Upgrading to support a Fridge and TV at night.

Kalani · June 2021

I have read multiple times that an array should not be more than '100 x Voltage' as a rule of thumb, but no one has stated why they say this.
Is it true? Why?
Just trying to understand.

BB. · June 2021

These are one of the "rules of thumbs" out there that folks use to do a quick back of the envelope design. It isn't one that I use, but this is why (I think) it is around...l

One of the equations for power we used is Power = Voltage * Current (or "V*I"). If you do a little high school Aglerbra (I think I still remember that from 40+ years ago):

P=V*I
I=P/V

Let's say that a "standard" solar panel is Vmp~17.5 volts (this is for a "12 volt" panel that is/was commonly used on smaller (and older) 12 Volt battery solar power system.

Power (I am guessing that is the "rule") = 100 X Voltage = Power
Power = 100 * 17.5 volts = 1,750 Watt Array max "rule of thumb"
I= P/V =1,750 Watts / 17.5 Volts = 100 Amps

If you look for 3% max voltage drop @ 17.5 volts Vmp-array that is 25 feet from your "battery shed" using a voltage drop calculator:
https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=9&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=17.5&phase=dc&noofconductor=1&distance=25&distanceunit=feet&amperes=100&x=40&y=17&ctype=nec

1/0 AWG Cable

Voltage drop: 0.59
Voltage drop percentage: 3.40%
Voltage at the end: 16.91

To "move" 100 Amps, you would need at least 2 AWG cable--And to move it 25 feet with ~3% max drop, you would need 1/0 or heavier cable... That is not small stuff.

But "today" with modern MPPT (maximum power point tracking) Solar Charge Controllers... You can run a 100 VDC array, or even a 300 VDC Array (300 VDC+ solar charge controllers are not cheap)... An MPPT charge controller is able to take high voltage/low current and efficiently down convert to low voltage and high current to charge your battery bank.

For example... Say you want an 80 Amp MPPT charge controller and are in Hawaii (no sub freezing weather) on your 24 volt battery bank:

80 Amp charge controller * 1/0.77 panel+controller deratings * 29.0 volt battery charging = 3,014 Watt array "max suggested" array
3,014 Watt array / 105 volts Vmp (that is 3 * 35V Vmp panels in series) = 28.7 Amps Vmp-array

So, a "higher" voltage array is 1/3rd the current and almost 2x more Wattage on your proposed 24 Volt battery bank.... The size wire needed for 3% drop, 25 feet @ 28.7 amps @ 105 volts:
https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=1&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=105&phase=dc&noofconductor=1&distance=25&distanceunit=feet&amperes=28.7&x=54&y=24&ctype=nec

12 AWG:
Voltage drop: 2.89
Voltage drop percentage: 2.75%
Voltage at the end: 102.11

And using the NEC chart for wire Ampacity...:
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

10 AWG is a pretty comfortable choice... And much, much, smaller than 1/0 AWG... And if your array is "farther" from your home, you can use a bit heavier AWG to carry the current the longer distances (on much smaller AWG wire than at 17.5 volts for a "12 volt" battery system)... The distance between the charge controller and batteries, and AC inverter and batteries is always kept very short--To keep wire AWG "small" and voltage drop "low". We run the longer distances (from array to charge controller, and from 120/240 VAC inverter to loads) at higher voltages--Current is "low" and voltage drop is low (relative to the higher MPPT input and AC output voltage).

With "modern" systems... A rule of thumb we use is for Arrays 400 Watts or less, a PWM type charge controller is probably less expensive/simpler to design and install. For arrays >800 Watts, then an MPPT charge controller will be easier and cheaper to design...

Anyway... This is "part" of getting loss in the details before you have your "loads" defined, and have done a couple paper designs (basically block diagrams)--This gives you a rough bill of material for your system and you can do some quick costing to see if this makes sense for you (and look at different brands/models/etc. vs costs vs functions of hardware).

-Bill

wellbuilt · June 2021

I have to say bill ,photowhit ,Dave , and others inspired me and helped me go off grid .

I could not of done it with out there encouragement .

After all the discussions we had in the first 2 years I ended up buying a outback flex power one unit from AWS .
It’s 48 volts 3600watts and I have 4500 watts of solar panels .

I think it cost 2700 bucks at the time or a bit more .

I use bills # 3200 watts a day in power or a bit more , I use 2 strings of Costco battery’s so 16 215 ah
I could of run just one string and stayed under 20% discharge no problem except my sun gets turned off for most of December and January and then I’m buried in snow till April’s
My system will reach full charge even on rainy dark days and on a good sun day I can charge up in a few hours .

I think this could really work well for you .

I think 2000 watt of solar should be good four you . Just my 2 cents .

Kalani · June 2021

I've been talking with my brother about "the future" - like if I get married (which is likely), that maybe I should come up with a solar power system that will support both of us. So I'm really trying to get my loads down, even so much to the point of how many hours a day the bathroom light alone might run. Silly I know, but I only want to do this once. Good thing is, neither of us are big power hogs.

Thanks for chiming in wellbilt, it's nice to see another system similar to what I need.
And thanks BB, for putting up with me so far.

Another question to help me wrap my head around things:
When playing with battery calculators, it seems that by going from 24v to 48v, one would need less ah in batteries. Is that true?

