The Prepper Journal

Affordable Off-Grid Solar Electric

Editor’s Note: This post is another entry in the Prepper Writing Contest from John D. If you have information for Preppers that you would like to share and possibly win a $300 Amazon Gift Card to purchase your own prepping supplies, enter today.

Would you like to add off-grid solar to your preps, but think it’s too expensive?  I’ll show you how to build an inexpensive system that can grow, as funds become available.  You’ll find your small system very useful, and you’ll look forward to the added capabilities that each upgrade brings.  If you can spare as little as $50 per month, you’ll have a substantial system in less than a year, and one that meets all of your needs for electricity much more quickly than you might imagine.

Before we look at the system itself, let’s consider your electricity usage.  As anyone who’s ever dabbled in off-grid solar will tell you, the first thing to do is to look for ways to reduce electricity consumption.  If you can reduce the amount of electricity you use, you can build a smaller system, saving you money, while still meeting your needs.  Since we’re talking about a small system, you can forget about central heating and cooling, and you won’t be using an electric water heater or range.  Those items require a lot of electricity, and I doubt that you want to cover your entire roof with solar panels.  But, that doesn’t mean you have to do without or be uncomfortable.  As a prepper, I suspect that you’ve already considered alternatives.  You may have a fireplace, with a heat exchanger, that serves as an alternative to your furnace.  Or, maybe you have a wood-burning stove and a good supply of firewood.  An outdoor fire pit can be used for cooking.  Perhaps you’ve considered using the sun to heat water for dishes and for bathing.  And, of course, who doesn’t have an ice chest for keeping food and drinks chilled?

If your alternative system has the capacity to power a refrigerator/freezer, you have the ability to keep food and medicine from spoiling.  However, because it runs 24/7, the overall use of electricity is high.  In the event that your alternative electricity system can’t handle that, consider an energy-efficient chest freezer instead.  A chest freezer uses much less electricity.  Besides keeping food frozen, you can use it to make ice for an ice chest.  Placing it in the coolest part of your home will also help to reduce electricity usage.

Kill A Watt Electricity Usage Monitor

Your alternative source of electricity will also be used for lights.  Since you’ll be using lights for several hours each evening, this is an area where cutting back is very beneficial.  Instead of 60-watt incandescent bulbs, use either compact fluorescent bulbs (CFL) or LED bulbs.  A 13 watt CFL puts out as much light as a 60-watt incandescent bulb.  Even better, a 10 watt LED bulb puts out as much light as a 60-watt incandescent bulb.

An electric frying pan also uses a lot of electricity, but perhaps for only an hour a day.  If using an electric frying pan, or if an electric deep fryer is important to you, then you must build a system large enough to supply that amount of electricity.  A microwave oven also uses a lot of electricity, but items cook much more quickly in a microwave than they do in an electric skillet, meaning that the overall use of electricity when using a microwave oven is much less.

Make a list of all of the items you’ll want to use in a grid-down situation.  Calculate the daily power requirements in watt-hours (watts times hours), for each device.  If you don’t know how many watts a particular device requires, measure it with a Kill-a-Watt meter.  A Kill-a-Watt meter is an inexpensive device, available from Walmart, and many other sources.  For devices that use power intermittently, like a refrigerator, measure the total power used (watt-hours), over a 24 hour period.  Add all of those, to give you a rough idea of your daily electrical needs.  You may come up with a number between 1,500 and 3,000 watt-hours per day.  With that in mind as your ultimate goal, you’re now ready to build the starter system.  Add to it over time, as funds become available until it meets all of your needs.

Affordable Off-Grid Solar Electric - The Prepper Journal
The starter system, as described, can provide 450-watt hours per day or more.

While you could start with just a battery and a solar panel, the basic system described here is a complete system.  Because it’s not tied into existing house wiring, you’ll also need extension cords and power strips.

