# Small Cabin System

Registered Users Posts: 9
I am putting together a small system just to run some florescent light and fans. I am thinking 2- sharp ND-230QCJ , morningstar sunsaver Mppt 24v. 4 -T105 Batteries to make 2- 24V banks a magnum MM-AE inverter.

I have been told that the 2 panels isn't enough to charge a 24v system, I should make it a 12v system. Is this correct?

Please bare with me while I am learning.

• Solar Expert Posts: 5,400 ✭✭✭✭
Re: Small Cabin System

4 T 105s will only make one string of 24vdc batteries.

You need to keep PV charge current between 5-13% of ah capacity. In a 24 volt system, your batteries would be ~225 ah/24vdc, so you would need ~ 12-25 amps from 24 vdc.

Panels need to be matched to batteries, and m out importantly batteries need to be matched to loads. Start with a good load estimate, and don't buy anything until you can reasonably calc the loads. Remember the few cardinal rules of off grid. First, people at the same time underestimate thier loads, and over estimate thier solar input.

A goo rule of thumb for off grid, is to take the name plate rating of the PV, divide by two to account for all cumulative systems loses, then multiply that by 4 to represent the adage hours of GOOD sun you can expect, per day, on average over the course of the year.

200 watts of PV would look then like this 200/2=100*4=400 watt hours per day.

One can adjust the numbers a bit, but seldom more than 49 hours on average.

Good luck, welcome to th e forum, and keep in touch,

Tony
Re: Small Cabin System

The same batteries arranged as 12 volts or 24 volts is still the same amount of power... The big advantage with 24 volts is the reduction in current (1/2 the current vs 12 volts) and the additional voltage drop (2 volts vs 1 volt for typical 24/12 volt systems) for wiring losses makes higher voltage systems easier to wire and sometimes less costly (for a charge controller like the very nice MorningStar 15 amp MPPT controller), they are rated for a maximum charging current:
• 14.5 volts * 15 amps = 217.5 watts into battery bank
• 29 volts * 15 amps = 435 watts into battery bank
So, the same controller can manage ~2x the number of panels/wattage with a higher voltage battery bank.

• 2- sharp ND-230QCJ
• morningstar sunsaver Mppt 24v
• 4 -T105 Batteries to make 2- 24V banks (actually makes 1x series string 24 volt bank)
• magnum MM-AE inverter
For the panels:
Maximum power (Pmax) 230 W
Open-circuit voltage (Voc) 36.9 V
Maximum Power Voltage (Vpm) 29.3 V
Short-circuit current (Isc) 8.45 A
Maximum Power Current (Ipm) 7.85 A
Module efficiency 14.1 %
First, realize that solar power does not generate near as much energy as most people assume. And that most people use much more power than they realize.

I normally suggest that people start by defining their loads (peak wattage, average wattage, and Watt*Hours or Amp*Hours per day needed), where the system will be used/installed, and what months/seasons it will be used (plus weekend or full weeks).

At this point, lets assume that the 4x T105 6v 225 AH batteries is your starting point. What would the "ideal system" look like and how would it perform.

First rule of thumb is 5% to 13% rate of charge with 10% being a healthy nominal rate of charge:
• 225 AH * 29 charge volts * 1/0.77 solar+controller derating * 1/0.13 Rate of charge = 1,186 watts of solar panel "cost effective maximum"
• 225 AH * 29 charge volts * 1/0.77 solar+controller derating * 1/0.10 Rate of charge = 848 watts of solar panel "nominal"
• 225 AH * 29 charge volts * 1/0.77 solar+controller derating * 1/0.05 Rate of charge = 424 watts of solar panel "minimum"
Next, what is the amount of power such a system could output. Assuming Delta Rat means you hang out around the Sacromento Delta, using PV Watts for Sacramento, 1,000 Watts of panels (1 kW is minimum software takes) and 0.52 system derating:
"Station Identification"
"City:","Sacramento"
"State:","California"
"Lat (deg N):", 38.52
"Long (deg W):", 121.50
"Elev (m): ", 8
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 38.5"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:","12.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (\$)"
1, 2.75, 41, 5.12
2, 4.23, 57, 7.12
3, 5.27, 79, 9.88
4, 6.33, 90, 11.25
5, 6.83, 97, 12.12
6, 6.93, 93, 11.62
7, 7.23, 99, 12.38
8, 7.30, 101, 12.62
9, 6.96, 95, 11.88
10, 5.67, 82, 10.25
11, 3.68, 52, 6.50
12, 2.71, 39, 4.88
"Year", 5.50, 927, 115.88
So, 1kW of solar panels in Sacramento will generate around 39kWH to 101 kWH per month (fixed array) depending on the month. I use the 9 month estimate as the minimum sized system that will run without backup genset--In this case, February at 57 kWH per month or:
• 57,000 WH / 28 days per Feb = 2,036 WH nominal per day for February (+/- 10-20% depending on weather)
If you choose a 424 watt array (minimum size) then the math would look like:
• 2,036 WH per Feb day * 0.424 kW array = 863 WH per day
If your there 12 months of the year or only 3 month for summer, you can pick the numbers that make sense for your usage. Note, don't plan on using 100% of your power every day--You should only plan on using 50-75% of your daily power unless you "micro manage" your power usage and make up with a genst (more fuel).

