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Old 07-13-2020, 09:55 PM   #1
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Question Calculating Power Usage

I,m trying to figure out my daily power usage to properly size my battery bank and solar system for boondocking off grid.
I understand Watts=Volts x Amps. and Watts/Volts=Amps
I have two, 100AH flooded batteries, that I know I don't want to discharge past 50%, leaving me 100 AH to play with.
My wife's blow dryer is 1875 W and draws 15.62 Amps when is use. I verified this with a clamp on meter and it proves the formula 1875 W/120 V=15.6 A. She uses it for 10 minutes a day.
Am I correct to say that 15.6 Amps divided by 60 minutes, x 10 minutes is 2.25 Amp hours used?
And, that a 32" LED TV using 2.2 Amps for 4 hours, uses 8.8 Amp hours? I verified the 2.2 Amps.

I'm not sure if I'm butchering the concept of Amps (Amperies) and Amp Hours.

I tried using an online calculator and they are either very conservative or just trying to sell larger solar systems. (They always seem to be tied to Solar Manufacturers)

I made a way to measure with clamp on meter without pulling the cords apart. I already had two extension cord repair ends. One 3 prong plug and one 3 prong socket. I opened the outer sleeve of about 10" of Romex to be able to isolate the black wire and assembled that. Then I just plugged each AC/DC item into that and was able to measure them all. Pretty sure I'm not the first one to think of that!

Here are some of the other values I found.
Keurig- draws 11.5A for 9 minutes to make 3 cups coffee. Ah used 1.72
VitaMix- 7.5 A for 2 minutes. Ah used .24
CPap machine- draws 8-12 Amps variably so I averaged on high side at 11 Amps for 8 hrs. Ah used 88! Holy Cow!
CPap was my biggest power suck if I figured it right

I have other stats if desired.
I do plan to use an MPPT controller with Monocrystalline panels. I may add 3rd battery or go Lithium. $$$
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Old 07-13-2020, 10:46 PM   #2
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My wife's blow dryer is 1875 W and draws 15.62 Amps when is use. I verified this with a clamp on meter and it proves the formula 1875 W/120 V=15.6 A. She uses it for 10 minutes a day.
Am I correct to say that 15.6 Amps divided by 60 minutes, x 10 minutes is 2.25 Amp hours used?

You can use watts and compare them to each other regardless of the voltage. So 1875 watts at 120 volts is still 1875 watts at 12 volts. There will be some conversion loss.
Your 15.6 amp calculation is correct. However, it is unlikely the hair dryer uses that much current. It is probably closer to 13 amps. They are all designed to work on 15 amp outlets. Even so, it is close.
1875 watts at 12 volts is 156.25 amps. You would of course need a 2000 watt inverter to run the hair dryer from the batteries. The inverter would try to draw 160 or so amps which the batteries could not supply. You would have to double the battery bank to make it work or triple it for better efficiency. 10 minutes is 0.17 hours. So 0.17 x 157 = 2.6 amp hours.

And, that a 32" LED TV using 2.2 Amps for 4 hours, uses 8.8 Amp hours? I verified the 2.2 Amps.
A much smaller maybe 180 watt inverter could easily handle the 27 watts. Batteries would have no problem with this draw.

Keurig- draws 11.5A for 9 minutes to make 3 cups coffee. Ah used 1.72
1380 watts at 12 volts would be more than 115 amps. Again the 2000 watt inverter would overtax the 200 amp battery bank.

VitaMix- 7.5 A for 2 minutes. Ah used .24
900 watts at 12 volts would be more than 75 amps. A 200 amp hour battery bank could do this for a short period. If the battery was at a low state of charge, the inverter may trip due to low input voltage. Installation of the inverter would require short oversize wire.

CPap machine- draws 8-12 Amps variably so I averaged on high side at 11 Amps for 8 hrs. Ah used 88! Holy Cow!
Batteries will have the same problem with the CPap as with the Keurig.
Many CPap machines will run directly on 12 volts. You would need a 12 volt cord for it. The humidifier in the CPap draws significant current. If you do not use it the power draw will be extremely low. Without the humidifier, a 180 watt inverter would do fine.

I do plan to use an MPPT controller with Monocrystalline panels. I may add 3rd battery or go Lithium.
You need a large solar array to recharge a much larger battery bank for these kinds of amp hours and amp draws. Remember, the rated solar output is at peak full sun exposure. That lasts for a short time each day. Outside of that output drops or stops altogether.
A generator with a 100 amp charger run a few hours each morning followed by long slow solar charge would do for a 400 amp hour battery bank.
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Old 07-14-2020, 01:32 AM   #3
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Arizona is a zone 6 insolation area, that means there is more than a 'short time' daily solar gain.
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Old 07-14-2020, 11:21 AM   #4
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Interesting article.

