New to solar have a question on panel configuration.

Stix · June 2015

Well, First off I would like to say hello to all. Secondly I will give all the information on what I have and what I am doing so it will make it easier to help me. My system is setup on a gentrol controller as I can switch from line to solar/backup. I cannot get natural gas and LP is to expensive for my taste so a generator is out and that is why I have come to install a solar backup. I put it on a gentrol controller because I want to use it even when the power is on to save money and I can control what runs off of it and to get by alot of redtape this makes it alot easier. I run basically all my LED lights and tv/radios/laptops and all my plugs except high drain areas like bathroom and kitchen. This is where my question comes in. My equipment is as follows:

2 - RENOGY® 250W Watt Monocrystalline Black Solar Panel UL Listed
1 - Renogy 40amp mppt
8 - 6V batterys 24V config
1 - Go Power! GP-SW3000-24 3000-Watt Pure Sine Wave Inverter (hard wired)

I want to add 2 more panels to this setup to be able to take care of the higher draw areas and still charge the batteries if needed 450AH.

So far this system has lowered my bills on average $40 - 80 and is working good. I want to be able to take it a step further. I think I know the answer but want confirmation. to keep the voltage higher and within the chargers 100v can I have 2 in series and 2 in parrell to the same controller? I know its a dumb question but I have searched and searched. I say dumb question but would be dumber not to and get it right. This is on a homemade tracker.

Thanks
Joe

BB. · June 2015

Welcome to the forum Joe... As soon as you add a refrigerator to your off grid power system... You are now in the "medium" size system and these are not inexpensive if done right.

Just out of curiosity, if you saved $80 in power... What time frame is this. Say 1 year:

$80 / $0.15 per kWH = 533 kWH worth of power
533,000 WH / 365 days per year = 1,461 Watt*Hours per day
1,461 WH per day * 1/0.52 off grid system eff * 1/4 hours of sun per day = 704 Watt array (guessing)

So--Depending on how much sun you get, and the time period for $80 savings--It is possible that, at least some of, your savings are the results of you paying attention to how much power you use (and "waste") and you are actually "conserving" energy too. A typical off grid 500 Watt array system would be hard pressed to save ~$80 worth of power in one month.

And a 500 Watt system would be pressed to its limits to supply a typical refrigerator/freezer with reliable power for ~9 months a year (during winter, not enough sun).

Do you have a Kill-a-Watt type meter to measure your daily loads? I really need to understand your daily loads to help you design an off grid system that will, reliably, meet your needs.

By the way, what Amp*Hour ratings are your battery/battery bank (24 volt battery bank?).

-Bill

Stix · June 2015

BB. wrote: »

Welcome to the forum Joe... As soon as you add a refrigerator to your off grid power system... You are now in the "medium" size system and these are not inexpensive if done right.

Just out of curiosity, if you saved $80 in power... What time frame is this. Say 1 year:
$80 / $0.15 per kWH = 533 kWH worth of power

533,000 WH / 365 days per year = 1,461 Watt*Hours per day

1,461 WH per day * 1/0.52 off grid system eff * 1/4 hours of sun per day = 704 Watt array (guessing)

So--Depending on how much sun you get, and the time period for $80 savings--It is possible that, at least some of, your savings are the results of you paying attention to how much power you use (and "waste") and you are actually "conserving" energy too. A typical off grid 500 Watt array system would be hard pressed to save ~$80 worth of power in one month.

And a 500 Watt system would be pressed to its limits to supply a typical refrigerator/freezer with reliable power for ~9 months a year (during winter, not enough sun).

Do you have a Kill-a-Watt type meter to measure your daily loads? I really need to understand your daily loads to help you design an off grid system that will, reliably, meet your needs.

By the way, what Amp*Hour ratings are your battery/battery bank (24 volt battery bank?).

-Bill

Hi Bill,

Well you are right as to why it dropped due to LED lighting/laptops instead of desktops. Lights on sensors and or timers. I consider that all part of the total system and savings. That is a monthly savings. My average bill before starting this was $240 - 320 a month. After installing solar system and all said above I have dropped to an average of $160 - 180 a month.

