# How much power do I have

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Registered Users Posts: 27 ✭✭
Looking at Pvwatts December for my area gives me 4.71hrs at 60°H. At 6° from horizontal in June gives me 6.64 hrs.  With 780 watts of pv. And 420 ah @12 volts. How much daily  power do I have in June compared to dec?

• Super Moderators, Administrators Posts: 33,440 admin
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Assuming that your PVWatts is accurate for your location (remember that PV Watts is something like a 20 year long term average of "hours of sun" per day, including "average weather"). The basic equation for off grid harvest would be:
• 780 Watt array * 4.71 hours of sun per day (Dec) * 0.52 off grid AC system end to end eff = 1,910 Watt*Hour per day
• 780 Watt array * 6.64 hours of sun per day (June) * 0.52 off grid AC system end to end eff = 2,693 Watt*Hour per day
That is the "optimum" predicted energy harvest (20 year average). And if you are within 10% of the above, that would be pretty much dead on (getting measurements that are within 10% of the "real numbers" is "accurate" for off grid power systems--lots of variables here).

I would suggest that you "plan" your base loads (things that run every day like a refrigerator, laptop computer for work) at 50% to 65% of "predicted" values. On sunny days, you can run your "optional loads" like LED TV, pumping to cistern, vacuuming, washing clothes.

For a full time off grid power system, the "cost effective/optimal" battery usage is ~25% of its capacity per day (2 days of "no sun", 50% maximum discharge for longer battery life). For your 420 AH @ 12 volt battery bank:
• 420 AH * 12 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max battery discharge = 1,070 WH of battery energy per (or over night)
And, the average suggested supported AC inverter rating would be around 500 to 1,000 Watt rated inverter (larger inverters "waste" more energy running small loads; also for larger loads/surge loads, you need a larger battery bank). For a 12 volt battery bank, suggest the maximum AC inverter is 250 WH per 100 AH (at 12 volt) FLA battery bank.
• 420 AH * 250 Watts * 1/100 AH (at 12 volts) = 1,050 Watt "maximum" suggested AC inverter (1/2 that at ~250 Watts is generally a "good fit")
For an off grid power system, suggest 5% to 13% to 20% rate of charge for a battery bank. 5% can work for weekend/sunny weather system, 10%+ suggested for full time off grid system:
• 420 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 395 Watt array minimum
• 420 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 791 Watt array nominal
• 420 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,028 Watt array "typical cost effective" maximum
That should be a good review of your system.

Questions?

-Bill

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 27 ✭✭
edited July 2019 #3
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On pv watts I put in my location and changed the tilt to get the best of summer and winter. I assume it's correct. On the inverter size 1050 watts max you said ~1\2 that 250 ?? Was that meant  to be 525 ? Then I'm sized for TV, led  lights, fan, charging phones. Small stuff. To be able to run a washing machine or microwave what size battery bank and voltage would I need? 800 AMP HOURS @ 12 VOLTS 8*250=2000 watts @166 amps.  Wouldn't it be too much amperage for 12 volts?
• Registered Users Posts: 27 ✭✭
edited July 2019 #4
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Checked my washing machine 783 peak with watt meter.   Not sure of surge 830 wh consumed on heavy cycle.     Microwave  just for no more than 5 minutes.   heating water or warming left overs.   1035 running watts 87 wh.   A 1700 genny will run either one. when the powers out.
• Super Moderators, Administrators Posts: 33,440 admin
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I am sorry... You are correct... 525 Watts. I have fixed my post (just was thinking about another number and typed that instead).

Problem with solar power systems... To run "large" surge currents and heavy loads for a short period of time, you still need a large battery bank and AC inverter (i.e., microwave 20 minutes a day, washing machine 2x per week, etc.).

If you have other loads that run "all the time" (refrigerator, lighting, TV, laptop computer, water pumping, etc.)--Then the larger power system and battery bank required to run those loads can also run the big/short term loads without too much issue too.

Sort of like between a mini-van that is used all weeks, vs a Mac Truck that is only used for a little bit a few times a year.