MarkP · June 2021

I have a Schneider Conext 4048 inverter and eight golf cart batteries. I went to 48 volts right off the bat because my first inverter was a 48 volt UPS obtained free. I have never looked back and feel it was the right decision. I recommend that you go to 48 volts too. You should also just buy a regular 120 VAC fridge of any normal size and a regular washing machine. Just don't get crazy and you should be fine. I have 15 panels that add up to around 3500 watts. My charge controller is a Schneider Conext 60-150 MPPT, so can put out 60 amps to the batteries and take in power at up to 150 volts from the panels. I occasionally see 60 amps coming out of the charge controller although being in sunny Upper Puna often much less. On a typical reasonably sunny day you can see the current increase steadily as the sun rises and moves from a glancing angle on the panels to hitting them more perpendicularly. At a certain point usually well before noon there is a sharp peak as the current starts to drop. This is because until that peak the charge controller was passing through everything it could until the absorption voltage of 56 volts was reached whereupon the charge controller throttles the power to the batteries to limit the voltage to the 56 volt absorption voltage for one or two hours. After that the charge controller throttles even more to drop the output to the float value of 54 volts. Every clear day looks like this although given the frequent cloud cover in my neighborhood the overall pattern is often barely recognizable behind the overlay of sharp spikes and troughs due to periodic bouts of liquid sunshine. On bad days the system pulls in a few hundred watts and I don't reach float. Peaks of power usage also print through.

When I started out I ran a refrigerator and my TV off of three 250 watt panels and a 2700 watt APC SmartUPS3000 with the tiny batteries removed and wired to eight golf cart batteries. For full disclosure I am on my 4th set of batteries in 8 years.

wellbuilt · June 2021

Battery’s ahave the same AHs
4 12 volt 215AH battery’s have 860AH 4 12 volt battery’s run in SP at 24 volt would have 430AH 4 12vt battery run in series would have 215AH at 48 volt but it’s the same amount of power .

It’s really hard to figure you power usage based on hours of use like your bath light 💡
The thing is my system uses 2 300watts of power day time and 100 watts over night for about 3200 in 24hours .
Lights burn little power I have 68 led lights with all of them on I’m burning 280 watts a hour .

My 5’ ceiling fan use about 35 and 55 watts on low but they are on 24/7 so 90 watts a hour so 2160 watts a day
but its powered off the solar panels in day light hours . Fridge uses 125 watts an hour for 12 hours a day but it gets powered by the sun in the day time .
Every thing is turning on and off all day so most thing don’t run all the time
I think in your area you could use 8 golf cart battery’s with a 48volt inverter and 2 3000watts of solar and not have to worry about power .

Most people are not using power at night I’m sitting here streaming tv off my phone typing on my iPad with 10 lights frig freezer cell booster 60” tv and some other small loads. and when I looked I was using 250 watts ? When I go to bed I’m down to 100 or less if I run the inverter in search mode I use up 6/7% of my battery over night .

Photowhit · June 2021

Kalani said:
Another question to help me wrap my head around things:
When playing with battery calculators, it seems that by going from 24v to 48v, one would need less ah in batteries. Is that true?

Much easier to work with watts, but yes using the basic calculation of amps x volts = watthours, yes a 24 volt system with a 400ah battery bank is the same watthours of storage as a 48volt system with 200 ah of storage.

mcgivor · June 2021

Photowhit said:

Kalani said:
Another question to help me wrap my head around things:
When playing with battery calculators, it seems that by going from 24v to 48v, one would need less ah in batteries. Is that true?

Much easier to work with watts, but yes using the basic calculation of amps x volts = watthours, yes a 24 volt system with a 400ah battery bank is the same watthours of storage as a 48volt system with 200 ah of storage.

Still the same number of batteries are required to achieve the same storage capacity, there is no savings battery wise, but the lower current on the DC side will allow smaller conductors and over current protection, thus less expensive ballance of system.

jtdiesel65 · June 2021

In my experience, the little things that are required to put everything together end up pushing up the budget. Panels are cheap, racking is not. Combiner boxes, breakers, wire, nuts and bolts, and bunch of other stuff end up pushing the cost up.

In your budget, you want to itemize everything that you will need right down to every screw.

For the washing machine, you can plan to get it and just run it when the batteries are full and you have sun. I'm not sure if it really needs to be thrown into the loads calculation unless you plan to use it regardless of battery/sun state. When you live off grid, you have to think like that. Sun comes out and it's time to run the washer and vacuum.

wellbuilt · June 2021

Boy mark that sure seams like a lot of battery’s ? I get 5 years out of mine and I have a set going on 7 years that still work in my trailer
they are starting to use more water and I noticed a cell that had darker water in it .

I did get low on water but my plats where 1/4” fro the top
I’m hopping for 5 years from my battery’s
I’m running 2 sets of 8 for a total of 16 sams club battery’s

Kalapanatic · June 2021

Our first house in Puna was 400 square feet. We had a propane refrigerator that used a lot of gas and we had to refil the tanks often. I would suggest looking into a chest type fridge. This type might be more efficient. There is at least one model configured for 12/24 volts.

There's a little solar place in Kurtistown, right on the main highway, that could advise you. They sell panels and cables and stuff, but don't have a lot on display. I bought some batteries there.

Where our house is, it can be rainy and/or overcast for days. In 2018 we had reduced sunlight due to the smoke from the eruption. You'll want to size your batteries accordingly.

Upgrading to support a Fridge and TV at night.

Comments

Categories