Need Sanity Check

Cougfan · December 2015

I am planning to install a 1.2 KW off grid solar system. A lot of this is new to me, so I would greatly appreciate a sanity check on the proposed system.

The solar panels will be four Canadian Solar 310 watt panels (around 37 bolts, 9 amps). They will be located on a south facing roof on an outbuilding in my pasture. Since it is about a 220 foot run to my house from there, the panels will be wired in series (about 150 volts) to minimize the costs of the wire run. Looks like #10 wire might work, although will probably run #8. The rest of the components will be in the garage that is attached to my house.

The voltage controller will be a Midnight Classic 200. This should be able to handle the 150 volts coming in.

The battery bank will be four 6 volt L16 Deka lead acid batteries wired for 24 volts.

The inverter will be 2000 watt Samlex True Sine wave (SA series), or Go-power, or Cotek. I chose 2000 watts as it is the minimum size that can run my power tools in the garage if I had the need to.

I plan to install a sub panel with a manual transfer switch and move key 110 volt lighting/outlet circuits to the subpanel. During power outages, I would transfer the subpanel to the inverter and run selected small loads like some lighting (compact fluorescents) and electronics. During normal operations, these loads would be powered by my main panel on utility power.

My chest freezer in the garage would be plugged directly into the inverter full time. It uses about 800 watts per day. Fortunately, in the winter, the garage temp goes down into the 50's, so lets assume the load is 600 watts per day in the winter.

My intent is to set up my house so that I have a modest power source in the event of an extended power outage, and to be able to run my freezer independent of the grid. I have a wood stove, so I have a heat source and a means to cook. In the event of an extended outage the freezer would be the priority, and I would only run what lights and such I had excess power for.

I live in the Pacific Northwest, so only average about 3.8 sun hours per day.

So my questions are as follows. Will this system meet my stated needs? Is it adequately balanced (size of components relative each other)? Any fatal flaws in component choices, or are there better affordable choices? Any fatal overall system design flaws?

Regarding component choices, other than the voltage regulator, you will see that I have chosen second tier products. This is because this system absolutely maxes out my budget. For example, I would love to have a better inverter, but do not have the money. My goal is to get the maximum capability at the minimum cost, with "good enough" quality. That said if I had to spend more, I could go without the system for another year and save up some more, if it was crucial to spend more on any given component.

Thanks in advance for any input.

Cougfan

zoneblue · December 2015

Hi and welcome to the naws forum. Your gear sounds pretty reasonable, and your thinking overall sound.

The question really boils down to 'define extended power outage'. If its less than 48hrs, then the PV doestn really add anything to the equation. Even if its more than 48hrs, given that its usually storms that knock grids down, how much charging are you gong to get? You might do some math on just a battery and inverter as an alternative design. Or battery, inverter and genset for that matter.

But solar systems are good fun, and you do learn a whole heap load building a system like this, even if you do have grid. Others might say poohbah, I say enjoy.

- peak charge rate = 4 * 310W /24V*0.77= 40A. 0.11C into 370Ah.
- peak discharge rate= 2000W/24/0.85= 100A, 0.26C
- skip the go-power the others are ok

vtmaps · December 2015

Welcome to the forum,

I believe those panels are 72 cell panels with a Voc of 44.9 volts. Four in series is too high a voltage for a Classic 200, especially with a 24 volt (rather than 48 volt) battery. On a 24 volt system, the Classic 200's hyperVoc only goes to 224 volts.

There are many other reasons not to use the Classic 200... The Classic controllers are not efficient when the difference between input and output voltages is very high. The Classic 200 should be used with 48 volt (or higher) batteries.

Consider this: if you put your panels in a series/parallel configuration (Vmp = 72.8) and use #6 gauge copper cable over a 220 ft run, you will have a voltage drop of 4%. On the rare and brief occasions when you are producing full nameplate power, you would lose 51.5 watts in the cable. If you use #4 gauge copper cable, the voltage drop is 2.6% and the maximum cable loss is 32.4 watts.

If you double the voltage (four panels in series, as you propose) you will reduce the cable loss, but you will lose all or most of those 50 watts in the controller. Losing the power in the controller means the controller will heat up more, and that will shorten its life.

Furthermore, the cable losses that I calculated vary as the square of the power being produced. Those calculated numbers will be much lower at typical power levels... At half power the cable loss is 1/4 of what it would be at full power. Meanwhile, the decreased efficiency of the controller at high voltages occurs at all power levels.

There is a price to pay for transmitting power over long distances. There are some very expensive controllers (different design than the Classic) that work efficiently with high voltage DC input. Another option is to use a Classic 150 with heavier cable between the panels and the controller.

Try to keep things in perspective... If your cable loss is 50 watts, what would it cost to increase your array size by 50 watts? versus What would it cost to put in a heavier cable? You will pay a price one way or another.

Another option to consider is buying 60 cell panels (Vmp about 30 volts, Voc about 39 volts) and putting them 3 in series. The cold temperature Voc will rarely (if ever) drive a Classic 150 into hyperVoc, and will certainly not ever damage the Classic. You might be able to find six 200 watt, 60 cell panels that you configure as two strings (three panels in series to make each string). The voltage drop on #6 gauge copper cable would be 2.7% and the maximum cable loss would be 33.8 watts.

--vtMaps

westbranch · December 2015

Have you considered having the batteries in the barn ? (outbuilding) and sending 120V AC to the house?

Cougfan · December 2015

Thanks for the input from all.