And the battery bank--The larger the battery bank, for many reasons, the larger the solar array to keep it property charging and equalization charging. Most FLA deep cycle vendors recommend 10% minimum rate of charge (i.e., a 800 AH battery bank should have 80 amps of charging current minimum). However if you have a weekend summer home, a 5% solar charging system can work too.

Regarding the battery bank--Again for many reasons, highly suggest that an 800 AH battery bank is about as large as you would want to go for smaller systems... I.e., you have a choice between 800 AH at 12 volts or 400 AH at 24 volts, usually the 24 volt system is the better choice (smaller wire sizes, fewer parallel battery strings, smaller solar charge controllers, etc.).

Two ways to proceed here... We can look at you present system and discuss ways to improve it to support your desired loads. Or start with your loads and design a system that supports it.

It is, many times, difficult to "reconfigure" an existing system to support your heavier load requirement. It is a mix and match project, and many times, the existing pieces don't work in the "optimal" designed system (i.e., your 7,000 Watt inverter sounds way too large for your needs... But its operating voltage (12/24/48 volts) may not match an "optimally sized" battery bank AH and Voltage.

AC inverters and gasoline gensets can be cheap--And the bigger they are, many times, the cheaper per Watt they are. With solar, the solar arrays and battery banks--2x larger system, 2x more \$\$\$ spent. And a good quality AC inverter designed for off grid solar may cost many times more than an inexpensive AC inverter purchased off of EBay.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 27 ✭✭
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I get what  you're  saying.  Bigger inverter bigger  battery more panels charge
controller. To just use the extra expense a couple times weekly for washing. My system  keeps up with  needs fairly  well. But it is a little aggravating to go out crank the genny come back use the microwave for 5 mins  then go out to shut it down again. Knew I should  have gotten a remote start generator. I only use the RV  4-5 months in summer. Was trying to figure a way to use the system at home. We have frequent power outages. And the batteries  will probably age out before they cycle out. Might as well  use it than let it collect dust for 7 months. Guess I could run the big screen.
• Registered Users Posts: 4,496 ✭✭✭✭✭
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You can run a microwave for a short time on a smallish bank at 12v, but there are some things to be  aware of.  Say the unit is 1200w running, so ~100adc.  You turn on a properly wired inverter capable of that output, and run it for 5mins.  That's 1200÷60x5=100 watt-hours.  On say a 225ah@12v house bank, that's not a lot.

While running though, the bank voltage will sag.  If the bank is pretty full, it may be able to sustain the load without dropping too low (~10.5ish).  If the bank is already at a fairly low state of charge, maybe not.  I wouldn't turn the genny on for 5 mins just to run the nuke or whatever.  If I turned it on for the nuke, it would be because the bank was low on needed charging anyway, and leave it on for a bit to charge.

I'd rather decide myself when to start the genny than rely on autostart, and IMHO there are too many things to go wrong
Off-grid.
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
• Registered Users Posts: 27 ✭✭
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@Estragon I was thinking of a remote controlled start generator so I could  sit inside and start it. not auto start. Don't like the thought of it running unattended. So @12volts 250 watts per 100 ah. 480 ah for 1200 watt microwave. In my situation 6 6 volt 210 ah batteries 2s 3 p 630 ah at 12 volts. 6.3x250=1575 watt inverter potential of 131 amps.  What about surge? How does that figure in to the equation? And if 24 volts 500 watts per 100 ah? Say 4 L16 330 ah. All in series. 3.3x500=1650 68.75 amps. Seems safer at 24 volts and none in parallel. and as for pv  12 to 1600 watts?
• Registered Users Posts: 4,496 ✭✭✭✭✭
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My boat house bank is a pair of GC2s for ~225ah@12v.  It occasionally runs a microwave (~900w rated IIRC).  The heating output is notably reduced when on battery power, but I'm pretty sure that's because I was sold a MSW inverter before I knew any better
Off-grid.
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
• Solar Expert Posts: 3,854 ✭✭✭✭✭✭
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Microwaves have lousy power factor which some inverters may have problems with, running a 800W microwave the apparent power observed is ~1300W, my Schneider 2,5Kw with a transformer has no issues, a cheap 2KW inverter I have will run it for a few seconds then shut down. The wattage/current  draw from the batteries however would be the same as rated wattage of the load, minus efficiency losses, as power factor plays no part in the DC side, only the AC generation portion, the inverter.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Registered Users Posts: 27 ✭✭
edited July 2019 #11
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I'm starting to understand solar is complicated. @Estragon what size inverter do you use? If a 225 ah bank runs a 900 watt microwave. I would think my 420 ah should be able to for 5 mins once or twice a day.