1 – 100 watt solar panel$113
1 – Battery, deep-cycle, 12v, 100ahlocally from Costco, SAMS or WalMart$86
1 – Battery boxlocally from Costco, SAMS or WalMart$19
1 – Charge Controller, PWM, 30a$50
1 – Inverter, PSW, 300 watts$144
1 – Meter/Switch assembly(DIY):  parts from$18
1 – 40 amp circuit breakerNAWS store$10
Misc wire, connectors, fuses, hardwarevarious sources$20
Total starter system cost: $460

The starter system, as described, can provide 450-watt hours per day or more.  You may be far from your goal (1,500 to 3,000 watt-hours), but it’s a good start.  You’ll be able to charge cell phones and portable electronic devices.  You’ll have an abundance of light, using LED or CFL bulbs.  You might use a table-top fan, power a TV, cable box, game machine, and a wide range of other devices.

I’ve listed a 30 amp charge controller, while a less expensive 10 amp charge controller would be just fine.  I suggested the larger controller so that it doesn’t have to be replaced when installing additional solar panels.  A 30 amp charge controller will handle the current for up to 4 – 100 watt solar panels.

Make sure that the wire you use can handle the expected current, and that all circuits are properly fused.  A single solar panel might produce 6 amps of current at 12 volts, so a 15 amp automotive fuse will work fine.  Fuses, and in-line fuse holders, are available from any auto supply store.  10 gauge wire can handle the solar panel current.  For battery interconnects, and wiring from the battery to the inverter, use 7 gauge or heavier wire, and a 40 amp breaker.  If you can’t find wire that heavy, consider cannibalizing a set of automotive jumper cables.  For best performance, keep wires as short as possible.  For roof-mounted solar panels, ground the frames, and a lightning-protection device is recommended.

The battery box is oversized for the battery you will be using, and it includes a spacer.  This extra space provides plenty of room for the charge controller and fuses.  The battery box is properly vented and should be located outdoors, in close proximity to the solar panel.  The manufacturer is listed as NOCO, and it’s called a Snap-Top HM318BK Group 24-31 Battery Box.

For roof mounted solar panels, ground the frames, and a lightning-protection device is recommended.

The Meter/Switch assembly is optional but highly recommended.  The meter displays either the battery voltage or the output from the solar panel(s), selectable via a two-position switch.  This allows you to get the most out of the system and tells you what you need to know in order to protect the battery.  The parts can be ordered from  Part numbers are 2152323, 2135857, and 675489, for the meter, switch, and enclosure.  You’ll also need some 3 conductor wire.

Knowing the battery state of charge (SOC) at any particular time is important for two reasons.

  1. To make sure that the battery is fully charged, each sunny day.
  2. To avoid over-discharging the battery.

Chronically undercharging, or over-discharging the battery can shorten its life.  Unfortunately, measuring the battery SOC is not a straightforward process.  However, after observing the system over time, and with a little practice, it’s not difficult.  First of all, what we consider a 12-volt battery is not really a 12-volt battery.  It’s a 12.7-volt battery.  When a battery is neither charging nor discharging, it is considered to be “at rest”.  With that in mind, here’s what to look for:

  1. When the solar panel voltage is significantly higher than the battery voltage, it’s an indication that the charge controller is doing its job, and the battery is fully, or nearly fully, charged.
  2. Expect the fully charged battery reading to be 12.6 to 12.7 volts, while at rest.
  3. When the at-rest voltage drops below 12.1 volts, the battery is considered to be about 50% discharged.
  4. While charging the battery, and for a period of time soon thereafter, expect to see a battery voltage reading far above the “normal”, fully charged reading, perhaps in excess of 13.0 volts.  This is normal.  It’s called “surface charge”.  However, it makes determining the actual state of charge a bit more difficult.  You can “burn it off” by applying a load for a short time, or simply wait until it dissipates.
  5. The load (devices drawing power from the battery), affects the voltage reading.  Determine the SOC when no load is connected, and when the battery has been at rest for 1 hour or more.

Copy this chart, and place it near the meter:

Battery VoltageLoadApproximate SOCComment
12.8 or higherNoneunknownBattery is charging
12.6 to 12.7None100% 
12.1None50%Do not connect load

For best results, battery should be at rest for 1 hour before taking a voltage reading.  Readings are taken at night when solar panels are not generating power.

Your results may be different than those listed above because your batteries may be different than the ones I use, and the temperature has an effect on battery voltage.  Until you’re comfortable with determining battery SOC by reading the meter, it’s best to estimate SOC based on usage, as explained below.

If you start with a budget of $50.00 per month, you’ll have a complete, and substantial system in about 9 months, and a more powerful system in about 14 months.

