# Sizing

plongson
Posts:

**113**Solar Expert ✭✭
I'm pulling a dummy moment and can't quite figure what I need here...panel size and battery.

I have a little device I call a "call tower" on our ranch that is essentially is an internet gateway (1/2 mile away) to control an entry gate from the main house.

Total power consumption of all the components in the control box is 8a 12v 24/7.

I have a left over 50w panel from another project installed and a brand new automotive battery (I know...wrong battery for the application, but it was free) that was sitting around running everything now and it's worked great all summer.

Now the days are shorter and sometimes cloudy, the little inverter shuts off on low battery voltage and the system goes down.

Without getting too crazy, how much battery and how much panel would you recommend to stay above 50% DOD for say 48 hours??

Zip is 84721

Do you have a calculator?

THANKS!! Paul

I have a little device I call a "call tower" on our ranch that is essentially is an internet gateway (1/2 mile away) to control an entry gate from the main house.

Total power consumption of all the components in the control box is 8a 12v 24/7.

I have a left over 50w panel from another project installed and a brand new automotive battery (I know...wrong battery for the application, but it was free) that was sitting around running everything now and it's worked great all summer.

Now the days are shorter and sometimes cloudy, the little inverter shuts off on low battery voltage and the system goes down.

Without getting too crazy, how much battery and how much panel would you recommend to stay above 50% DOD for say 48 hours??

Zip is 84721

Do you have a calculator?

THANKS!! Paul

3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options

0

## Comments

2,840Registered Users ✭✭✭✭✭8a x 48hrs = 384 amp-hours ÷ 50% DOD = 768ah battery. In most areas, to recharge to full or close to it the next reasonably sunny day, you'll likely want about 1/10 of capacity charge rate, so ~75ish amps. Assuming 75% of STC rating output, 75a ÷ 75% x 14.5v charging = 1450w array.

Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer

Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter

27,898Super Moderators, Administrators adminHere 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

Measured in kWh/m2/day onto a solar panel set at a 38° angle:Average Solar Insolation figures

(Optimal winter settings)

- 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&eres=1&x=0&y=0

Voltage drop:

7.94Voltage drop percentage:

3.31%Voltage at the end:

232.06I will stop here and let you pick your jaw up off the floor... Please ask your questions and challenge my math/estimates.

-Bill

15Registered Users ✭✭27,898Super Moderators, Administrators adminAnd, 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

113Solar Expert ✭✭113Solar Expert ✭✭27,898Super Moderators, Administrators adminTry again with 8 Watt load:

First, your daily energy usage:

- 8 Watts * 24 hours per day = 192 Watt*Hours per day

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

- 192 WH per day * 2 days storage * 1/0.50 max discharge * 1/12 volt battery bank = 64 AH battery bank @ 12 volts

Size the solar array. Two calculations, 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).- 64 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 60 Watt array minimum (weekend/summer)
- 64 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 121 Watt array nominal
- 64 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 157 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

Measured in kWh/m2/day onto a solar panel set at a 38° angle:Average Solar Insolation figures

(Optimal winter settings)

- 192 Watt*Hours per day * 1/0.61 average DC off grid system eff * 1/4.63 Hours of sun = 68 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:- 68 Watt array * 1/0.50 base load "derating" = 136 Watt array

Now, you need to figure out what is important to you... A 2x larger array (68 Watt vs 136 Watt) to 157 Watt array, your choice (I suggest >136 Watt)--That could also dramatically reduce your need for a genset during bad weather (and keep your battery bank healthier).That is a much more reasonable system...

-Bill

113Solar Expert ✭✭Paul

113Solar Expert ✭✭Ubiuiti

27,898Super Moderators, Administrators adminDepending 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