How much power would a 225 AH 24 volt battery bank support? Normally, we assume that a battery bank will support 1-3 days of no sun and 50% maximum discharge in normal use. For yours, I will assume 2 days and 50% maximum discharge.
• 225 AH * 24 volt * 1/2 days of no sun * 50% maximum discharge = 1,350 WH per day
• 225 AH * 1/2 days of no sun * 50% maximum discharge = 56 AH 24 volt per day
Again, just a starting point using rules of thumbs and other assumptions.

Sizing of the inverter... Normally, a flooded cell battery bank is rated for ~C/8 for continuous current discharge maximum:
• 225 AH * 24 volts * 0.85 inverter eff * 1/8 rate of discharge = 574 watt inverter average output
And the maximum surge is typically around C/4:
• 225 AH * 24 volts * 0.85 inv eff * 1/4 = 1,148 watts maximum surge load
So, typically, I would suggest a maximum of ~500 watt inverter with 1,000 watt maximum surge rating.

Next, looking at the solar panel you have choosen with the MorningStar 15 amp MPPT charge controller... There is a problem running a 24 volt battery bank. A single panel with Vmp~29.6 volts is too low for a 24 volt battery bank (need ~35-37 volts or so Vmp). And two panels in series to too high of voltage for a MorningStar controller on a cold day (will over voltage the controller).

Your choices (with hardware given) is to put the panels in parallel and use a 12 volt battery bank (really too big of array for a 15 amp @ 12 volt MPPT controller). Or choose a higher voltage controller (MorningStar TS MPPT series at 45 or 60 amp output, or similar from other vendors).

I will stop here for the moment... There are probably more than enough numbers and assumptions that I have made. Some feedback from you will help us in designing a system that will meet your needs.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 8,973 ✭✭✭✭✭
Re: Small Cabin System

I would think the Rogue 30A controller - no, too low of input voltage for those panels, same problems as the small MS MPPT.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 9
Re: Small Cabin System

Thank you for the responses, My mistake on the batteries, (We were originally going with 8 batteries to make 2 banks of 24v.) Lots of numbers to decipher. I'll have to take a closer look, I'm sure I will have even more questions.
• Registered Users Posts: 23
Re: Small Cabin System

I keep seeing such low amount of hours for "sun time". If the sun shines 8-12+ hours a day, wouldn't a simple motorized suntracker increase hours significantly?
• Solar Expert Posts: 249 ✭✭✭✭✭
Re: Small Cabin System

I get about 20% gain from my trackers. They are economical for me because I built them from scratch, but if you had to buy them, I wouldn't......with the price of panels now days, install more panels.

• Solar Expert Posts: 5,400 ✭✭✭✭
Re: Small Cabin System

Themax,

Remember, the short hours of sun average also takes into account cloudy days, partly cloudy days, rain events, snow events etc, shift of seasons etc. So, as I have said before, you can adjust the number up or down to suit local conditions,, but without proper analysis you take the real chance of over estimating your potential harvest.

