Off grid system for remote cabin

Re: Off grid system for remote cabin

AC or DC does not matter too much for our scale of systems... Volts, Amps, Resistance pretty much. The issue is usually DC may be 12-48 volt batteries vs 120-240 VAC for line power... About 10x the voltage and 1/10 th current for AC vs DC comparison (and this has confused more than a couple of licensed electricians on their first solar power installs who end up using to small of wire for an installation).

I use a simple voltage drop calculator and look at 1% to 3% wiring drop typically... But it really depends about what section of the system you are talking about (voltage drop is not as series an issue for wire from panels to charge controller--The drop from charge controller to battery bank should be very low to ensure the controller is measuring the "correct" battery voltage during charging, etc.).

What we really need to know is the Array specifications (how many panels, Vmp/Imp of the panels), brand/model of solar charge controller, battery bank voltage/AH/configuration, Inverter Brand/model/ratings, distances for each wire run, etc.

Regarding battery installation--make sure it is easy to get too... You will want to check the batteries physically every week or month (electrolyte levels, state of charge, corrosion, current balance if parallel strings, voltage readings of cells/batteries, etc... Basically looking for anything "going wrong" before something is permanently damaged.

-Bill

Re: Off grid system for remote cabin

Your other option is to run the panels in series... The higher Vmp-array you can get, the smaller gauge wire you need.

For higher end MPPT charge controllers, they can run Vmp-array upward of 100 VDC (or even more for a few controllers).

So--it is a trade off. More money on copper wire, more money on charge controller, or having battery+equipment in cabin (security/heated environment in winter/easier to maintain batteries/allows use of DC power directly for lighting, car type electronics, etc.) for easy access vs 60' away in clearing.

If your system stays small, it might be better to keep everything at the array and just run 120 VAC wire back to the cabin.

The downside is that you probably want to turn the DC power to the inverter on and off--Even smaller inverters draw around 6 watts just on and idling. Trudging back and forth to the remote power station is going to be a pain for the longer term.

One option would be to change from the Exel inverter to the MorningStar 300 watt 12 volt TSW Inverter (600 watt 10 minute surge). It has both power save mode (search mode--pulse 120 VAC once a second to look for >6 watt loads) and a remote on/off switch (12 volt pair of signal wires you can attach a small on/off switch to). The MorningStar is probably the best 12 volt inverter out there right now because of relatively low price, rugged construction, and the power control options.

If you will be going with a larger system eventually, then go with the MPPT charge controller and higher voltage Vmp-array. And install the battery bank/power system in/under/next to the cabin (remembering batteries do out gas hydrogen and electrolyte fumes, and wire up for safety with fuse/breaker/etc. as needed--And I like to recommend sheet rock/concrete backer board for some additional fire proofing on walls and wood subfloors in case things go pear shaped).

-Bill

Re: Off grid system for remote cabin

You have a very good inverter--It is just so easy/cheap to buy a "big inverter" but then get into a money pit to support it with lots of battery bank, heavy cabling, and lots of solar array.

Lets go back to two x 6 volt 220 AH golf cart batteries--2-4 of those batteries are hard to ignore as cheap/reliable batteries for a first time system.

We start with a 5% to 13% rule of thumb for battery charging (good life, quicker battery charging). So picking 10% rate of charge (nice round number and good rate of charge):

• 12 volts * 220 AH * 1/0.77 panel+controller derating * 0.10 rate of charge = 343 watts of solar panel

Using the "Cheapskate" $$$/Watt listing from our host:

2x Kyocera KD215GX-LFBS panels is a nice price ($1.36 per watt + shipping--don't forget shipping+handling to your front door--different panels/suppliers can dramatically affect the delivered price).

