Building my first Solar System... HELP!

120 VAC Refrigerators also have relatively high starting/surge current. So a 120 Watt (average running wattage) fridge will take >600 Watts to start. To "reliably" run a generic refrigerator needs around a 1,200 to 1,500 Watt AC inverter.

You need to get a handle on all of your power consumption. A 100 Watt transmitter (taking 200 Watts or ~17 amps @ 12 volts--And the whole issue of radios being spec'ed for 13.8 volts nominal for full rated output and an off grid storage battery system seeing around 11.5 to 15.0 volts, discharging to charging) probably only takes 10 watts (~1 amp @ 12 volts) on receive (wild guess). 5% duty cycle for transmitting--The average overall energy usage (AH/WH per day) is not bad. However, you talked about some other 10 amp loads--If those are 8-24 hour per day, they are not insignificant.

And will you be using buck/boost for 10.5-15+ volt input to 13.8 volt DC to DC converters for the radios (a bit more losses, and higher average power usage at higher radio bus voltage)?

Two ways to go here--Design a system for your loads... Or use some part of your system as the limiting factor (i.e., xxx watts of solar panels or yyy AH of battery bank) and then design the balance of system--And see what your available (paper) output power would be.

end of part 1

OK... Not much current draw. And, by the way, I like to type 0.5 amps instead of .5 amps -- It is easy to miss the decimal point.

There are different ways to figure out the "optimum" system for you... If we know your loads, we would start from there. However, your limitations may be battery bank or roof area for the solar panels. And, you may choose to mount them flat to the roof, or pitch them for your location/summer/winter usage.

For charging, you can run 5% (good minimum for weekend/emergency usage+genset backup) or 10-15% rate of charge on your battery bank (full time off grid/minimal genset+fuel usage).

Running the fridge from propane is usually the best deal for smaller systems. Wire up a DC line from the tow vehicle to run the 15 amps while moving (may not even be worth it if you are only going a few hundred miles or less). Run the battery bank for 2 hours is only 30 AH out of your (I guess) 436 AH battery bank--Then setup for propane on site (or AC utility if available).

436 AH @ 12 volts to run a 0.5 amp radio, 1 amps of lighting, and a (for example) 3 amp CPAP machine would look like:

• 0.5 amp radio * 24 hours = 12 AH
• 1 amp lighting * 12 hours = 12 AH
• 3 amp (@12v) CPAP * 8 hours = 24 AH
• 5 amp radio * 0.33 duty cycle * 24 hours = 39.6 AH

===============================================
87.6 AH per day
87.6 AH * 13.8 volts nominal = 1,209 Watt*Hours per day

And 1/4 of your bank AH capacity is ~109 AH... So, you could run the above a bit more than 2 days straight just from the battery bank.

If you want to run 110% from solar with a tilted (worst case December), you would break even with:

• 1,208 Watt DC loads * 1/0.61 DC solar system eff * 1/3.17 hours of sun (December) = 625 Watt array (break even December)

And if you only wanted panels mounted flat to the trailer (Boise ID):
http://www.solarelectricityhandbook.com/solar-irradiance.html

Boise
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
1.72	2.66	4.07	5.38	6.36	7.24
Jul	Aug	Sep	Oct	Nov	Dec
7.37	6.44	4.99	3.43	1.97	1.49

You can see that mounting the panels flat will really kill your December output:

• 1,208 Watt DC loads * 1/0.61 DC solar system eff * 1/1.49 hours of sun = 1,329 Watt array Flat December Array
  

So... That gives you an idea of the upper end of what you could want from solar with my estimated loads... However, you could also design for floating the system (parked, no AC power)--For that, I would suggest a 2% rate of charge (minimum). Such an array would look like:

• 436 AH battery bank * 14.5 volts charging * 1/0.77 panel+controller derating * 0.02 rate of charge = 164 Watt array minimum float charging

And, for example, if you are deploying spring/summer for tornado and/or fire seasons--That give you more sun, and more solar power. If you are deploying for winter/spring rains/snows/floods--Different seasons, different sun.

I am sorry to keep throwing math at your questions--But it is a good place to start. Can you tell me more about your loads/deployment/what you are looking for (i.e., hardly any genset use, or short term deployment+10 gallons of gas will keep the batteries charged with an eu200i for 5-10 days is "good enough"). How much room on trailer roof? Will you tilt panels? Are you looking to minimize cash outlay? Will you have AC power for float charging the battery bank? etc...

-Bill

You are very welcome Scott.

Mounting the solar panels is a "big thing". They need to withstand the winds when driving, and tilt (if you want them to).

In winter (November through February), you are loosing a fair amount of "hours of sun" with the panels flat on the roof. If you do not plan on traveling in the winter (or do the snow-bird thing and head south)--Flat panels are not as big of draw back (play with the solar electricity hand book, and see where and how tilts affect your hours of sun). More or less, 4 hours of sun is pretty good, and less than 3 hours of sun is not great for solar.

http://www.solarelectricityhandbook.com/solar-irradiance.html

For the number of panels, just sq ft wise (using random, less expensive, solar panels):
SolarWorld SW-285 Mono All Black

Pmax: 285 Watts
Voc: 39.2 Volts
Vmp: 32.0 Volts
Isc: 9.52 Amps
Imp: 9.0 Amps
Silicon type: Monocrystalline
Efficiency: 17%

Amphenol UTX locking connectors
Weight: 39.7 Pounds
Dimensions: 65.95" x 39.4" x 1.30"

And your trailer is:

7.5 feet = 90 inches

10 feet = 120 inches

At best you can fit ~3-4 of these "full size" panels. That is ~855 to 1,140 Watts of solar panels.

Just to give you a feel--To supply ~1,208 Watt*Hours per day

• 1,208 Watt DC loads * 1/0.61 DC solar system eff * 1/1.49 hours of sun = 1,329 Watt array Flat December Array
• 1,208 Watt DC loads * 1/0.61 DC solar system eff * 1/3.17 hours of sun = 625 Watt array 32 degrees from vertical (or 58 degrees from horizontal) December Array

Only you can make the decision on what will work best for you (I can only give you the "numbers"). If you will be using the system a lot in winter, a tilted array gives you about 2x the daily power vs flat to the top of trailer. Of course, you can install more solar panels, or even use more genset fuel (a Honda eu2000i generates about 3,300 Watt*Hours per gallon of gasoline--loaded with 400 watts or more of loads). Or, 1 gallon of fuel (charging your batteries, no sun) supplies about 2-3 days of usage. Solar panels (and tilting framework)--They cost significantly more than a few gallons of gasoline (if you are not in the field that many days in the winter in the far north).

-Bill

I think you have the basics (panels, batteries, loads) documented--And the limitations (solar array size, battery bank size/weight).

Just remember to keep the battery bank floating (ac power+float charger). If the batteries are allowed to sit for days/weeks under charged, or months of no-charging, they will sulfate and die.

Plan for the ultimate configuration. Do not buy something "on sale" that does not fit into your long term plans (unless you have made the decision for short term vs long term). Many times, it is very difficult to mix and match differing equipment into one unified system without wasting money on those issues (i.e., small charge controller now, add panels, then need new charge controller. Or too large of AC inverter for what is starting out as a small system, etc.)..

-Bill