Sizing

Paul,

Here is the "relatively" conservative rule of thumb design (just like designing for an off grid cabin or small home).

First, your daily energy usage:

• 8 amps * 12 volts * 24 hours per day = 2,304 Watt*Hours per day

That is pretty much what a full size energy star rated refrigerator uses per day (~1-2 kWH per day).

So, my first question--Is there any way of reducing energy usage? Assuming not, to continue...

Flooded Cell Lead Acid battery, generally the optimum design is 2 days of "no sun" storage, and 50% maximum discharge (for long battery life):

• 2,304 WH per day * 2 days storage * 1/0.50 max discharge * 1/12 volt battery bank * 1/0.85 fudge factor (DC to DC converter, battery is actually at ~13 volts and 8.5 amps, other issues) = 904 AH battery bank @ 12 volts

Generally, for any battery bank ~800 AH capacity, I would round up to the next higher battery bank voltage (904 AH @ 12 vs 452 AH @ 24 volts). Same cost of batteries, but lower current saves on wiring costs and charge controller (an 80 Amp charge controller is 80 amps at 12, 24, or 48 volts--Higher bank voltage, larger array support with just the one controller).

Anyway, to continue on... Next to size the solar array. Two calcuations, one based on battery bank AH capacity... 5% to 13% rate of charge typical solar. For "full time" off grid power, highly suggest >10%+ rate of charge (unless you live in a really sunny region).

• 904 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 851 Watt array minimum (weekend/summer)
• 904 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,702 Watt array nominal
• 904 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,213 Watt array "cost effective" maximum

And then there is sizing the array based on your loads and hours of sun per day... Assuming you have a clear southern exposure (no shading, not in a deep valley). Fixed array tilted for optimum winter harvest (you will have way more panel than you need fo summer):
http://www.solarelectricityhandbook.com/solar-irradiance.html

Saint George Utah
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 38° angle:
(Optimal winter settings)

Jan	Feb	Mar	Apr	May	Jun
4.63	4.76	5.84	5.85	5.65	5.61
Jul	Aug	Sep	Oct	Nov	Dec
5.33	5.51	5.91	5.92	5.11	4.67

Worst case "average" month is January at 4.63 Hours of sun per day (long term average)...

• 2,304 Watt*Hours per day * 1/0.52 average DC off grid system eff * 1/4.63 Hours of sun = 816 Watt array minimum (January)

Now, you should never plan on getting 100% of your average predicted daily energy from your system. Generally, 50% to 65% is probably a better estimate for 24x7 base loads (ignoring days/week+ of bad weather). So, a "conservative" solar array would be:

• 816 Watt array * 1/0.50 base load "derating" = 1,632 Watt array

My suggestion would be to go with either a 24 volt to 12 volt DC to DC converter (or even a 24 VDC to 120 VAC inverter--I already have a 0.85 "fudge factor" in the above daily load estimate for battery bank sizing. The Off Grid System Efficiency I used above is 0.52 -- Is used when an AC inverter or DC to DC converter is used... If you do not use any converter, you can use 0.61 for the DC system eff, and remove the 0.85 fudge factor for battery sizing.

Now, you need to figure out what is important to you... A 2x larger array (1,632 Watt vs 816 Watt) is ~$800 more (plus racking)--That could also dramatically reduce your need for a genset during bad weather (and keep your battery bank healthier).

I would guess this is a WAY LARGER off grid $olar $ystem than you initially planned. Those 24x7 base loads (radios, refrigerators, servers, etc.) are real "killers" for off grid power systems. Does laying a 1/2 mile of direct burial cable (or strung on a set of power poles) look more interesting?

You can send 1 amp @ 240 VAC (240 Watts) a 1/2 mile on 12 AWG wire with 3.3% voltage drop. That is something like $1,500 worth of cable from Home Depot (~$487 per 1,000 feet of 12/2 cable).

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=240&phase=ac&noofconductor=1&distance=2500&distanceunit=feet&amperes=1&x=0&y=0

Voltage drop: 7.94
Voltage drop percentage: 3.31%
Voltage at the end: 232.06

I will stop here and let you pick your jaw up off the floor... Please ask your questions and challenge my math/estimates.

-Bill

Drakyla, welcome to the forum.

And, yes, you are correct. 8 amp load sounds like it might be an overestimate/not clear specifications. Or other things happening.

The 8 amps may be the maximum current (radio transceiver + operating gate)... And the rest of the time just the radio transceiver at closer to 0.5 to 1.0 amps or so...

Plongson, have you measured the average current and/or actual AH used per day?

https://www.amazon.com/gp/product/B019CY4FB4 (DC Current Clamp DMM)
https://www.rc-electronics-usa.com/ (DC AH/WH meter)

-Bill

Neat. Always wanted a Ubiquiti system, but never really had a need for such equipment.

Depending on your power needs, up sizing the system with a pair of 6 volt @ 200 AH batteries would give you closer to 6 days of storage with relatively cheap and rugged golf cart batteries.

And if you ever needed some AC power, a 300 Watt Morningstar true sine wave inverter could bea nice option.

Of course, larger battery bank needs more solar panels...

Bill