A typical residential-size solar system installation will include properly sized and installed AC and DC electrical wiring to avoid electrical fires, a proper grounding system to avoid shock and lightning damage, proper battery installation and venting to avoid gas explosions, and a properly installed solar array to maximise performance while avoiding roof damage.
I’ve covered many various types of solar power systems in practically all of my previous articles, but I’ve avoided going into depth on how to instal them yourself because most systems should be sized and wired by qualified solar professionals. However, numerous e-mail enquiries about smaller do-it-yourself solar projects for distant weekend or vacation homes in locations without power connections continue to flood the Backwoods Home website. If this describes your circumstances, and you’re prepared to stick to the fundamentals, I’ll show you how to set up a very basic battery-based off-grid solar system to power a few lights and potentially a DC well pump or freezer.
I’m sticking with 12-volt DC equipment because it has a low risk of shock and allows you to use a lot of common electrical components. But first, a word of caution. Even though everything will be the same low voltage wiring as in your automobile or boat, there will still be some safety considerations.
If not sized and placed appropriately, any deep-discharge battery contains hundreds of amps of stored energy, which can easily melt heavy gauge wire or electrical components. Due to an improperly fitted battery, I’ve seen electrical cable the size of your thumb quickly flame red and melt. Any neighbouring walls, floors, or roofs might easily be ignited by this lighting cable.
This implies that if I tell you to utilise a certain type or size of component, there’s a valid reason for it. You should not presume that anything you have on hand that is “nearly the same” would suffice. All of the components required are available in pre-packaged solar lighting kits of various sizes if you truly want to simplify the equipment choices.
Battery sizing
The size of the system is the first decision we must make. One or two 12-volt batteries will do if you simply need to run a few lights in two or three rooms in a distant lodge. You’ll need two to four 6-volt batteries if you want to run a small DC freezer or a DC well pump. Under no circumstances should you use a conventional car or truck battery, as they have very thin lead plates to save weight and will not last long while cycling on a daily basis.
Normally, I would recommend either 12-volt RV/marine batteries or 6-volt golf-cart batteries for this use. These can be found locally and are reasonably priced. If you select a sealed GEL or AGM battery, however, you will considerably limit the possibility of explosive gases being vented and the requirement for battery maintenance. Sealed batteries are over 40% more expensive than open cap cells that need to be watered, and they don’t last as long. However, with a smaller cabin installation with limited storage space, not having to create a specific battery room or vented inclosure will be an advantage. In colder climates, regardless of battery type, the battery bank should be protected because battery charge declines dramatically below 35 degrees.
Solar array
The size and number of solar modules required are determined by the capacity of your battery bank and the location of your off-grid cabin. If at all practicable, the solar array should face south. Because most mornings in my area are foggy and late afternoons are bright and sunny, I prefer a somewhat south-west orientation.
With a tilt angle equal to your latitude, a solar module provides the optimum year-round performance. Summer output will be better with a lower angle, and winter output will be better with a steeper angle. This is between 37 and 42 degrees Fahrenheit for most of the United States. If your cabin will only be used for a portion of the year, choose the tilt angle that will create the most energy during that time.
Your solar array can be installed on the roof of your cabin, on a nearby pole, on a raised structure in the ground, or on a neighbouring storage shed. Because solar modules are quite light, the main mounting problem is wind uplift rather than collapsing in your roof. Because a high wind will readily take out any screws that simply pierce sheeting plywood, any mounting method should only utilise stainless-steel bolts or lag screws penetrating into rafters or blocking.
Array sizing
The majority of 12-volt solar modules on the market today are under 100 watts. The current market trend is for larger modules with a nominal output voltage of 24 volts. When a deep-cycle 6-volt golf-cart battery or 12-volt RV battery is depleted 50%, it stores about 1 kWh of electricity. This indicates that, assuming five hours of direct sunlight, you’ll need around 200 watts of solar array to recharge one battery in one day.
This is usual during most summer months, although during short winter days in many places of the northern United States, you may only get three hours of direct sunlight. This means you’ll either need to raise the solar array to battery ratio or minimise your power consumption during cloudy weather.
