First, this article is for entertainment purposes only. I have used all this equipment in the ways I describe, but I am not a licensed electrician. I am professionally trained in off-grid solar electric systems and have installed, consulted on, or maintained hundreds of systems, the most remote of which were in the jungles of Papua New Guinea. I do not advise setting up your own PV system without consulting an experienced and knowledgeable source.
After perusing the survivalblog archives for new ideas and methods in off-grid solar, and finding very little at all in the way of solar power explanations, I decided to add a little to the survivalist community by writing my own article. I figured “it cant be that hard… after all, I have written 1000’s of emails explaining these principles to individual clients.” But what I found is that it IS very difficult to summarize all aspects of solar power in one article, especially with an audience whose background and experience is as varied as this one. I am sorely disappointed in the outcome, but I do hope this article helps enlighten some of you to the possibility of off-grid solar, and saves others of you some costly mistakes. So forgive me for not being able to explain all the details, as that would be a book.
I do want to mention this article from survival blog as I agree with and have tried not to restate most of what was said.
For the sake of simplicity, I will use a specific set of equipment in the scenario described below. Obviously there are hundreds of different panels and charge controllers that can be used, but they are not all compatible with this example. In this example (and what I am currently using), we will use four UNI-SOLAR PVL-136 panels and one Midnite Kid charge controller.
Imagine you have arrived at your bugout or bugin location and have your KID charge controller connected to a 250 Amp Hour battery. You have rolled your flexible panels out on a south facing roof or lawn (provided you are in the northern hemisphere.) Now you will connect the panels. Connect the positive wire from panel 1 to the negative wire of panel 2. You now have one 48v string. Do the same to the other panels for a total of two pairs or “strings”. Connect the negative wires of each string together, along with up to a 30’ length of UV inhibited #10 wire. Do the same for the positive wires of each string. Bring two the #10 wires down to the PV positive and negative inputs on your KID charge controller and connect them (don’t reverse polarity). You’re done! You now have up to 30 amps of power charging your 12v battery.
Using Your System
With this system, you can expect to be able to run about 1.6 kwh. This is equivalent to a small DC freezer, a 12vdc pump for 1 hr, a laptop for 3 hours, a Ham radio for a half hour, and several 12v LED lights for several hours per night. Or you can just run a single super-efficient AC Upright fridge/Freezer (not recommended)!
If you were to upgrade to 6 of these panels, an Outback FM80, and a 500 AH battery bank, you would have the ability to a large (8 CuFt) DC Fridge and a small DC Freezer, 5 hours on a laptop, 2 hrs from a12vdc water pump, a couple hours on the Ham radio, a couple hours of TV/DVD, and several LED lights for several hours per night. This is about 2.9 kwh.
However, you would need larger wires and fuses. The complexity rises some, though it’s far from prohibitive.
All this is obviously dependent on the amount of sun you get, which varies greatly based on your location and time of year. For the above example I used an average of 4.5 hrs of usable sunlight per day. You can find daylight information specific to your location by going here. During the months of November, December, and January when you may only average closer to 3 hours of sunlight per day, you can get by with disconnecting the freezer at night
PV Panels and why UNI-SOLAR?
The Uni-Solar are lightweight, easy to transport, and much more resistant to breakage. There are lots of other panels out there that will work, but they are heavier, fragile, and hard to transport. Four Uni-Solar panels can be rolled up and stuffed in an oversized duffel. Be careful not to roll them up too tightly as they CAN CRACK! The downside is that Uni-Solar panels are no longer manufactured. They are also amorphous silicon, and therefore take up more space per watt than any other type of panel. There are other Amorphous Silicon Roll-up panels available on the market, but they are generally very expensive.
All panels HATE shade, but the UNI-SOLAR can tolerate it better than most. The output of a monocrystalline or polycrystalline panel will drop by 33% by simply placing a quarter in the middle, while the Uni-Solar will drop by only about 10%.
Charge Controllers and Why the Midnite KID?
PV panels can be directly hooked to a battery… BUT there are LOTS of problems with this. One, without blocking diodes, the batteries will back-feed the panels at night. Two, most panels (30 cell panels are the exception) can quickly overcharge and destroy your batteries. I have connected 24v panels directly to a 12v battery in an emergency, and using a multimeter to constantly monitor the voltage and amperage, was able to charge the battery without a problem. A charge controller does this for you.