For \$40 I can get a 700 watt. Sure I'd have to wait a little longer to heat with a 700 but it would  be much cheaper than growing my whole system. And save me alot of steps when the s/o wants to heat her plate of left overs.

And as for the washer 783 watts running 830 wh per load. My thoughts  were to only use it on sunny  days  Not so much strain on the batteries.

What size quality pure sine would  I need to run the washer and a small microwave? One at a time of course.
• Registered Users Posts: 27 ✭✭
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@mcgivor surely I wouldn't need a 2500 inverter? Would I?
• Solar Expert Posts: 3,854 ✭✭✭✭✭✭
edited July 2019 #13
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@mcgivor surely I wouldn't need a 2500 inverter? Would I?

Not necessarily there are so many inverter manufacturers out there so it would be wrong to blanket all as being inappropriate, but as a general guide the phrase "Good not cheap, Cheap not good" comes to mind. The cheap 2KW one I have was less than \$300 which has bèen amazingly reliable in every aspect, other than the microwave incompatibility and serves well as a backup. The 2.5Kw was \$1300, much more sophisticated and better serves my needs as I'm 100% off grid..

The cheap 2KW was a replacement for a 2Kw Cotek inverter purchased from Samlex, which powered the 800W microwave without issues, it however net it's demise unrelated to quality, but that's another story.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Solar Expert Posts: 2,045 ✭✭✭✭✭
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mcgivor said:
@mcgivor surely I wouldn't need a 2500 inverter? Would I?

it however net it's demise unrelated to quality, but that's another story.
Care to share?

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

• Registered Users Posts: 4,496 ✭✭✭✭✭
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Mine runs off a Xantrex Freedom MSW 2.5kw IIRC, but might be 2kw.  It's fairly heavy, so transformer type.  I have a cheap (Canadian Tire) 1kw which, though I've never tried, I'm pretty sure wouldn't run it.
Off-grid.
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
• Super Moderators, Administrators Posts: 33,440 admin
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The rule of thumb assumes a 1,000 Watt inverter will surge to 2,000 Watts for at least a few seconds (2x rated power for surge).

Also, AC line voltage math is complicated. Basically an 800 Watt microwave with 0.50 (50%) power factor will draw around 900 Watts from the battery bank (assuming 15% loses).

But the VA rating of the inverter needs to be (at least)

800 Watts / 0.50 PF = 1,600 VA

Most residential inverter ratings Watts=VA

Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 27 ✭✭
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2500 watt inverter on a 225 volt bank? What size fuse do you use? There goes the 250 watt per 100 ah recommendation.  @BB a 700 watt microwave would need a 1500 or 2000 to be sure. And on the DC side it would pull ~66 amps?
I have read 100 amps max on a battery bank for safety reasons.
• Super Moderators, Administrators Posts: 33,440 admin
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To clarify my point above about VA and Watts... Watts is actual power (and Watt*Hours is Energy) used... Kind of like burning fuel 1 gallon of gasoline burned gives you 120,000 BTU (roughly).

Power = Volts * Amps (RMS, DC, approx AC) = Volts * Amps * Cosine phase angle between volts & amps = Volts * Amps * Power Factor

VA or Volts*Amps is how you have to size the AC wiring, AC inverter, Genset, etc.