Multiple “parallel” wired solar modules and batteries have numerous design issues, therefore larger-capacity solar modules and batteries are much easier to utilise than smaller ones. When buying batteries and solar modules, keep this in mind. In the long term, ordering larger unit quantities will save you money over buying smaller, less expensive brands offered locally.
Wiring safety
You must understand that at 120 volts AC, the same watt size electric load requires 10 times the amp current at 12 volts DC. Because watts for a particular load do not change with voltage, two 100-watt light bulbs requiring 1.7 amps at 120 volt AC (200 watts/120 volts) can be wired with a #14 size wire rated at 15 amps. This identical load will draw 17 amps (200 watts/12 volts) at 12 volts DC, above the #14 wire’s 15 amp rating.
Furthermore, these two 100-watt light bulbs would only last five hours before exhausting your deep-cycle battery (5 hours x 100 watts x 2 bulbs = 1 kWh). So, from the outset, use only the most energy-efficient DC lighting and appliances available, and don’t size your wires based on 120-volt AC demands.
We do not require fuses or circuit breakers just because we are using low voltage DC electricity. To avoid overload and possibly fire, each wire supplying a load in your cabin must have an appropriately sized fuse or circuit breaker.
A DC rated fuse or circuit breaker is usually physically larger and more expensive than an AC device of the same amp size. It’s also far more difficult to locate a local provider of DC circuit breakers. However, we’re in luck because the Square D “QO” line of AC circuit breakers is also dual rated for up to 48 volts DC, but only for the “QO” line.
I’m not aware of any other low-cost circuit breaker rated for low-voltage DC service that can be found at a local builder supply. For all of your load wiring, I recommend using the eight-circuit Square D “QO” subpanel with single-pole 10 or 15 amp “QO” circuit breakers. You might be tempted to use cheaper automotive-style DC fuses, but they aren’t allowed for residential usage. A great assortment of 12-volt DC lighting fixtures may be found at your local RV or boating supply store, which should easily cover all of your lighting requirements. For outdoor lighting applications, these are also available in waterproof designs. Because DC rated switches are hard to come by and make wire installation more expensive, look for DC light fixtures that incorporate an inbuilt on/off switch.
Water system
An on-demand RV or boat 12-volt DC pressure pump can deliver pressured water from a storage tank to a kitchen sink or shower. This reduces the cost of plumbing as well as the high power demand of a deep well pump. Rainwater, a local brook, or a spring can be used to refill this tank. Drain valves should be easily accessible at all low locations to allow for quick system draining when the house is not in use. Obviously, without adequate treatment, this water is unfit for drinking or food preparation, but you may always consume bottled water brought with you on each visit to avoid the cost of drilling a well or the added solar cost of powering a well pump.
Refrigeration
Having an electric refrigerator or freezer in an off-grid rural cabin dramatically increases the size and cost of the solar power system. Bringing along a high-quality ice chest full of ice is the most obvious answer for shorter periods of cabin use. The better insulated ones, I’ve discovered, can keep ice for up to four days.
Several 12-volt DC and propane-powered refrigerators and freezers are available in the RV and boating industries, although most have a very high daily electrical load. You might wish to read my piece in Issue #102 (Nov/Dec 2006), which contains a lot more information.
information on refrigerators and freezers that run on DC electricity. There are some great 12-volt DC refrigerators and freezers developed expressly for off-grid solar homes that require significantly less solar electricity to function if you are ready to invest $900 to $1,500.
Low-energy 12-volt DC refrigerators and freezers from SunFrost and SunDanzer are available for off-grid solar applications. These super-efficient models are pricey, but they will save you thousands of dollars because you can get by with a lot smaller solar array and battery. If you plan on staying in your off-grid cabin for an extended amount of time, the solar refrigerator should be your first priority.
