24 volt to 12 volt

If you are planning on using AGM batteries--They will support higher discharge currents (C/2 or even more)... AGM tend to be more expensive and have a bit shorter life (it seems to me).

With off grid solar power, you really need to conserve. It is sort of like having a 15 gallon gas tank, that you want to (for example) supply 2 days of fuel to 50% maximum discharge... For say 5 hours a night (7.5 gallons / 2 days = 3.75 gallons per day then divide by 5 hours per night, means 0.75 gallons per hour of average usage.

You can supply a 1,200 HP motor at 150 Gallons per hour for ~1/10th of an hour from a 15 gallon tank. It works, but usually not very useful unless that is your only load.

The microwave is possible--It is only ~1,000 Watts or so (if you get a smaller one). And if you only run it for 10 minutes a day--It is not a lot of energy.

Use a thermos to hold your coffee and boil the water on the stove, and you may get away with no electricity at all.

I would suggest starting with flooded cell batteries for your first bank or two--Most people "murder" their first bank of batteries on solar.

If the array is not going to be mounted on the vehicle--What about making a fixed solar system shed (or on a trailer, if living location is short term) and simply (for the time being) just use the RV power system when traveling (which you are not planning on doing a lot for now?). The size and weight of a "big&cheap" lead acid battery bank.

The details of your system design can be more flexible that way. Put the panel and shed close to each other and run your 120 VAC back to your RV spot.

Or, we need to choose an MPPT solar charge controller and array configuration that optimizes the Array to RV/charge controller/Battery Shed configuration. It is much easier to send higher voltage/lower current longer distances than low voltage/high current (from solar array) those 60+ feet.

Choice of 120 VAC (remote shed), or Vmp~100 Volts (150 VDC MPPT max input controller), or higher voltage MPPT controllers (the higher Vpanel input controller voltage, the much more expensive MPPT controller--There are models that will take Vmp-array~400 VDC as solar input--Great if you need to send the power longer distances from the array).

A 1,000 WH per day system is a "small". Good for LED lighting, RV water pump, running a Laptop computer, charging a cell phone, etc.

A 3,300 WH per day system is a "medium" size system that will give you a "near normal" electric lifestyle (using propane/etc. for cooking, hot water, heating). The above system plus a refrigerator, well pump, and clothes washer.

Say you don't want to use very much genset in the winter, a 3,300 WH per day system could look like this:

• 3,300 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/24 volts batt = 647 AH @ 24 volts

Basically, 4x 6 volt @ ~220 AH golf cart batteries in series times 3 parallel strings = 24 volts @ 660 AH bank (12 GC batteries total). If you use "golf cart batteries", plan on them lasting 3-5 years full time off grid use. Although, your first bank may only last a few months or a year if you are not "careful" (no over discharging, no deficit charging, getting >~90% state of charge at least once per week, keep batteries watered and monitor specific gravity once a month--more often SG monitoring as you first start).

Then to charge the battery bank, 5% to 13% rate of charge (20 hour battery bank capacity). 10%+ highly recommended for full time off grid:

• 660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,243 Watt array minimum
• 660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,486 Watt array nominal
• 660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,231 Watt array "cost effective" maximum

And then sizing based on hours of sun per day (by season). A fixed array facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Hutchinson Kansas
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 52° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.76	4.01	4.83	5.19	5.37	5.56
Jul	Aug	Sep	Oct	Nov	Dec
5.93	5.68	5.44	4.84	3.90	3.54

Say you want a minimum usage of genset, pick December as your "break even" month (need to run genset during bad weather):

• 3,300 WH per day * 1/0.52 off grid AC system eff * 1/3.54 hours of sun per day = 1,793 Watt array minimum (Dec break even)

I would suggest an array from 1,793 Watts minimum. And optimally 2,486 to 3,231 Watt array (batteries are "expensive" these days and easy to kill--Extra solar panels makes batteries "happier" and you don't have to manage your loads on a daily basis (run the washer unless cloudy weather is forecast, etc.). As well as allowing you to use less genset time/fuel.

The above is just a stick in the ground to set expectations. If you only need 1,500 WH per day, the above would be cut by ~1/2.

Anyway, the basics on designing a system. Once you have your energy needs and the basic system math penciled in, then start picking the hardware. Don't buy anything until you have done a few paper designs first.

-Bill

I think you understand how this all works... But to give you an example. Normally, we suggest a 1% to 3% voltage drop. For longer cable runs a 1% drop is usually too expensive (lots of heavy copper wire).

Let's do a 2,486 Watt array. One at Vmp-array=36 volts (used with PWM charge controller on a 24 volt battery bank). And a 120 VAC run.

• 2,486 Watt array / 36 volts Vmp = 69 Amps Imp-array @ 36 Volts Vmp-array
• 2,486 Watt array / (3 panels * 36 volts) = 23 Amps Imp-array @ 108 volts Vmp-array (may be too high for some controller on a sub-zero F day)

Using a generic voltage drop calculator, enter 60 feet one way run, and adjust AWG until we get around 3% voltage drop:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.4066&voltage=36&phase=dc&noofconductor=1&distance=60&distanceunit=feet&amperes=69&x=59&y=13
1 AWG for Vmp~36 volt array and 3% drop maximum:
Voltage drop: 1.03
Voltage drop percentage: 2.85%
Voltage at the end: 34.97

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=108&phase=dc&noofconductor=1&distance=60&distanceunit=feet&amperes=23&x=38&y=20
10 AWG for Vmp~108 volts and 3% drop:
Voltage drop: 2.76
Voltage drop percentage: 2.55%
Voltage at the end: 105.24

Every 3 AWG change in wire size is a 1/2 (or 2x) change in square inches of copper (copper face of wire in square inches/mm^2/etc.).

10-1 = 9 AWG steps or 3 factor of 1/2 = (1/2)^3 = 1/8th the amount of copper.

This is a "worse case" design comparison (worse PWM vs best 108 Vmp for MPPT controller).

120 VAC would be a little better (slightly less losses).

If you have 120/240 VAC split phase inverter and can run (for example) a 240 VAC will pump or spread your 120 VAC loads between L1 and L2 (with neutral/white lead)--You could go down 3 AWG to 14 AWG or so.

Many people like the batteries and charge controller next to their home... Saves trudging out in all weather to check the local equipment gauges. The more expensive MPPT controllers frequently have an Ethernet server, so that makes remote monitoring/control much easier.

Anyway--Why we ask the questions we ask.

-Bill