How to utilize solar components for use with RV and homestead

Welcome rxeghtr.

rxeightr wrote: »

G'day all. We have 2- 125 watt panels and 4- Trojan T105 batteries in our RV with an elcheapo PWM 30 amp controller. Recently we acquired property that has an off grid cabin in the woods. Due to shade trees we cannot place panels on the cabin roof. The RV is parked some 300 feet from the cabin and has ample sun hours. The distance to ample sun hours is too far away to make it economically feasible to connect panels on property for the cabin and have a battery bank in the cabin.

First thing... How much power do you need? Watts peak and Watt*Hours per day at the cabin? 120 VAC or 12 VDC for the cabin power? etc...

Next--It is possible to put a decent sized array 300 feet away if you are prepared to spend a few bucks on a nice MPPT type charge controller. For example... Say you want 24 VDC at the cabin. One of these Midnite Kid controllers ($313 each):

http://www.solar-electric.com/inverters-controllers-accessories/chco/misoclchco/midnite-solar-kid-mppt-charge-controller/midnite-solar-kid-mppt-solar-charge-controller.html

The size solar array it could run would be around:

• 29 volts charging * 30 amp max charging current * 1/0.77 panels+controller deratings = 1,230 Watt array "optimum maximum array @ 24 volt battery bank)

A solar array could have Vmp ~ 100 VDC (on a 150 VDC max input voltage controller):

• 100 Volts / 17.5 Vmp panel (this is a common "12 volt" panel) = 5.71 ~ 5 panels maximum in series
• 1,230 Watts / 5 = 246 Watts per panel -> 2 parallel strings ~ 123 Watt panels (common size for "12 volt" panels) or 10 x 123 Watt panels
• 5 * 17.5 volts Vmp = 87.5 Vmp-array
• 2*123 Watt parallel panel strings * 1/17.5 volts Vmp per panel = 14 amps Imp-array
• 100 volts / 30 Vmp panel (this is the larger format "GT type" panel) = 3.3 ~ 3 panels maximum in series
• 1,230 Watts / 3 = 410 Watts -> 2 parrallel stings of 205 watt panels (common GT panels) = 6x 205 Watt panels
• 3 * 30 Vmp panels = 90 Vmp-array
• 2 * 205 watt panels strings / 30 Volts Vmp = 13.7 Amps Imp-array

To make it easy, roughly Vmp-array 90 volts and Imp-array 13.7 amps (either array type). 3% to 1% voltage drop, using a generic voltage drop calculator:

• 90 volts * 3% = 2.7 volt drop; 13.7 amps, 300 feet one way run ->

6 AWG
Voltage drop: 3.25
Voltage drop percentage: 3.61%
Voltage at the end: 86.75

• 90 volts * 1% = 0.9 volt drop; 13.7 amps, 300 feet one way run ->

Voltage drop: 1.02
Voltage drop percentage: 1.13%

Voltage at the end: 8.98

So, in copper wire, you could run around 6 AWG to 1 AWG (a pair of wires) for that array. If you use aluminum wire (special care in terminating to copper ends), you could probably save 1/2 the cost of the copper wire (the aluminum wire would have to be 4 AWG or larger for similar voltage drop to 6 AWG copper.

And a 1,230 Watt array will charge at 5% to 13% charging rate (based on battery 20 Hour Rated capacity):

• 1,230 Watt array * 0.77 panel+controller derating * 1/29 volts charging * 1/0.05 rate of charge = 652 AH @ 24 volts maximum AH battery bank
• 1,230 Watt array * 0.77 panel+controller derating * 1/29 volts charging * 1/0.10 rate of charge = 327 AH @ 24 volt nominal battery bank
• 1,230 Watt array * 0.77 panel+controller derating * 1/29 volts charging * 1/0.13 rate of charge = 251 AH @ 24 volt minimum battery bank

Depending on where the cabin is--If reasonably sunny area, you can get ~4 hours a day minimum sun for ~9 months a year

• 1,230 Watt array * 0.52 off grid system eff * 4.0 hours of sun = 2,558 Watt*hours per day

That is a pretty goodly amount of power--Can probably run an efficient refrigerator, small water pump, laptop computer, lights, cell charger....

I will stop here on this post--Don't want to erase all my work before posting...

-Bill

rxeightr wrote: »

Thanks for the prompt and thorough replies. The cabin power requirements have been calculated to be 2.0kw per day, utilizing 12VDC primarily and turning on a 1000 watt or 300 watt Inverter only when needed.

Sizing the battery bank for 1 (seasonal/weekend use) or 2 days (full time off grid recommended) of storage and 50% maximum discharge:

• 2,000 WH per day * 1/12 VDC battery bank * 1/0.85 ac inverter eff * 1 day storage * 1/0.50 max discharge = 392 AH @ 12 volt battery bank (2 days storage)
• 2,000 WH per day * 1/12 VDC battery bank * 1/0.85 ac inverter eff * 2 days storage * 1/0.50 max discharge = 784 AH @ 12 volt battery bank (2 days storage)

And, to power a ~1,000 Watt AC inverter, you would need around a 400 AH minimum battery bank (to supply AC inverter with enough surge/operating current).

So, based on your needs, roughly a 400 to 784 AH @ 12 volt battery bank minimum.