[TABLE]
[TR]
[TD]Max Rated Power (Pmax)[/TD]
 		[TD]215 Watts[/TD]
 	[/TR]
 	[TR]
 		[TD]Voltage at Max Power (Vmp)[/TD]
 		[TD]26.6[/TD]
 	[/TR]
 	[TR]
 		[TD]Current at Max Power (Imp)[/TD]
 		[TD]8.09 Amps[/TD]
 	[/TR]
 	[TR]
 		[TD]Open Circuit Voltage (Voc)[/TD]
 		[TD]33.2[/TD]
 	[/TR]
 	[TR]
 		[TD]Short Circuit Current (Isc)[/TD]
 		[TD]8.78 Amps[/TD]
 	[/TR]
 	[TR]
 		[TD]Length x Width x Depth (inches)[/TD]
 		[TD]59.06 x 38.98 x 1.8[/TD]
 	[/TR]
 	[TR]
 		[TD]Weight of Panel (Pounds)[/TD]
 		[TD]39.7[/TD]
 	[/TR]
 	[TR]
 		[TD]Connector Cable Type[/TD]
 		[TD]MC4 Latching Cables[/TD]
[/TR]
[/TABLE]

So--two of those in series would be ~Vmp-array=53.2 volts and 8.09 amps

Using a generic voltage drop calculator and playing with numbers (3% of 53.2 volt drop = 1.6 volt max; 1% drop would be 0.5 volt drop):

• 60 foot one way run, 8.09 amps => 10 awg + 1.2 volt drop
• 60 foot one way run, 8.09 amps => 6 awg + 0.5 volt drop

Either of those wire gauges will give you a satisfactory system with a 430 watt array.

Rogue is coming out with a new MPPT charge controller--We have to wait and see if Marc will have a good solution/price point for this system (March?).

Otherwise, you are looking at:

Morningstar TriStar 45 amp MPPT Solar Charge Controller for $410 + options (LCD Meter, remote bat temp sensor, etc).

Or for a much larger/more features at $400-$600+ for the Midnite Classic series.

I always like to push the Battery Monitor (like the Trimetric or similar) as a great way to monitor a battery bank for spouse/kids/guests who you don't want taking specific gravity readings from your bank...

Good thing this is not my money. Hope this helps (information is a "starting" point--You are welcome to adjust, change vendors/suppliers/etc. to better meet your needs...).

-Bill

Re: Off grid system for remote cabin

Caveman wrote: »

Yes it does help, and thanks again, but I'm still not clear (bear with me) if I should try to return my inverter for a smaller one or if this one will work with the stuff i bought. I already purchased the panels, controller, and batteries as well.

Larger inverters consume more power when operated at light loads (i.e., 10-20 watts just "turned on"). If you can use a 300 watt TS Inverter (600 watt surge), the MorningStar is hard to beat... Plus it has the remote on/off and "search mode" to reduce power usage without spending a lot of extra money on large DC switches, etc.

If you need more power, then the larger inverter is nice (run a refrigerator)--But then you are looking at a larger battery bank--Which then means a larger solar array--And then a larger charge controller--And then a larger backup battery charger+genset, etc...

It is a vicious circle. So, based on what you know now--How much power (watts, Watt*Hours per day, etc.) do you want to run (from batteries/inverter). That will drive the rest of the equipment and make your choices more obvious (probably not ea$ier).

I was planning on making this a 12V system, if I install the panels in series, will it be a 24V system?

PWM charge controller are "simple" on/off switches. If you run a "24 array" (really ~Vmp=35 volts) through a PWM controller into a 12 volt battery bank, you will be "wasting" 1/2 the energy from the solar array. (current into the controller=current out of the controller).

MPPT charge controllers are "DC Power Down Converters"... Usually built around a DC Buck Mode Switching Power Supply (think the DC equivalent of a variable AC transformer). Power IN = Power out. For example:

• 2*140 watt panels = 280 watts = 17.5 volts * 16 Amps = 35 volts * 8 amps = 14.5 volts * 19.3 amps Batt Charging

Of course, the MPPT charge controller is only ~95% efficient, and solar panels only output around 81% of rated power, so the output will be less than name plate ratings.

For a first approximation, with a MPPT charge controller, you could put the two panels in parallel (Vmp=17.5 volts) or in series (Vmp=35 volts). And the battery bank will see the same charging current (power).

The difference is the current in the 60' of wire going to the solar array/charge controller will be 1/2 also--Allow you to use smaller gauge wire to carry the power.

If you can use your existing Inverter and PWM charge controller--We have to get back to the loads you want to power, and your expectations.

To send 16 amps 60' with less than 0.5 volt drop (Vmp=17.5 volts), would require around 3 awg.

To send 8 amps 60' with less than 1.0 volt drop (Vmp=35 volts) would require around 8 awg wire (~0.7 volt drop).

-Bill