It is like---If you pull a truck with a rope. If you pull straight on, 100% of the pull is used to move the truck. Cosine 0 degrees = 1.00

If you are off to the side by 60 degrees, then Cosine 60 degrees = 0.50 ... Or only 50% of your pull is used to move the truck forward. If you want to pull 2,000 lbs forward at 60 degree angle, then the rope needs to be 1/0.50 or 2x heavier because of the offset pull.

PF = Cosine phase angle... You can also have "non-linear" loads too (non-sine wave). Sort of like pulling on that truck... 100% of 2,000 lb pull (steady pull) = PF = 1.00  If you jerk on the rope (4,000 for 1 second and 0.0 lbs for 1 second, the average is 2,000 lbs of pull, but the rope needs to withstand 4,000 lbs of pull). (note -- More expensive meters measure RMS voltage and current -- Root Mean Square -- RMS is the pure "DC" equivalent value--Another "fudge factor").

I do not have your microwave, but assuming 50% Power Factor (0.50--Which is a "poor" power factor, yours could be better-- 1.00 PF is "perfect"), then it would look like this:
• 800 Watts * 1/0.85 inverter eff * 1/14.75 volts (battery in Absorb) = 63.8 Amps (technically Amps RMS)
• 800 Watts * 1/0.85 inverter eff * 1/13.60 volts (battery in float) = 69.2 Amps
• 800 Watts * 1/0.85 inverter eff * 1/12.5 volts (battery in discharge) = 75.3 Amps
• 800 Watts * 1/0.85 inverter eff * 1/10.5volts (battery in near dead and/or max current draw under load) = 89.6 Amps
AC Inverters are "constant power" devices... And as you see above, the lower the battery bank voltage, the higher the current draw (to keep power on the AC side stable).

And then there is the 1.25 NEC derating (or 1/0.80 = 1.25). NEC (national electric code) is pretty conservative... But even then, if you pull maximum amp rating continuously (hours), the wiring and stuff can still get hot... And typical fuses/breakers are rated to not trip below 80% of load, and trip 100% of the time at 100% rated load (may take minutes to hours to trip--Breakers and fuses are "not that accurate").

On DC Power Systems, charging the battery bank can be at rated current for many hours... Not at all like running your vacuum or hot water heater for 20 minutes.

For off grid solar, we are suggesting heavier wire for two reasons. One is to derate the NEC rating (no false trips, wiring/insulation/fuses/breakers/tc. last longer at cooler temperatures) and the second is to reduce losses (Power = I^2 * R --- 2x current = 4x more losses at the same resistance)... So, for the above 800 Watt mythical AC microwave load, the conservative rating would be:
• 800 Watts * 1/0.85 inverter eff * 1/10.5volts battery cutoff * 1.25 NEC derating = 112 Amp minimum rated breaker+branch circuit wiring (round up to next available standard breaker/wiring).
So... You can see that a ~64 Amps circuit draw when the sun is shining, can "devolve" into a 112 Amp rated branch circuit when you have a heavy load on a small/discharged battery bank and throw in a 1.25 NEC derating factor.

1,200 Watts (100 amps nominal * 12 volts nominal = 1,200 Watts) to 1,800 Watt system, when you take "everything" into account (to design a reliable system that will work over time/temperature/solar and battery conditions/etc.), the highest I would suggest for an AC inverter on a 12 volt battery bank would be:
• 1,800 Watts * 1/0.85 AC inverter eff * 1/10.5 battery cutoff voltage * 1.25 NEC derating = 252 Amp rated branch circuit
And if you look at a (simplified--the real NEC table has more derating factors) NEC max rated current table:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

4/0 high temperature insulation cable is rated for 260 Amps (and copper 4/0 cable is something like \$4.40+ per foot from Home Depot)... And 250 Amp rated breakers/fuses/fuse holders are not cheap either.

Anyway, rules of thumbs help us to size a system quickly to a 'workable' paper design. Yes, you can push the rules of thumbs, but you have to do a bunch of calcualtions and lookups to figure out what will work for you (cost, size, what is available locally, etc.).