Wire sizing
Use the same wiring processes and materials as the National Electric Code for your lighting and DC appliances (NEC). Because we are working at 10 times the current necessary at 120 volts AC, I only make one exception: I upsize the wire size to reduce wire resistance. The NEC amp rating for the most common size house wiring is shown in Figure 1, followed by my recommended wire size for all 12 volts DC wiring:
Even if the load is still minor, I would proceed to the next larger wire size if your wire run is greater than 50 feet. Use only copper wire as well. Aluminum wire is less expensive, but for the same wire size, it has a lower amp rating than copper. All aluminium wire connections require specific anti-corrosion joint treatments, so stick to all-copper wiring and solid copper wiring devices and connectors to keep things simple.
You can simplify the wiring by placing your circuit breaker panel near the front entrance and using the circuit breakers to turn on and off loads that don’t have internal switches. This is due to the fact that 120-volt AC wall switches purchased at any hardware or construction supply store will not work on a DC electric service. Because DC current has a steady flow characteristic, it’s not uncommon for AC switch contacts to fail “When utilised in DC wiring systems, the arcing can “weld” or even melt the wires together.
Since most electrical loads will be wired to terminals located in each item, you won’t need any perimeter wall outlets because your simple DC electric system will only supply a few DC lights and DC appliances. If you really need to, though, “They do make a wall receptacle and plug designed for low voltage DC service, so you can “unplug” an appliance. To avoid accidentally putting an appliance into the wrong voltage service, the two prongs of DC outlets and plugs are turned 90 degrees in comparison to a typical 120-volt AC outlet.
Solar and battery wiring
Proceed with the interconnection wire after you’ve mounted your solar array modules. Solar modules with higher wattages are now offered with prewired male and female connectors and many feet of wiring. Because smaller modules still have an electrical junction box on the back, you can connect them with normal flexible PVC weatherproof conduit and conduit connectors.
Keep in mind that these solar models will be hardwired “Because you’re using a 12-volt battery system, you’ll want to use “parallel.” This means that each module’s wiring may require its own wire run back to a central roof-mounted combiner box. A solar charge controller is also required, which regulates the rate of battery charging and avoids overcharging. Any solar module should not be connected directly to the battery without a charge controller in between.
If you’re utilising sealed GEL or AGM batteries, a high-quality charge controller with a battery monitor is essential “A switch or jumper labelled “GEL/AGM” changes to a lower charging voltage. The typical wet cell charging voltage setpoint will destroy sealed batteries, thus this will protect your new battery bank.
If you require more than four solar modules, wiring them all in parallel becomes difficult, therefore a charge controller that allows you to use a higher-voltage solar array with a 12-volt battery may be a better option.
This implies you might be able to connect your solar modules in series instead of parallel, resulting in a 24 or 48-volt solar array that can power a 12-volt battery bank. This is entirely dependent on the solar charge controller model you select.
You’ll most likely be installing two to four solar modules with a total capacity of less than 100 watts apiece, although larger systems can benefit from a solar combiner box. This panel functions similarly to a fuse or circuit breaker panel, except it’s designed to be mounted outside next to the solar array. When you have many solar modules, you connect them all to a nearby combiner box with separate terminals to make wiring easier.
Finally, a fuse should be installed between the solar array and the charge controller, as well as between the charge controller and the battery. On systems this small, a two-pole Square D fused-disconnect can be used, with each fuse used separately for each wire, allowing a single disconnect to destroy both charge controller connections.
System grounding
Although your off-grid system will never be subjected to an electrical inspection and the low 12-volt wire will not electrocute anyone, this is no excuse for not employing safe wiring standards, including grounding. All professionally installed roof-mounted solar arrays must have a ground-fault circuit breaker and a separate grounding wire connecting each solar module directly to an earth-ground, according to the National Electric Code.
Because many grid-connected solar arrays output over 400 volts, any electrical short to the frame might electrocute someone or start an electrical fire if it arcs. Due to the much lower voltage, smaller 12-volt DC solar systems like this that are built for boat, RV, or camping applications don’t normally contain this added safety mechanism, but I can’t advocate leaving it out of your solar array wiring. Even if you determine that your small solar array has a low risk of lightning damage, because you are fastening huge metal objects high up on a roof in an open region, it is still necessary to provide a good array ground to lessen the chance of lightning damage.