I think you are off on the CPAP machine. It should not draw that much current.

https://www.cheapcpapsupplies.com/bl...ampere%2Dhours.
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Old 07-14-2020, 12:31 PM   #5
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Originally Posted by sundancer 87 View Post
Arizona is a zone 6 insolation area, that means there is more than a 'short time' daily solar gain.
Can you translate that into amp hours for the OP?
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Old 07-14-2020, 12:38 PM   #6
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Originally Posted by persistent View Post
Can you translate that into amp hours for the OP?

On a good day, let's say in the southwest area, a 6 or 7 hour day could be expected. Amp hours will depend on the system of course. I lived 300 miles south of the Arizona border on solar. I remember long days of sun, sometimes so long I was wishing it would go down so it could cool down a little.


This company has the charts, tools and resources for building a system.

https://www.solar-electric.com/?utm_...RoCQC0QAvD_BwE
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Old 07-14-2020, 05:58 PM   #7
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You can use watts and compare them to each other regardless of the voltage. So 1875 watts at 120 volts is still 1875 watts at 12 volts. There will be some conversion loss.
Your 15.6 amp calculation is correct. However, it is unlikely the hair dryer uses that much current. It is probably closer to 13 amps. 1875 watts at 12 volts is 156.25 amps. The inverter would try to draw 160 or so amps which the batteries could not supply. 10 minutes is 0.17 hours. So 0.17 x 157 = 2.6 amp hours.


So it doesn't matter that im using 1875 Watts at 120 Volts? I guess I don't understand the Inverter then. I've read that there is about a 10% loss converting 12v to 120v. Is the inverter then always extracting a 12v conversion then?

Then; 1875 watts / 120 volts = 15.6 A +10% loss= 17.16 A is wrong.
Isn't that 17.16 A being drawn continually for an supposed hour but divided by 60 min = .286 per minute x 10 minutes = 2.8 A total Amps used. This is wrong?
I thought the Inverter was supplying me with 120V power, according to watts drawn, at a cost of about 10%. Thats wrong too apparently.

Geez I need a flow chart!

Many CPap machines will run directly on 12 volts. You would need a 12 volt cord for it. The humidifier in the CPap draws significant current. If you do not use it the power draw will be extremely low. Without the humidifier, a 180 watt inverter would do fine.

But it really wouldn't matter at 12 V would it? It would require even more power!
If 11 A x 120 V = 1320 Watts Then 1320 Watts / 12 V = 110 Amps which is alot higher. 110 Amps x 8 hours would require 880 Amp hours.

From these figures I wonder how anyone does solar. Looks like its going to be beyond my reach. Our trailer is an Ultralight being towed by a 1/2 ton truck. Even without the hair dryer (which is a boondocking deal breaker she says) even the Keurig requires more than we have.

I was in the Army so I can boil cheap coffee and drink it from a mud cup but she can't! And yes, she's worth the fuss!
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Old 07-14-2020, 07:16 PM   #8
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Originally Posted by persistent View Post
However, it is unlikely the hair dryer uses that much current. It is probably closer to 13 amps. They are all designed to work on 15 amp outlets. Even so, it is close.
Ok I checked the blow dryer again and yes, you're correct, it is 13.43 Amps when running on 120 V.

Quote:
Originally Posted by franktafl View Post
I think you are off on the CPAP machine. It should not draw that much current.
https://www.cheapcpapsupplies.com/bl...ampere%2Dhours.
Quote:
Originally Posted by persistent View Post
The humidifier in the CPap draws significant current. If you do not use it the power draw will be extremely low.
You are both correct. I wasn't thinking that I had the Humidifier on as well as the Heated Tubing. With them on the figure was correct but with them both off the current draw is substantially less at .13 A on an inhale and .08 A on an exhale. A fair average is .1 Amp per hour I figure.
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Old 07-15-2020, 07:47 PM   #9
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Originally Posted by Thomas View Post
[ ...

Then; 1875 watts / 120 volts = 15.6 A +10% loss= 17.16 A is wrong.
Isn't that 17.16 A being drawn continually for an supposed hour but divided by 60 min = .286 per minute x 10 minutes = 2.8 A total Amps used. This is wrong?
I thought the Inverter was supplying me with 120V power, according to watts drawn, at a cost of about 10%. Thats wrong too apparently.

...
15.6 A at 120 volts is 1875 watts. At 12 volts it becomes 156 amps plus 10% conversion loss. That equals 172 amps.

A 2000 watt inverter could supply that much, but a 200 amp hour deep draw lead acid battery probably cannot. Output voltage of the battery would drop well below 11.2 volts while that current is flowing. The inverter would likely trip due to low input voltage.