Another way to determine approximate battery SOC is by monitoring the load.  With one 100ah battery, the expected usable storage capacity is about 450-watt hours.  (More about determining battery capacity later in this article).  If you add all of your loads, multiplied by the hours of use, you can determine how much of that 450-watt hour capacity you’ve used.  For example, if your load includes only a 35-watt fan for 12 hours, and a 10 watt light bulb for 4 hours, the total drain on the battery is:  (35 x 12 = 420 and 10 x 4 = 40) or (420 + 40 = 460), exceeding the recommended shut-down limit by 10-watt hours.  Until you’re very comfortable using the meter to determine battery SOC, it’s best to use this method instead.

Is this system something you would consider building?

If you start with a budget of $50.00 per month, you’ll have a complete, and substantial system in about 9 months, and a more powerful system in about 14 months.  This system will power AC-operated devices, up to a maximum of 300 watts.  The AC will be “clean” power, allowing you to run devices without performance problems, and without the fear of damage to sensitive devices.  With a budget of only $50.00 per month, you could have a system that meets all of your needs (1,500 to 3,000 watt hours per day), in less than three years.

Here’s an upgrade suggestion:

1 – 100 watt solar panel$113
1 – Battery, 12v, 100ah$86
1 – Battery box$9
Wire, connectors, fuses, hardware, and misc items$22
Upgrade cost:$230

With this upgrade, system capacity is doubled.  You’ll have 900-watt hours of power available after a full day of sunshine.  While you still may not have met your goal, your system will have the capacity to run additional devices, and for longer periods of time.  Partly cloudy days will have less of a negative impact.

By adding solar panels and batteries, you’ll be able to use more devices, and increase runs time.  If, for example, you’ve used half of your available power in the evening for lights and TV, you still may be able to run a fan for 8 hours, while you sleep.

Also, at some point, you may want to add a more powerful DC to AC pure sine wave (PSW) inverter.  If you add a larger PSW inverter, you’ll be able to run devices that require more power, like a microwave oven, toaster, or vacuum cleaner.  I’m very pleased with the Exeltech 1100 watt PSW inverter I’ve been using for the past several years.  I paid about $570.00 for it.  However, I’ve seen high-power DC to AC PSW inverters advertised as low as $200.00.  Before considering an expensive inverter, read the reviews.


So there you have it.  In just over a year, on a budget of $50.00 a month, you could have a robust system that meets most of your needs.  In less than two years, you could have an even more robust system that meets all of your needs.  You’ll have plenty of light, you can keep frozen items frozen, you’ll have cold drinks, you can cook food and boil water, circulate warm or cold air, take a warm shower, watch TV or listen to the radio, use a computer or mobile computing device, use power tools, pump water, and run a vacuum cleaner.  In other words, you can power almost anything.  Until the next power outage, or before the SHTF, you can use your system daily, cutting your use of grid-supplied electricity, saving you money.  After all, it’s not like using a generator, where you’ll have to buy gasoline.  Solar electricity is free power from the sun!

The system capabilities listed here are estimates, but representative of the results you can expect.  Short winter days and extended cloud cover are your worst enemy.  You can either put up with that, cutting back on electricity usage when necessary, or build a system large enough to compensate for those things.  Until you can reliably power every device on your list, you’ll probably want to keep adding solar panels and batteries.  You can use the formulas I’ve provided, or simply test the capacity of the system, once built, and each time you upgrade it.

You may be asking yourself if you really need this.  Think about what life will be like when your food stockpile is depleted.  Out of necessity, you might have to spend most of your time finding food?  Having a convenient way to process it (canning), and store it (freezing), will make post-SHTF life much easier.  For me, this means having the ability to cook, without needing an open fire, and to have reliable power for a chest freezer.  Assuming that I’ll have some success hunting, trapping, fishing, and growing crops, I need not find food every day.  I’ll have ample time to take care of my other needs and to rest.  I plan for comfort, not just survival.

As a backup to the backup system:

Although I already have a robust solar electric system, I’ve decided to build a smaller one, as described here, for my own use.  After I’ve completed the system, and thoroughly tested it, I’ll pack it up and store it.  Having a portable system, packed and stored, offers several advantages:

  1. It will be stored in a Faraday Cage, eliminating the chance of damage due to an EMP. My existing solar electric system is exposed, and might be damaged by an EMP. If that happens, this system can replace my EMP-damaged system.
  2. In the event that my existing system is not damaged, this system can supplement my existing system.For any grid power outage, or when the SHTF, more power means more comfort.
  3. In the event that I need to bug out quickly, this system will be packed and ready to go.