Other issue with large >800 AH battery banks, is that you have to purchase 2 or more large solar charge controllers, find crimp connectors and crimping tools for large cables, estimate voltage drop (short/heavy cables to keep voltage drop low--12 volt systems have lots of voltage drop issues--That 24 and 48 volt system do not/or at least not as bad)...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 27 ✭✭
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Wow inverters can be expensive.
For a  \$40 microwave and a \$100 washer. I'm looking  at msw.
I have a 300 puresine for TV. Dvd stereo Fan, nebulizer and phone charging.
• Super Moderators, Administrators Posts: 33,440 admin
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There used to be 12 volt Microwaves... But they seemed to have all (?) dissappeared from retail land.

That is the problem... You have to size your system for the largest load(s) you want to support. If those loads only run for 20 minutes a day, and the rest are a few LED lights and a laptop/cell phone charging station, it is difficult to design a system that will do all that without spending a bunch of money.

If you have a large 4-8+ kWatt system to run a modern home, then a 1,200 Watt microwave is not a problem.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 27 ✭✭
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Sounds much too complicated and expensive. Seems it would  be easier and cheaper to sell my 2 generators and get a champion inverter wireless remote start. That way I can start it flip the main breaker.  And run everything.
• Super Moderators, Administrators Posts: 33,440 admin
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"We" generally here suggest using a backup genset if you have grid power and/or are just weekend cabin usage. The Honda and similar smaller inverter-generators are quiet and don't burn a lot of fuel.

If you want something in the middle... Build a system out of 2-6 Golf Cart Batteries in a 12 volt system with a 300 Watt or so AC inverter (the MorningStar 300 Watt TSW inverter is nice--Has remote on/off and "search mode"--Not many small inverters have those features, although, it looks like there are some out there now).

https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html

Run the genset for heavy loads, and run the smaller 300 Watt inverter for evening/over night/quiet times. An AC battery charger (from the genset) and/or some sort of smaller solar array--And you get the best of both worlds. Quiet power for lighting, laptop, cellphone, and 12 volt RV water pump... And the genset when you need to power the larger stuff (and the genset is more portable than a solar power system).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 27 ✭✭
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In summer dc fans are my biggest loads ~ 600 wh.
Fridge and hot water on propane (silinoids) led lights RV water pump ~250 wh. 850 wh on 12 volt DC.
32 inch TV 30 watts 8 hrs 240÷85 inverter eff.~280 wh  charging 2 phones 30 wh.call it 1200 whs a day.
A little less than 25% of my 5000 watt hour batteries.
In spring or fall my daily watt hours will be less not as much fan usage  say 1kw per day.
11 months  of the year pv watts gives 5.07 hrs or more.
With a 780 watt array. 780x.77=600 ×5 hrs 3000.
Batteries  at 50% soc needing 2500 x 1.2 =3000.
I should be able to charge in one day. After a couple rainy days???
• Registered Users Posts: 27 ✭✭
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At 1kw per day  x 2 days 2000 wh x1.2 =2400 whs needed plus 500 day useage 2900 whs.
Since bulk takes 2 hours and  absorb takes 2-6 hours. I would  guess it's gonna take 2 days to completely charge??
• Super Moderators, Administrators Posts: 33,440 admin
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What is your battery bank? ~420 AH @ 12 volt flooded cell lead acid deep cycle or something different size/type?

Realistically, more or less, you can recharge an FLA battery bank ~25% in one day on solar (decent hours of sun). Assuming ~10% rate of charge (~2.5 hours of Bulk) and another 2-6 hours of absorb... And you are in 4-8 hours of sunlight. That is about it (winter away from the equator, that means more genset time).

Where is your system located (roughly, near what major city)? You can use this to see what your hours of sun per day by month works out to be (assuming that you don't have shading from trees, on back side of mountain, bottom of a valley, etc.)?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 27 ✭✭
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This is at 8° I can adjust tilt through out the year quarterly monthly etc.
• Registered Users Posts: 27 ✭✭
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