Right now I have the panels tracking the sun and average amperage on a sunny day from panels is 14.1 - 18.1 for about 6 hours+ and I keep having to add circuits because the inverter shows high incoming voltage.... This is when it is just float. Within specs just a prewarning. I have the fridge/freezers on the system during the day I just have them timed to go back to the grid 4 hours before sunset. By doing this the batteries never drop below 80% even through the night. I would like more panels and calculated I would need 6 minimum and 8 would be ideal at 250W. That is calculating winter and cloudy days.

The batter pack AH is 430 and should be 630 by my calculations at minumum. I use about 28.5kw a day with solar in use it was 34.4kw depending on season total KW for the month was 860. But that includes 220 which I do not use on solar yet just doing essential 120V for now. That is why I am here to see if my calculations are right. I am still wrapping my head around the KW month/day. I feel like I am rambling. It makes more sense in my head. If I try to explain it it all comes out in pieces.

Sorry if I edit alot and try to get all the details to you. If something make no sense just let me know I can clarify. I calculated I need about 2-3 kw a day at worst 2 normal.... Sorry first time I wrote I included 220V power.

BB. · June 2015

I have suggested before that folks that try conservation--That it is possible to cut energy usage by upwards of 1/2 of their bill. Neat to see you are approaching that savings.

Ok... Starting from scratch with 3,600 Watt*Hours a day of off grid power:

Battery 1-3 days of storage, pick 2 days as a good number, 50% maximum discharge for longer life (note, ~800 AH or larger battery bank, you should go to 48 volt battery bank for various reasons):

3,600 Watt*Hours * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days of storage * 1/0.50 max discharge = 706 AH @ 24 volt battery bank

To charge such a battery bank, recommend 5% to 13% rate of charge with solar.

706 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,329 Watt array minimum
706 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,659 Watt array nominal
706 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,457 Watt array "cost effective" maximum

Note that 5% is good for weekend/seasonal use... For daily use >9 months of the year, you should be looking at 10% or larger array.

And then based on the amount of sun you receive. Using PV Watts, fixed array, tilted to 44 degrees from horizontal, Rochester MI:

Month
Solar Radiation
(kWh/m²/day)

1
3.46

2
4.68

3
4.82

4
5.10

5
5.19

6
5.65

7
5.78

8
5.57

9
4.88

10
3.91

11
2.98

12
2.75

Year
4.56

If you are looking at deep winter average sun of December:

3,600 Watt*Hours per day * 1/0.52 end to end off grid system eff * 1/2.75 hours of sun December = 2,517 Watt solar array "break even" for December

The above are just our typical rules of thumb against a typical off grid home... And the numbers are (or should be) a bit on the conservative side.

8x 250 Watt panels = 2,000 Watt array--A bit small in the winter and even a bit small for a 705 AH @ 24 volt battery bank (2 days storage, 50% maximum discharge)--But you are certainly in the realm of a "doable project" with the basic equipment you have suggested. If this is close to what you were expecting--Then you can start looking at selecting equipment. Picking equipment before you have sized the basic system--You can start wasting money very quickly.

-Bill

Stix · June 2015

Very well and thank you for your response and help. I included by accident the 220V side of things. I need more like 2-3kw a day at worst. Average 1.5 - 2kw for 110v side of things. I can add if I wish 220v later but really for me and what I need I only use 220v for A/C and range and well pump. Not essential for power outages. We have natural springs and woodstove/grill for cooking. That is why 220v is not really a concern for now. Just a question if ya will. What is the reason for 48V? Smaller wire more efficiency? I can switch at anytime. I am doing this as a work project as well as people are always asking about solar and I had no idea about it. Now I am in the processes of learning and will be going to school for it. I wanted some knowledge beforehand and not go in blind. I am licensed in all mechanicals and do HVAC/Refrigeration/taking test soon for Journeymen plumber/starting electrical field apprentice soon. So this all fascinates me and as far as wiring it is a piece of cake. What I do not get yet is the configuration.

I live in Davisburg area. I am at a high elevation above the trees and it works to my advantage for sunlight on sunny days and tracking helps a lot. Winter time is my Achilles and the need for so many panels/AH.

