Teach me how to calculate power consumption

[Toes]

So I've been studying the plugs on my various electronic devices so I can get familiar with the amount of power each item will be using when they go in my van.
But I'm still not getting it...

I'm going to be using my fan and computer plugs as examples.

Fan:
Input: 100-240v~
Output: 12v[DC symbol] 1.0A

Computer:
Input: 100-240v~ 1.5A
Output: 18.5v[DC symbol] 4.6A, 16.5v[DC symbol] 6.3A

Lot of questions...

So I understand you can calculate watts by multiplying amps by volts, and you can figure amps by dividing watts by volts.
So are we just concerned about the "output" here?

Why is the output in DC and not AC? Don't things that plug into the wall run on AC?

Since the fan is rated at 12v, does that mean I could just cut off the wall wart and wire it directly to a 12v battery and it'll work just the same?

Why is there two sets up volts and amps readings on the computer plug?

Is there a difference between amps and amp hours? And if so, how do amp HOURS fit into all this?

Sorry for the stupid questions, guys. I need a crash course here. :blush:

 

[SternWake]

Many devices run off of DC electricity, but come with the power transformers( wall wart) so that they can be plugged into the wall. Your fan is one of them. You can wire it directly to 12 volts, but must observe polarity. Some items will just not work if polarity is reversed, some others are burnt up by reverse polarity, so best to not take a chance and test the device with a digital multimeter and figure out which wire is (+) and which is (-). There will either be a white stripe or ridges on one of the two wires. I've seen the marked wire be both (+) on one device and negative on another.

A device can draw one amp. Over the course of an hour, it will have consumed one Amp hour from the battery. The battery capacity is rated in amp hours. Reserve minutes is another measure of capacity which in my opinion is only good at confusing people.

Watt hours is a more accurate measurement of actual consumption, as it accounts for changing voltages, but this is rarely used.

I'm not understanding what you mean by a DC symbol. Usually with standard barrel type connectors they how that the internal contact is (+) and the outer contact is (-).

The laptop power supply is rated for the maximum load the power supply, can supply. This occurs with a dead laptop battery and while doing some CPU intensive task, and does not accurately reflect what the laptop will consume in actual use. It is the maximum it could ever use, but actual consumption can be 20% of the maximum, and the average can be anywhere in between, and this varies brand to brand.

If you really want to know what the laptop is pulling during your regular use, Google Kill a watt.

You can use the maximum consumption of the laptop to base your calculations but will have more battery, and hopefully more recharging current than you actually need, which is a good thing, as far too many people plan for just enough and find that it is indeed too little once their habits change.

 

[Mac J]

Ask RV Bob.

 

[akrvbob]

First, let me say that Sternwake has forgotten more about electronics than I will ever know!! I have a solid foundation in solar, but I never read one of his posts without learning something I didn't know. He is the Master!

My advantage is that being a dummy, and having learned everything the hard way, I can usually make things a little simpler!

Here's my plan, I'll go through this a little at a time and we can talk about it. Mainly because it will take a lot of typing!! I will then probably take all of my writing and turn it into a post for the website. As I go, I'll cover the basics but there is more that I won't cover, maybe we can cover that in the thread.

Here is the bottom line about calculations, we really only care about one thing, how much can I run off my battery without damaging it? So to understand, work backwards from the battery. Batteries are measured in Amp-Hours and (ah) since all we really care about is the status of our battery, we need to know three things:

1) How many ah does my battery have and how much can I use?
2) How many amps does an item draw?
3) How much time will I run the appliance?

So we don't care about watts and have to start thinking in amps which is one reason this is hard. Most of us don't think about electricity at all and never think in amps. The only reason we care about watts is to convert it to amps. And the only reason we care about that is because we want our batteries to last a long time and they are rated in ah.

Lets say you have a typical Walmart marine battery rated at 100 ah. It is supposedly a "deep cycle" battery (although in truth it's a compromise but that's another topic). What's a cycle? This is critical to understand!

A cycle is from fully charged to half charged. Tremendously oversimplified, a battery is at
full charge when it is at 12.6 and half empty at 12.2. Maybe everybody can explain that to him. A deep cycle battery is designed to go through hundreds or even thousands of cycles while a starting battery is designed to go through 5-8. After they go through that many, they fail, stop holding any charge and need replaced.

Even worse, if you take it below half empty, more damage is done, reducing the number of cycles it will go through. Fortunatley, if you are good to the battery and never take it down to half (say you only take it to 3/4 at 12.4 volt) then the battery will last through even more cycles since they weren't a full one.

So the only reason you care about any of this stuff is to take good care of your battery. If you have the money, and can just buy new ones or have so much solar they never run down, you don't need to do the calculations at all.

So our 100 ah battery can only go down to 50 ah. That means we have an allowance of 50 ah (100-50=50) between recharges. But it's preferable to use less, but we can use that much. Most of us will recharge everyday so that means we have an allowance of 50 ah every day.

Those numbers are theoretical. In the real world that battery probably only had 100 ah capacity once and never will again and we won't 100% recharge it every day (although Sternwake will!!) so that's why the calculations are so often wrong. But let's begin with that number and then at the end we will add in some wiggle room.

So, now you know why you need to know how many ah your appliance use and why we are converting to amps instead of watts. In the next lesson I'll talk about how to make the calculation if others haven't already done so.

Here is a post I think great simplifies the whole thing:
http://www.cheaprvliving.com/blog/solar-basics-understanding-batteries/

and another one:
http://www.cheaprvliving.com/blog/solar-basics-batteries-part-2/
Bob

 

[Lucky mike]

now I know why I have so many batteries in here.......( 6 )....

 

[Toes]

Wow! Thanks for the great info, everyone! I'm learning so much here!
Y'all make moving into a vehicle a lot less daunting.

Stern, sorry about the confusion. Here's the DC sysmbol I'm referring too. I can't actually type it out...

 

[Zil]

Some place Bob will mention, Volts at the battery is not the best way to determine state of charge, but for some battery chemistry it is the only one available with out higher costs.

 

[Ballenxj]

Thanks guys, (Stern & Bob) I too am trying to let this soak in.
Bob, Your simplifying this helps a lot.