The Midnite KID is a robust MPPT charge controller that is competitively priced, made in the USA and appropriately sized for small offgrid applications. If you are using 12v panels with a 12v battery, you can go with cheaper and simpler PWM charge controllers. But the UNI-SOLAR PVL-136 are 24v panels, and I have them hooked up in a 48v string. The KID has so far proven to be a very reliable unit. The KID can be over-paneled with no problems. In other words, you can hook just about as many panels as you want to this little unit (not to exceed 150vdc if wiring in series). But regardless of how many panels you use with a Midnite Kid, it will never output more than 30 amps to the batteries. Six panels (800w) will increase your charging capacity during cloudy weather, and allow you to charge a little more into the afternoons than if using 4 panels (540w). I tried 1480 watts of panels on a KID with a 12v battery bank for 2 months just to test it. The Kid did fine.
I have also used Steca charge controllers, which seems to do fine, but I find them a bit overpriced. I have used BZ (and other similar Chinese controllers) but they are not reliable enough to recommend. In my opinion, the cream of the crop for an offgrid system is the Outback FM80 or the Midnite Classic, though both will cost you well over $500.
Most MPPT controllers allow you the option to connect to a 12v, 24v, or 48v battery bank. A Midnite Kid on a 12v battery can pass through around 400 watts from the panels to the batteries. When hooked to a 24v battery, it can pass through almost 900 watts. However, I would discourage 24 or 48v battery banks in survival scenarios. If for some reason you need to power a 12vdc device (a likely scenario in a desperate situation), and you tap one of your 12v batteries (that you have hooked in series to make your 24v battery bank), it will become extremely difficult to balance your battery bank and most likely lead to accelerated battery failure. There are ways to power 12v loads from a 24v battery bank, but the simplest answer is just don’t do it.
A word on MPPT. Maximum Power Point Tracking is a more efficient way to transfer energy from the solar panels into your batteries. A MPPT controller constantly monitors the output of the panels and adjust the voltage to facilitate maximum power production. These controllers can take high array voltages (up to 150v in most cases) and convert it down to nominal battery voltages. The “other” kind of controller is PWM, Pulse Width Modulated. They work basically by turning the PV panels on and off until the battery is fully charged. They are typically 20% less efficient and cannot convert voltage down, which necessitates lower array voltages, which leads to higher array currents, which requires much larger wires between the charge controller and the panels.
Wires and Fuses
The Uni-Solar PVL-136 panels come with the older mc3 style connectors (not UL listed). Amazon and Ebay are full of MC3 style connectors. You can find MC3 branch connectors that will combine your pv positive and pv negative wires together. The “right” way to connect PV strings together is with a commercially available combiner box. But these are expensive and not as useful for small offgrid systems. They do incorporate DC breakers for each separate string, which will help to protect your wires and other components. A third option that I have personally used on some of my own smaller systems (shhh, don’t tell anyone) is to make my own “combiner.” I take a 4” section of PVC and drill a hole at both ends. I use 1/4×20 Stainless bolts through the hole and throw a nut on. I now have 2 studs to use to combine all my negatives and all my positive pv wires, as well as the main #10 cables going down to my charge controller. I simply cut the MC3 connectors off the string ends and solder on a suitable ring terminal. This may not be considered a safe practice by some, but it small, lightweight, and works in a pinch. If you use oxide inhibiting grease, the connection will last for several years. If you don’t use a UL listed combiner box, I highly suggest you install a DC breaker (or fuse) between the charge controller and the PV panels. In our example above, we have two strings of panels each capable of producing a short circuit max of 5 amps. A 10 amp DC breaker would be the smallest you could use, while a 30 amp would be the largest (#10 wire can safely handle up to 30amps). Also worth noting: anything over 60vdc can be an electrocution hazard and it is worth taking precautions.
You must install a breaker or fuse on the output of the Charge controller. You will use 10 gauge wire to connect to your battery (keep it under 6ft), and either a 30 amp (continuous duty) DC breaker or 30 amp DC fuse. If you wish to increase your charging ability, you can install a second KID for a max output of 60 amps.