BB. · June 2015

Energy usage is high personal--And that it why I show my work--You can take the numbers/formulas and adjust them to your needs.

More or less, an 800 AH battery bank really needs, roughly, a 10% rate of charge. At this point, the larger solar charge controllers are in the 60 to 80 amp range. If you want more charging current, you have to purchase another $600 solar charge controller. But, since they work at 12/24/48 battery bank, you can go to 48 volts and the solar charge controller will "magically" support a 2x larger solar array at the same price.

Magic=Power=Voltage * Current ==== 2x the working voltage, 2x the power.

Wiring wise, I like to suggest around 100 Amps is a handy point to design for maximum/nominal current. Yes, you can design for 200+ amps, but the copper cables, fusing/breakers, voltage drop issues, and general wiring issues become costly and a headache to connect that huge of cable together.

So, 100 amps * 24 volts = ~2,400 Watt AC inverter....

When you get an engineer/electrician involved, that 100 amps nominal becomes:

2,400 Watt AC inverter output * 1/0.85 AC inverter eff * 1/21.0 volts minimum bus voltage * 1.25 NEC derating = 168 Amp minimum branch circuit wiring+breaker rating

As you can see, all of the safety factors/design issues/losses become a 169 amp, and round up to 175 Amp branch circuit for that "100 amp" nominal design.

Remember that the AC inverter can manage about 2x rated power for surge current (well pump starting, etc.)--So now you have voltage drop issues to work with too.

For 12 volt battery bank, you want around 1 volt max drop (at maximum surge current). ~2 volt drop max @ 24 volts and ~4 volts max @ 48 volt...

So when you put it all together, higher power systems are generally "much happier" and easier to build/maintain at higher voltages (there are other issues which through a monkey wrench into the higher voltage is better equation--Just be aware--Ask questions, do the research).

-Bill

BB. · June 2015

And to add--It is generally not very easy to "grow" off grid solar power systems... It is somewhat like trying to "grow" a VW bug into a station wagon, then pickup, then semi--Could it be done--Yes--But many times you would have been better to design/buy the vehicle you needed vs to add on.

-=Bill

Stix · June 2015

Very well. I am surprised how cheap it was to get into this. I have all the wire I could need in any size as that is part of what we do. If I was paying someone to do this then cost would be a big factor. Installing and adding panels and controllers is not to big a deal. The real deal breaker is in the original choice of 12/24/48v system it looks like for inverter. From the savings of the panels and conservation methods. I could have the system paid off in a year. If I ever go 220V then I would go 48V. I do not see the need for it at this time. I will be switching out the well to a constant pressure DC system where the draw is minimal on start up and RLA. DC compressor that stages itself in would cure the large startup demands. DC fans would lighten the load even more.

So much can be done to save alot.

SolarPowered · June 2015

As a professional, I dont recommend renogy panels, also there have been numerous complaints about their customer support and satisfaction.

If you haven't made the purchase yet, I would recommend SUNIVA as it is better and similar.

Or step up to solar world.

BB. · June 2015

Yep. An "off grid friendly" well pump, while very expensive themselves, can still save you money over all with the Solar/Off Grid power system.

First savings is surge current reduction (virtual elimination of well pump surge current). Smaller pump motor that is typically 1/2 the power and more efficient than a typical well pump.

And, if you can pump direct to a cistern--You can get a well pump that connects directly to the solar array--Pump during the day to the cistern (low power, low flow, 4-6 hours per day), and no batteries, no charge controllers, no AC inverter... Then use a small DC (or AC) pump to pressurize the house plumbing and you can use a very small battery bank to power that pump nicely.

-Bill

Stix · June 2015

SolarPowered wrote: »

As a professional, I dont recommend renogy panels, also there have been numerous complaints about their customer support and satisfaction.

If you haven't made the purchase yet, I would recommend SUNIVA as it is better and similar.

Or step up to solar world.

I appreciate the information. I have bought 2 so not sure if I should keep and just switch new panels over or keep them all the same now.

New to solar have a question on panel configuration.

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