Because this system will be in storage at some point, I’ll keep the battery charged, using my existing system and a battery selector switch.

Cutting your electric bill:

You can power household devices for several hours each day, but be careful not to over-discharge the battery.  That process can be automated, by using an automatic transfer switch.  After building my own system, I’ll be building another one for a family member who lives in an area where electric rates are high.

To calculate the power generating capacity of a solar panel array (example):

If you have 2 – 100 watt solar panels, and you expect 5 hours of sun each day, you have the capacity for (5 times 200), or 1000 watt-hours per day.  This power can be delivered directly to the load or used to charge batteries.  You might have less, on some days, due to clouds, or because of shading from trees.  You’ll get an efficiency bump if you use power directly from the solar panels, during the day, instead of using power from the batteries at night.

For optimum performance, keep the panels clean, and at the proper angle for best exposure to the sun.  For mounted panels, this might include adjusting the angle twice a year, for summer and winter exposure.  Personally, mine are mounted on a second-story roof, and I don’t adjust for the seasons.  I would adjust, given a SHTF situation, if I needed to get the most from an undersized array.

About batteries:

For batteries, the best value is the lowest cost per amp hour.  For example, A 200 amp hour battery at $160.00 is a better value than 2 – 100 amp hour batteries at $90.00 each.  Make sure that you select deep-cycle batteries, not automotive starting batteries.

A 12 volt, 100 ah battery can theoretically supply 1,200-watt hours to the load.  (Watts = Volts times Amps).  In practice though, you should not discharge the battery more than 50%.  Therefore the usable watt-hour capacity is 600.  However, there are discharging inefficiencies, and the rate of discharge affects performance.  As a result, the actual power that this battery can provide may be 20% less than the calculated value, or 480-watt hours.  Battery manufacturer ratings tend to be optimistic.  They may be accurate under ideal conditions, but you and I will never achieve that level of performance.

In my effort to get the most bang for the buck, I opted for 6-volt batteries in my system.  They’re designed for electric golf carts and floor machines.  They’re rated at 215 amp-hours.  By connecting 2 of them in series, the result is 12 volts at 215 amp hours, per pair.  Here are my calculations for this battery pair:

12 volts at 215 amp hours (12 X 215) = 2,580 watt-hours
50% usable (2,580 divided by 2) = 1,290 watt-hours
Accounting for losses (1,290 x 80%) = 1,032 usable watt-hours

This suggests that I can:

Power a 100-watt load for a little more than 10 hours (100 X 10) = 1,000 watt-hours

Power a 43 watt load for 24 hours (43 X 24) = 1,032 watt-hours

In practice, I’ll tend to use the most electricity in the evening hours, and less while I’m sleeping and during the day.

As you perform upgrades to your system, aim for more battery capacity than solar panel capacity.  Batteries last longer and operate more efficiently if you don’t discharge them too deeply.  And, with additional battery capacity, you’re less likely to have a shortage in the event of cloudy conditions.

If your battery is the wet-cell type, check water level once in a while, and add distilled water as necessary.  Avoid spills, and don’t let battery water come in contact with skin or clothing.  Keep a container of baking soda handy, to neutralize battery acid, in case of a spill.

Other things to consider:

As an alternative to the pure sine wave (PSW) inverter, you may have considered a less expensive modified sine wave (MSW) inverter.  Don’t do it!  MSW inverters may damage sensitive devices.  It’s also likely that a noticeable buzz will be heard in audio devices, such as radios.  Motors may run at the wrong speed or overheat.

There are a great many things to consider in the design of an off-grid solar electric system.  I’ve tried to get the most bang for the buck, while not ignoring safety.  You might have noticed that a pre-packaged system costs many times more than the one I’ve described here.  Because you’ll be making your own cables, assembling all of the components, and not buying things you don’t need, you’ll save a small fortune.

I’ll be happy to answer questions, via comments to this article.  Good Luck with your system!

John D

Here’s a link to a video that you might find helpful:

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