Every solar module has a predrilled and labelled ground wire hole in the frame, and you should connect each module with a bare solid-copper ground wire. Do not think that simply connecting the ground wire to the array mounting rack will ground the entire set of modules. When aluminium is attached to other metals, it corrodes quickly, resulting in a weak electrical connection.
Connect each module’s grounding point with a stainless-steel sheet-metal screw without severing the bare copper wire, then route down to a standard 8-foot copper-clad 1/2-inch steel groundrod driven next to the cabin’s foundation. A separate bare copper wire from this same groundrod must also be connected to the grounding buss bar within the main circuit breaker panel and array fused disconnect. A #8 or #10 bare-copper ground wire fulfils code for a system this small, but the code also requires ground wires smaller than #6 to be in conduit to protect the smaller wire. It’s frequently quicker to skip the conduit and just utilise the larger #6 wire.
What is the best way to power an off-grid cabin?
Off-grid power is the use of a renewable resource to generate electricity independent of utilities such as the electrical grid. Solar, wind, and micro-hydro are the three main types of off-grid power generation. Unlike the alternating current given by electrical systems around the world, all convert their energy source to direct current power.
The charged electrons in direct current electricity provide power that flows in just one continuous direction. The electrons’ direction is constantly switched in alternating current. This changeover occurs 60 times every second in the United States.
How much solar power does an off-grid cabin require?
The number of solar panels required to go off the grid is purely determined by the following variables:
To rely totally on its own energy output, the average off-grid home requires roughly 7 Kw (or 7000 Watts) of power.
Solar panels come in a variety of shapes, sizes, and designs. The amount of solar panels you’ll need to go off-grid is determined by two primary factors: your energy needs and the performance output of each panel.
- Standard testing conditions (STC) are used to rate panel performance: 1,000 W/m2, AM 1.5 sun spectrum, and 25 C module temperature.
A 100-watt solar panel, for example, is 47 x 21,3 x 14 inches in size. The dimensions of a 200-watt solar panel are 64 x 26 x 14 inches (these are rough estimates).
The larger the framework, the more photovoltaic cells may be installed inside of it, resulting in increased performance.
If your energy needs were the same as the average (7 kW), and you used 200-watt solar panels, you’d need about 35 panels to go off the grid. Alternatively, 20 350-watt solar panels might suffice.
You’ll need to calculate the total square footage to get an idea of how much space 35 solar panels will take up.
- 35 solar panels will take up around 389 square feet of roof space on your property. This gives you plenty of room if you decide to add more panels to your system in the future.
To make things easier for you, we’ve put up this chart that shows you how many solar panels you’ll need based on your situation.
How are you going to power the cabin?
To pay for the installation of electric lines to your cabin, set up a payment plan with your utility company. Depending on the company, you may be able to spread the cost out over several years, with payments applied to your power bill until you’ve paid off the total balance. They may, however, require payment in advance for exceptionally big quantities. Each business will have its own set of rules.
Determine how close your cabin is to a power source. Along the roads leading to the cabin, look for transformers on poles. Inquire of your neighbours about their electricity sources.
Make contact with the local public utility that distributes electricity. They’ll figure out how far away your cottage is from utilities. You will have to pay to have poles planted and lines run for your power if your cabin is more than a certain distance from the power source. The power company may charge you a price for installing power lines on your land, or they may require you to engage an independent contractor.
What generator size do I need to power a cabin?
A portable generator is useful for off-grid or island cabins that require power from time to time to run tools or other small appliances. It’s also a fantastic solution for cottages with electricity that might lose power owing to storms a few of times a year.
The smallest generators are the lightest and easiest to travel, but they also produce the least amount of power. A 1500-watt generator, for example, might power the modem, a few lights, PCs, and possibly the television.
At the very least, you’ll need a generator of 3500 or 4000 watts. This will also power a small refrigerator and the water pump. It’s also powerful enough for tools and equipment used for construction jobs around the property.
Once the units reach this size, or greater, they become heavy and difficult to move about. If your home isn’t completely flat, keep this in mind.
Portable generators typically run on gasoline or diesel fuel. They are noisy and emit a foul odour. It is critical that the generator be operated outside and at the recommended distance from the building.
