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Power for Dry Camping
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RV Electric Power for Dry Camping
Sizing and Design for Solar, Battery Bank, Inverter,
etc.
Read This Disclaimer First
I've written what I personally did, and my opinions. Don't assume what I
did was safe, and don't assume it will work for you. Do more research, and
make your own choices. I'm not responsible for your outcomes! :)
If you haven't read
Intro to Dry Camping you may want to click on it and start there first... or
not.
How Much Electricity do you Need?
To find that answer, you need to know how much
electricity you will use. The way to accomplish this is
to do an energy audit. On the other hand, you could just learn by
trial and error. If you go camping and run out of power add more batteries
and/or solar panels ;)
Electrical capacity is expressed in Amp Hours.
If you use a 2 amp appliance for 5 hours that's 10 Amp Hours (make sense?).
Batteries have an Amp Hour Rating, and your battery bank is the heart of your
electrical system. So doing this energy audit is a really important step.
I know it may look complicated, but it's not as bad
as you think.
Doing an Energy Audit
Use a spreadsheet, or a legal pad and draw 4
columns.
Column 1: List every item that uses battery power in your RV. This
includes not only DC lights, radio, etc, but also AC items that run off the
inverter, like your Microwave, the TV, Desktop computer, etc.
Column 2: List the DC AMPS that this
appliance draws. There are two ways to do this. You can actually
measure the current draw, or you can use the rated current on the appliance
label.
Some of you will be interested in meters to make these
measurements. If you are,
click
here, otherwise continue reading.
Using the rated current of an appliance will give
you a big cushion in your calculations. That's because ratings are always
greater than the appliance actually uses on average. (Those doing actual
current measurements will need to add in a 10% cushion).
Remember, we're powering those AC appliances from
12v DC batteries through the inverter, so you'll need to do a little math to convert AC
amps to DC amps. It's easy!
6 Amps AC = 60 amps DC.
60 watt AC bulb is 60/12 = 5 amps *
10 watt DC bulb is 10/12 = 0.83 amps
* 5 amps @ 12VDC  powering your AC bulb through an
inverter
Item 
DC Amps 


Front TV (AC) 
6.0 


Rear TV (DC) 
4.0 


Stereo 
1.5 


DirecTV Receiver 
1.5 


DVD / VCR 
2.0 


Laptop Computer 
4.8 


Desktop Computer 
8.5 


Microwave 
85.0 


Fluorescent Light 
1.3 


Reading Light 
2.0 


Water Pump 
5.0 


Etc... 



Now that you have all that, it's time for the next step:
Column 3: List how many hours in a 24 hour day you'll use that
item. This is different for everyone. Say 1 hour for the big TV, 2 for the
little one, 1.5 for the stereo, etc. If you have more than one of an item
multiply times that number, for example, you have 4 fluorescent lights and you'll use them about 2 hours each,
that's 8 hours. See the table below.
Column 4: Multiply column 2 (Amps) x column 3 (Hours). Now you have
Amp Hours.
Item 
DC Amps 
Hours 
Amp Hours (AH) 
Front TV (AC) 
6.0 
2 
12 
Rear TV (DC) 
4.0 
2 
8 
Stereo 
1.5 
4 
6 
DirecTV Receiver 
1.5 
4 
6 
DVD / VCR 
2.0 
2 
4 
Laptop Computer 
4.8 
6 
28.8 
Desktop Computer 
8.5 
3 
25.5 
Microwave 
85.0 
. 5 
42.5 
Fluorescent Lights 
1.3 
8 * 
10.4 
Reading Light 
2.0 
4 
8 
Water Pump 
5.0 
. 2 
1.0 
Etc... 



TOTALS 


152.2 
* 4 lights, 2 hours each
Total up column 4, now you know how many Amp Hours (AH) you'll probably use on
an average day. Let's round it to 150 for discussion.
Conclusions from Energy Audit
Now that we've done our energy audit let's see what
it tells us:

Battery Bank Size If we want to
minimize generator use, we need a battery bank capable of giving us 150 AH
each day.

Batteries should never be drained lower than 50% of
full charge because that greatly shortens their life, so the minimum size
battery bank for us is 300 AH (2x our daily usage).

If we don't mind running the generator more, we
could get by with less battery capacity. However, in this example four
golfcart size 6v batteries would be more than enough (420 to 440 AH).

What type of battery? There are several
choices and it's a fairly complex discussion.
Click here
for a lot more
information on batteries. A new window will open.

Charging Systems: We need
to be able to replace 150AH of battery power each day. We can do this with
Solar, with a Generator, or with a combination of the two.

Solar panels only produce their maximum output at
noon, with the panel pointed directly at the sun. They also produce less
output in the winter, and farther North. For this reason, we have to
consider the time of year and whether panels will be tilted to figure typical
output.

A 120 W panel will produce between
40 and 55 Amp Hours of power each day. Take the value of 50, and
we need to install three 120W panels if we want solar to do all our charging.

Even with 3 x 120W panels we'll still need to run
the generator on cloudy days.

For generator charging a 3 stage charger
rated at 50 amps should do well.

If you need to buy a small, efficient generator a
2000 Watt generator will power your 50 Amp charger along with a few other
small items, however, you'll need to turn off the battery charger to run the
microwave, hair dryer, etc..
For a more thorough discussion of these topics:
Battery Charging and Chargers
click here.
Solar Panels, Controllers, and Systems
Click
Here
Portable Generators
Click Here

Inverter Selection: If your RV
doesn't have an inverter you'll probably want to add one. To determine how
large the inverter needs to be refer to your energy audit.

Decide which AC powered items you want to run with
an inverter. Then, add up column 2, DC Amps, for each item and multiply by
12. This tells you the power requirement in Watts. Round this up to
give you a cushion, and that's the size inverter you'll need if you want to run
all the AC items at the same time.

You can see that the microwave at 85 Amps will
require 1000 Watts itself. All the other appliances together require about
300 Watts. So the minimum size inverter for our example is about 1500
Watts.

If you plan to run the microwave and other high
power items from the generator instead of the inverter you can use a smaller
inverter.
You can get more detail about inverters at this
link:
click here
Links
Here's a good resource for more advanced discussion
of RV Electrical topics. Especially if you own an HDT and 5th Wheel
or other truck/trailer combination.
Jack & Danielle
Mayer's web blog.
Here' a link concerning shore
power (electric service) in RV's. Both 30amp and 50amp circuits are
covered:
www.myrv.us/electric/index.htm
Continue on to Meters
 or choose another topic from the menu in the left sidebar at the
top of this page.
