 VDC battery. That 12 VDC must be converted (more correctly, inverted) to 120 VAC. To do that, you need an inverter. To "make" 8.3 amps at 120 VAC (1000 watts), the inverter is going to pull 1000 watts from the 12 VDC battery and that equals 80 amps at 12.5 VDC.

Still ignoring conversion losses, you could run this device for 3 hours before your battery is depleted (240 Ah used and down to 50% SOC).