To keep such a battery bank happy, you should have 5% to 13% or so rate of charge from solar (5% can work for weekend/seasonal, 10%+ for full time off grid use). Based on 400 AH @ 12 volt battery bank (an 800 AH @ 12 volt battery bank would be 2x larger array):

• 400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 377 Watt array minimum
• 400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 753 Watt array nominal
• 400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 979 Watt array "cost effective" maximum

Then there is the amount of array to support your loads.

Summer sun hours are 5.2 hours, winter is 3.14 hours, with average of 4.45 hours. Our power consumption will lower in the winter as we use radiant wood heat. Part of our summer power needs are for 12VDC fans used to help cool.

2 kWH per day on "average sun" day would be (assuming most is AC inverter power usage):

• 2,000 WH per day * 1/0.52 AC system eff * 1/4.45 hour of sun per day = 864 Watt array minimum (2 kWH per day, ~6 months a year)

Say you only want 1.0 kWH per day for average winter day:

• 1,000 Watt*Hours per day * 1/0.52 AC off grid system eff * 1/3.14 hours per day winter = 612 Watt array minimum (AC power)

If you are mostly using DC power, then the off grid system efficiency is closer to 61% vs 52% (no AC inverter losses):

• 1,000 Watt*Hours per day * 1/0.61 DC off grid system eff * 1/3.14 hours per day winter = 522 Watt array minimum (DC power)

Lots of numbers here... A 400 AH seasonal cabin battery bank (minimum sized battery bank): If you are going to run the genset a fair amount--You could get away with a 377 Watt array and keep the batteries relatively happy. If you don't want to run the genset very much, then you are looking at 864 Watt or larger array.

If you want closer to an 800 AH @ 12 volt battery bank, then you really should be looking at 2x377W= 754 Watt array minimum (need more array to meet minimum battery charging requirements).

On the stand alone solar system option for the cabin, it was mentioned to supply 24VDC from the new panels to the MPPT Charge Controller at the cabin. Is the "24v" panels with correct wire size more cost-effective than using "48v" panels and smaller AWG wire, given the 300ft distance the panels will be from the cabin?

Wire sizing is very dependent on the size array you choose... But for your needs, you really need to look at a 150 VDC minimum input charge controller (which usually works out to a 100 Vmp-array maximum array for a 150 Volt input controller in colder climates)... So, whether you put 5x "12 volt panels" (really Vmp~18 volt panels) or 3x Vmp~30 volt panels (not really "24 volt" panels as we call them for battery systems)--Is your choice. Based on best $$$/Watt pricing (usually 30volt Vmp panels are cheaper per watt than 18 volt Vmp panels).

You need to keep Vmp-array very high to so you can keep wire AWG relatively small. A "12 volt" or "24 volt" Vmp-array would be cost prohibitive for 300' wire run from array to charge controller.

One thing to think about--Many remote cabins are subject to theft--Keeping costs down for your installation (battery bank, solar array, copper wiring) can draw less attention and keep losses lower (if that is a problem for your area).

In the 2nd reply it was mentioned to use an 10awg extension cord from the RV to the cabin to power a Battery Charger. Am I correct that the extension cord is supplying 110VAC to the Battery Charger? If so than we will need an Inverter at the RV to provide the 110VAC. Can you advise on a Battery Charger suitable for charging the cabin batteries to near 100%?

Yes--That was the thought--However, your present trailer array is way to small to really support charging a relatively large battery bank (and supplying 2kWH per day) unless you really beef it up.

The question was asked if we wanted to have separate solar systems for the RV and the cabin. Based upon the small capacity of the panels on the RV, I do not see them providing enough wattage to supply power sufficient for the cabin Battery Charger. We do have room on the RV to add a 3rd solar panel, and would prefer to consider that along with a Battery Charger for the cabin batteries in lieu of the expense of separate solar panels for just the cabin needs. The cabin is our home. When not at the cabin we travel in the RV. When at the cabin the RV is there too.

I agree--Your estimated loads at 1-2kWH per day are just too much to supply from your RV solar system as it stands now... An option is to run the RV solar at night (and when not much power is needed during the day).

Be advised we do have a 2000w Inverter Generator that can be used as backup for cloudy days, or additional needs to recharge batteries.

And run a local genset during the day when power usage is higher/to keep the battery charged during bad weather. Stick with a 300 Watt AC 12 volt input inverter and run LED lights, laptop, cell phone charger, etc. from RV on 10 AWG 300' extension cord.

Run the inverter-generator when you need 1,000 Watts of power.... Don't bother with a larger battery bank (and solar array) at this time.

The cabin will have some AC wiring and a transfer switch for genset (or two AC circuits--One for 300 Watts from RV+AC transfer switch for AC genset power, and a second circuit that can take 1-2kWatts from genset and send power where needed). That will reduce your risk from theft (very little fixed assets for electrical at cabin)--And no 2nd battery bank at the cabin (remember, you really need a small solar array to keep lead acid batteries charged when you are not there too--1% to 2% minimum array to float the batteries).

I would "beef up" your RV a bit (more solar, 300 Watt Morning Star AC TSW inverter (with "search mode" and "remote on/off"). Don't put batteries in cabin at this time. Use some good quality LED lights (or flashlights with diffusers) and AA or AAA batteries for some backup lighting at the cabin--if needed (no RV power, and/or to keep cabin energy usage low).

Anyway--Some thoughts. The "system" needs to make sense to you and meet your needs.

-Bill