Great thoughts on the efficiency of the midnight classic 200 with all four panels in parallel. Will redesign to series/parallel, make the run to the house with thicker wire and go with a 150. Will save $40 on cheaper controller and only spend perhaps $70 more on wire. Almost a wash, and sounds better. Plus always nice to work around as low of voltage as possible.

Did consider the batteries in the "barn". Barn is actually just an animal shelter roof supported by logs, and is not enclosed. So moisture and some freezing temps were concerns for the batteries and inverter. In the end, I decided the components would be happier in the garage. Otherwise, bringing the power up as AC would be a perfect solution. Shelter temp range over the course of the year would range from as low as 15 degrees in the winter and as high as 90 degrees in the summer. The garage temp ranges from 50 degrees to 70 degrees with a steady humidity of 60%.

With regards to skipping the solar, and just going with an inverter/battery/generator combo. In the end, you are right, solar is fun, and this would spoil my new hobby... . Just buying a used Honda inverter generator and calling it square would make lots of sense, but still not as fun. Besides, I am in a fairly remote area and have lost power up as long as 5 days so far, and I live in earthquake country, so a longer outage very feasible (although a big quake might hurt my gear too). Let's see, earthquake, wind storm, global warming. I will just keep coming up with excuses to support my new solar habit.....

Cougfan

morpho · December 2015

Hey Cougfan,
I really can't give you much in the way of sound equipment advice, but if I were in a place where I lost power for 5 days and the risk of earthquakes, godzilla attacks etc...on top of the PV, I would have the back-up genset with a propane conversion kit so you can keep a couple of 100b tanks and not worry about gas going bad. By all means, get your PV system and enjoy because it is really cool and works great. But if there is anything I know about, it is how important it is to have redundant systems in place. But, then again, if your main concern is losing a freezer full of pizza pops with a value of a few hundred bucks, it probably makes more sense to keep enough dry and canned food on hand, compost what gets destroyed in the freezer, save the few thousand in difference. Thats what "makes sense", but like you said: not as fun.

Cougfan · December 2015

Morpho,

Thanks for the comments. Agree that the basics of dried food, etc. makes a lot more sense in terms of being prepared for a long term loss of utility power. I pretty much have the rice and beans part already covered though. Have some dried food, water source, wood stove, unlimited fire wood, chickens, big garden, etc. Frankly, it would be no trouble to not have any power at all for an extended period, although I do have a small gas generator with gas stored (I periodically burn it in my truck as a rotation plan - agree propane would be WAY better).

I would look at these basic preps as the cake, and the solar project as the frosting. The cake tastes pretty good by itself, but much tastier with the frosting.... . Definitely a "want" not a "need". While a plate of beans and rice with dried tomatoes and fresh garlic from the garden would be good, a nice piece of salmon from the freezer would be even better.

To be truthful there is even more fun in it than I have shared. I have not yet built the animal shelter in the pasture the panels go on yet, so I have the fun of doing that too. Just peeling the logs I cut for the supports. Project has got set back a bit by the 14 inches of rain we have received in the last week.

Hill_Country · December 2015

Cougfan,

For what it's worth, I fell into the category that 'vtmaps' talked about earlier in this thread....I have a 125 foot run between my pole-mounted solar array to my batteries/charge controller/inverter setup. However, I'm in the 48v camp, not the 24v camp, which is why I was able to get away with using a Midnite Solar Classic 200 like vtmaps references. I ended up doing a series/parallel setup with my 8 solarworld panels. Specifically, I used 2 strings of 4 panels each and so I was running just under 150volts, which is why I used a Classic 200. I went overkill and used a #6 AWG wire to minimize voltage drop over the 125ft. It also gives me some wiggle room in the future it I want to add additional panels...only having to bury/pull wire once is very nice. Think about what you need now, and what your goals are down the road...you don't want to have to pull wire over 220 feet more than once! Now, on to my point...

One thing I want to throw into the mix planning-wise is to take a good look at the topic of grounding (both equipment grounding and lightning-related grounding); especially given the long wire run you're planning between the panels and the inverter/battery/cc wiring (~220 ft.). You will want to make sure you are properly grounded! I ended up using a direct-burial, bare, solid #6 AWG copper wire along with the buried conduit housing the 125 ft. wire run between my panels and inverter/cc/batteries. You'll want to make sure you use ground rods (or equivalent) and tie all the ground rods together. You want to avoid having multiple independent grounds, as that is a quick way to get potential buildup between grounds if they are not tied together! Search this forum for grounding and you'll see some themes...

I'm happy to share my experience so far...we've been off-grid since the spring of 2015 and it has been great!!!

Cougfan · December 2015

Good point on the grounding. I will go research it and do it right. You got my attention on that as we took a lightening strike to a 100 foot Hemlock tree very near our house last summer. Took out our well pump controller, pressure switch, computer modum, computer and our phone system. Plug boards did nothing to protect the equipment. In fact, sparks came out of our computer plug board when the lightening hit. Shook our whole house pretty good as well and split the tree right down the middle. Found pieces of tree that flew over our house and were on the other side. My respect level for lightening effects is high as a result.....

Cougfan · December 2015

Hill Country,

I did some research on grounding and the general rule seemed to be one grounding source for all the equipment tied together, as you have done. However, the exception was if the panels were more than 50 to 75 feet away, then use a separate grounding rod for the panel frames at the remote location.

Did you put grounding rods in both in your remote panel location, as well as where the rest of your equipment is? If so, did you tie them into the #6 bare copper wire in the ditch going in between?

Cougfan

Need Sanity Check

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