For example, you might (like me) have a long summer day (18+ hours) but by 10 am on most summer days we get puffy cumulous clouds that cover much of the sky, forcing the panels to ramp up to over capacity with edge of cloud events giving way to full sun, and then having output drop to near zero in a matter of minutes,, over and over again.

I have found the 4 hour average is a pretty good rule of thumb at least to start.

As for the matter of tracking. Let's assume a 2 kw system off grid, at PV cost of ~ \$2/watt (not system cost but raw panel cost) A 20% increase (which would be a lot) would be a net of ~180 watts with standard off grid derate. That panel cost might cost ~\$360. Installation cost of both PV and Trackers is likely to be similar so that is a wash for the sake of this conversation. The question would be,, how much would it cost to buy a 2 KW tracker? I would guess more than \$360.

Tony
• Registered Users Posts: 23
Re: Small Cabin System

Well, it's sunny here in AZ MOST of the time. So if I had some panels on a tracker on a sunny summer day with the sun shinning directly on the panels for, say 12 hours, wouldn't that be 2-3x the "average". That's just an example, but it's sunny here 85-90% of the year. I'm just trying to get an idea of if it's that simple. Even in the winter, you got at least 8-10 hours of sun. Couple that with micro inverters, seems like a no brainer. ???
• Solar Expert Posts: 5,400 ✭✭✭✭
Re: Small Cabin System

Like I said before,, do the math. Figure out what the total life cycle cost of a tracker installation cost VS the cost of additional panels and I suspect you will find that adding PV is way cheaper. It is no surprise that you seldom if ever see large scale ground mount systems on trackers.

As for the 4 hour daily average, also like I said, feel free to adjust that up or down according to local conditions, but keep in mind over estimating will have consequences in performance. AZ of course is a much better PV environment than say Vancouver, both climatically as well as the seasonal changes in insolation. Rules of thumb are just that,, rules of thumb!

PV watts is probably your best resource in determining your daily/weekly/monthly/yearly average insolation.

http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

This might be useful as well:

http://solardat.uoregon.edu/SunChartProgram.php

Tony
Re: Small Cabin System

Using PV Watts for Pheonix Az, 1kW (1,000 watts) of solar array, first is fixed mount with "defaults", second is 2 axis tracking. Note I have used 0.52 system derating for off grid AC power:
"Station Identification"
"City:","Phoenix"
"State:","Arizona"
"Lat (deg N):", 33.43
"Long (deg W):", 112.02
"Elev (m): ", 339
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 33.5"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (\$)"
1, 5.09, 75, 6.38
2, 6.06, 81, 6.88
3, 6.61, 94, 7.99
4, 7.54, 102, 8.67
5, 7.53, 103, 8.76
6, 7.28, 93, 7.91
7, 7.13, 94, 7.99
8, 7.17, 95, 8.07
9, 7.15, 92, 7.82
10, 6.75, 94, 7.99
11, 5.59, 78, 6.63
12, 4.88, 73, 6.21
"Year", 6.57, 1074, 91.29

And 2 axis tracking:
"Station Identification"
"City:","Phoenix"
"State:","Arizona"
"Lat (deg N):", 33.43
"Long (deg W):", 112.02
"Elev (m): ", 339
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: 2-Axis Tracking"
"Array Tilt:","N/A"
"Array Azimuth:","N/A"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (\$)"
1, 6.40, 95, 8.07
2, 7.76, 105, 8.93
3, 8.65, 125, 10.62
4, 10.57, 146, 12.41
5, 11.39, 159, 13.52
6, 11.44, 149, 12.67
7, 10.39, 140, 11.90
8, 10.05, 137, 11.64
9, 9.61, 126, 10.71
10, 8.83, 125, 10.62
11, 7.17, 101, 8.59
12, 6.33, 94, 7.99
"Year", 9.05, 1503, 127.76

First column is Month, second is "average hours of noon time sun per day", third is average kWH per month, last is \$\$ worth of power per month based on 8.5 cents per kWH (cheap utility rates).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 5,400 ✭✭✭✭
Re: Small Cabin System

So about a 30% increase with two axis tracking. So for a 1kw system, another ~300 watts,, \$450-600.

A two axis tracker capable of ~1800 watts is ~\$3300

http://www.wattsun.com/prices/Wattsun_Tracker_Prices.pdf

Tony