I am assuming that you will be using standard Lead-Acid, AGM, or GEL batteries. There are more technologies out there, such as Nickel Iron and LFP, but they require more specific charging parameters. Most car batteries are Lead Acid batteries with liquid electrolyte. They WILL WORK on a solar electric system during an emergency but they are far from ideal. Deep Cycle batteries, such as marine and golf cart batteries, will work better and are still rather easy to come by. AGM and GEL batteries are the ideal choices for offgrid systems, but are more expensive and harder to find in a shtf scenario. All batteries can be permanently damaged without proper care. In my experience, discharging too far is the #1 cause of premature battery failure. High heat and low electrolyte levels are top battery killers as well. I always like to keep my batteries (AGM) over 65% full and ensure they get a full charge at least once a week. Discharging more than 50% can cause premature battery failure. Failing to fully charge at least once per week can cause premature failure. Most lead acid batteries are around 50% charged when they read 12.2 while resting. So if I wake up in the morning to a battery at 12.4 volts, I am happy. If I wake to a battery that says 12.2 volts, I know I need to manage my power use better.
Fridge / Freezer
I haven’t seen a lot of preppers planning to use a freezer long term, but I say why not?! I planned the above system around one to show that it is possible. The most costly part of adding a freezer to your system is the unit itself. Don’t skimp on this! Most of the “Cooler Style” dc freezers are too inefficient to bother with. The only brand I can currently recommend is the Steca chest style freezer, model PF166 and PF240. The Steca uses an ultra-efficient, computer controlled Danfoss DC compressor with a well-insulated cabinet and all aluminum coils. I have seen two kinds of failures with these units. Lightning induced surges will cook the ECU on the compressor. This needs to be well protected with whatever type of surge suppression you can dig up (ferrite?). The electronic thermostat in the handle can short out from water getting in the handle. Don’t set things on top of the fridge to defrost. Sundanzer makes a slightly cheaper alternative, but they use steel for their condenser coils, and this tends to rust out after a few years.
If you have sized your system for a fridge, it is possible to actually run a dehumidifier instead. “why would you do this” you ask? Well, after dumping gallons of water down the drain from my basement dehumidifier, I thought this might be an additional source of drinking water (after running through a filter!) I haven’t tested this long term, so I cant offer specifics.
Customizing Your System
This is where it can get tricky, but I can’t count on the above setup working for everyone. Skip this is your eyelids are feeling heavy, but here are some simple tips to keep in mind if you design your own solar electric system.
Keep your system balanced. Your array size, your loads, and your battery capacity should all be balanced with one another. Having a large array is not the problem it used to be, but having too small of an array is a death sentence for your off-grid electric system. Your array needs to be able to supply a charging amperage of at least the c20 rate (this is ABSOLUTE MINIMUM!). C20 is your battery bank Amp Hour capacity divided by 20. So if you have a 250 AH battery bank, you will need an array capable of producing 12.5 amps. Your power consumption can be about 70% of your array production, or about 20% of your battery size. So a 1000w array (assuming 4.5 hrs of avg insolation ) would theoretically produce about 4500 watt hours per day. So you could plan on using about 3100 watt hours per day. But if you have a 12 volt, 250 AH battery (which yields approximately 3000 watt hours), you should only be using about 600 watt hours from the batteries (at night).
Fuse your wires. Just because it is low voltage doesn’t mean it is safe. You will not get shocked (under 48v anyway) but the likelihood of a fire is extreme. You must fuse all your DC wiring with appropriately sized fuses or DC breakers.