Also, you will not get 200 amp hours out of the battery bank at that discharge rate because they become highly inefficient as current out increases above 10 amps (approximate 20 hour discharge rate). You could reasonably expect 40 amps from each 100 amp hour battery. Even so you would get less than 100 amp hours out of each. You would need to double your battery bank size to accommodate 170 amp current.
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Old 07-15-2020, 11:55 PM   #10
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Not to mention the extreme weight of the true deep cycle batteries one would need.
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Old 07-16-2020, 08:00 PM   #11
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Originally Posted by Thomas View Post
...From these figures I wonder how anyone does solar. Looks like its going to be beyond my reach. Our trailer is an Ultralight being towed by a 1/2 ton truck. Even without the hair dryer (which is a boondocking deal breaker she says) even the Keurig requires more than we have.

I was in the Army so I can boil cheap coffee and drink it from a mud cup but she can't! And yes, she's worth the fuss!
People don't use solar to operate their camper like an electric grid-tied home or camper plugged into shore power. People use RV solar to recharge the batteries for the most part, with small loads like phone chargers and LED lighting able to have their demand entirely met by solar. Solar allows you to extend the time your batteries can provide you with 12V power while boondocking, possibly even fully charging every day.

As described, watts are watts despite what your power source is. When you are tied to grid power, you have a 120V source so the current needed is 120V x Amps = Watts. When your power source is 12V batteries, the current needed is 12V x Amps = Watts. So your amps required are 10x more with 12V than with 120V.

Because there is the inefficiency of an inverter converting 12V DC into 120V AC, you can divide wattage by 10 for an estimate of how many amps your batteries will need to supply for the load. As you've read here, high electric draw items on AC power become pretty much infeasible for a hobby 12V system to provide via batteries and inverter. And even if you had the batteries, you have safety issues that come with wiring up a system capable of carrying 200+ amps. Problems that can be overcome, but this is beyond hobby level.

I personally love solar for how we use our camper. We never run out of battery, beer/food/water run out long before batteries will. :-)

Here is a picture from this weekend's solar powered, fun-filled weekend. And propane-powered.
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Old 07-23-2020, 12:43 AM   #12
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Power

So I can not do your calculations but on my previous Voltage 3970. I had 6 solar panels for a total of 990 watts and four gel cell batteries. When solar was installed we did a 24 hr test.During test we were to live as normally. Hair dryer and coffee pot, big 50in tv. Everything you are discussing. So Battery bank was equal to 600mahs of which 50% was 300 mahs. In a 24 hr period we used 150 mahs. So 600mahs in batteries was more than enough for all our uses. If that helps.
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Old 07-23-2020, 05:11 PM   #13
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So I can not do your calculations but on my previous Voltage 3970. I had 6 solar panels for a total of 990 watts and four gel cell batteries. When solar was installed we did a 24 hr test.During test we were to live as normally. Hair dryer and coffee pot, big 50in tv. Everything you are discussing. So Battery bank was equal to 600mahs of which 50% was 300 mahs. In a 24 hr period we used 150 mahs. So 600mahs in batteries was more than enough for all our uses. If that helps.
Sounds like a nice system! That is a lot of battery capacity and solar, it should suit full-timing lifestyle.

upon re-reading the OP and answers, I thought I'd offer another suggestion for how to approach solar for the OP. You're really thinking of two different standalone projects here that are certainly related but also independent from one another.
1. Sufficient battery and inverter to meet all of your power needs while boondocking. If your battery capacity is large enough, you may be able to accommodate your needs without running out of battery during your camp trips. But if not, then you'll need some way to get charge back into them while off-grid.
2. How to recharge those batteries while boondocking and don't have access to shore power. One option is using a generator and the camper's charging system - some upgrades may be necessary to maximize. Another is running your tow vehicle engine and using the trailer electric connection - some upgrades may be necessary to maximize. Another is using solar - a lot to consider depending on how much you want to spend vs how much charging you want/need to have to make it worthwhile.
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Old 07-23-2020, 09:44 PM   #14
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We have a new 2020 TT. It came with a 60 ahr battery and I added a 100 ahr deep cycle marine battery plus an AIMS 2000/4000 watt inverter. I run a Resmed S8 CPAP all night with no problem. I recharge the batteries each day via generator when boondocking. Just my experience.
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Old 07-26-2020, 12:10 AM   #15
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Boondocking? May I suggest: the wife can towel dry the hair. You can percolate or press coffee on a propane stove. Play cards or games instead of watching TV. Get a shaker and drink margaritas, with veggies. Have surgery to remedy the sleep apnea.

Lastly, gel batteries won’t cut it for what you want to do. Seriously look at two or more Li batteries that you can run to zero with no harm, with some 400W of solar panels. Go MPPT on the controller, that can charge Li batteries, with Bluetooth programming capability. Also, your onboard converter may need upgrade in order to charge LI batteries while on shore power. You’ll need to rewire to place them inside the cabin where you can lock them up. Costco just had a sale: Two 100 amp hour Lion Energy UT 1300 for $1400.
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Old 11-12-2020, 04:26 AM   #16
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In a pinch jumper cables from your tow vehicle will give you a good charge for about an hour or so.
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