Wire Size. When using low voltage, your wire size will increase dramatically. This is due to “voltage drop” caused by higher electrical currents in the wire. On a 120v AC system, a 1200 watt load is drawing about 10 amps (1200w/120v=10a). The same load on a 12v DC system would draw 100 amps (1200w/12v=100a). The more amps you draw, the larger wire you need. Try to keep all your wires sized to provide less than 0.5 volt drop. There are online calculators you can use to figure this out, but to be safe just stick with #10 wires for your 12v loads under 250 watts, #4 for 12v loads under 500 watts, and #00 for larger loads up to 1500 watts. These sizes assume your total wire length between the battery and said load is under 25’
PV array. Don’t mix your panels if you can help it. There are three main types of PV panel: monocrystalline, polycrystalline, and amorphous silicon. They don’t generally play well with each other. If you have to mix and match panels, make sure they are the same type. You will need to know that hooking up different wattage panels in series will reduce the output of each panel down to the output of the smallest panel. If you connect a 50w and a 100w panel together in series, both will output a max of 50w for a total of 100w. If you must, different size panels of the same type are best hooked in parallel, such as two 12v 50w panels in series, connected in parallel to one 24v 80w panel; effectively putting a 24v 50w in parallel with a 24v 80w for a total of 130watts at 24v. Bottom line: do your best to only use 1 type and size of panel.
What is “Series,” “Parallel,” and all this other vdc, vac, kwh stuff?
Series: connecting the positive of battery #1 to the negative of battery #2. You then get double the voltage between the negative of battery #1 and the positive of battery #2. If you have two 12v batteries in series, you get 24v out. The same is true of solar panels. Two panels, each capable of 12volts and 5 amps, when hooked in series, become a 24v array with a 5 amp output.
Parallel: hooking up the positive of battery #1 to the positive of battery #2 and the negative of battery #1 to the negative of battery #2. You get double the amps. So two 12v 100amp hour batteries in parallel give you a 12v 200 amp hour battery. Two 12v panels capable of putting out 5 amps each, when hooked in parallel, becomes a 12v array capable of putting out 10 amps.
DC: Direct current. Electricity flowing in one direction. Batteries and Solar panels provide DC power. Most DC devices are much more efficient than their AC equivalents.
AC: Alternating current. Electricity flowing back and forth at regular intervals. This is what you get when plugging into the grid or a generator.
Voltage: I like to think of it as the pressure behind the electricity. A high voltage power line is like high pressure water.
Amperage (Amp): I like to think of it as the volume of electricty. A high amperage device requires a very large diameter hose. A fire hose is able to move large amounts of water ( high amperage) at a very high pressure (high voltage). A pressure washer propels water at a very high pressure (high voltage) but doesn’t deliver as much volume (low amperage). Dumping over a 55 gallon drum delivers a very large amount of water (high amperage), but at a low pressure (low voltage).
Watt: without getting overly complicated, a watt can simply be said to be a measurement of electricity use. Watts can be calculated by multiplying Amps * Volts. (10 amps *12 volts = 120watts AND 1 amp * 120volts = 120watts). Power Factor can affect this measurement, but that’s beyond the scope of this article.
Watt hour (Wh): the amount of watts used in one hour. 1 kilowatt hour (Kwh) is 1000watts per hour
Amp Hour (Ah): the amount of amps used in an hour. A 100 amphour (Ah) battery can theoretically deliver 100 amps for 1 hour. It’s not a perfect measurement when sizing a battery. A 100 Amp Hour battery might deliver 120 Ah if its taken out slowly at 1 amp per hour for 120 hrs. There are different ratings like c20, c/10, 20hour, 5 hour, etc. Interested readers can look it up at BatteryUniversity.com
Inverter: Converts DC power from batteries to AC power (like 120vac) found in your home outlets. I didn’t include an inverter in the above example, but you certainly could. PSW (pure sine wave) inverters are a must if powering high efficiency AC fridges, Laser printers, and running power in wires over 75’ long. The outback FX1312 or FX2012 is my choice as it can run ANY 120v AC load, which includes table saws with over 10,000watt startup surge (yes, I have tried this…over and over and over…). But those are transformer based inverters, very reliable but very HEAVY (60lbs)! If you need portability, Samlex makes a decent PSW inverter for a good price. I keep one of these in a duffle with 3 unisolars and a Midnite Kid, with a weight of about 8o lbs. The even cheaper option is the MSW (modified Sine wave inverter). Some people swear by these, but I have had lots of problems. They ARE NOT reliable when used daily… I get about 1 year out of them. They don’t reliably run small motors and water pumps and solenoids. They don’t handle high startup surges. They are GREAT for Laptop charging (the switching power supplies love these things for some reason.)
That’s all folks. I wish I could help each of you individually design the perfect system, but that’s just not possible… May God bless you as go out into the Darkness and shine as Lights.