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Off-grid power 101

Do you know how to power your electrical devices without the grid? This guide explains the basics in simple terms, including what you can (and can’t) realistically do and what gear you need to make your own electricity for specific tasks — such as recharging a phone or flashlight without using a wall socket after a natural disaster.

Figuring out how to keep your gear running without the grid can be confusing. Just understanding the terminology is difficult enough, and after you figure out the basics, you still have to piece together a system that meets your specific needs and budget.

Good news: You don’t need an advanced degree. Just a few simple definitions and a lay of the land will guide your power preparedness.

Summary:

  • Think of the products in these markets in three buckets: portable (hiking), semi-portable (driving to basecamp), and fixed (roof installations).
  • The simplest off-grid power solution is just a high-capacity battery that you charge beforehand.
  • Even when things get more complicated than that, a battery will almost always be a part of the chain (even if it’s built into the device you want to use), acting both as storage and to smooth out wobbly supply from a generator.
  • So when you’re putting together your off-grid kit you should figure out your battery needs first, then build from there.
  • Always buy the biggest battery your budget and mobility needs will accommodate, because bigger batteries drain more slowly (even when the device draw is the same).
  • The energy that goes into a battery has to come from somewhere. If not from the grid, then you have to convert your own solar or rotational energy into electrical current.
  • Solar is the best renewable power supply, winning across every metric like cost, efficiency, and convenience.
  • Technology is rapidly improving in all fields, but wind-, water-, and crank-powered devices just aren’t good enough yet for most personal needs — the exceptions are crank-powered lights and radios in a backup role.
  • Gas-powered generators are another option, obviously limited by portability and the amount of fuel available.
  • You don’t need to worry about advanced gear and concepts like inverters, charge controllers, or AC vs. DC until you get into larger installations (eg. a house roof).

Putting the right gear together for your needs is generally broken up by the type of devices you need to power (e.g. your phone or space heater) and how portable you want to be (home vs. vehicle vs. go bags).

Devices and the gear needed to power them tend to stay in the same portability category. A cell phone is portable, so it can generally pair with a portable battery and generator. A space heater is technically portable, but you’d just carry it around the house or in your car to a basecamp, so it pairs with semi-portable power. Your full-sized kitchen fridge is fixed, so it needs a large fixed solar and battery installation in your home.

Products tend to stay in those lanes due to the nature of how electricity works. A phone, for example, already has a built-in battery, can charge via a simple USB cable and DC power, and doesn’t require a ton of juice to run. On the other hand, your laundry machine requires a lot of steady AC current and doesn’t have its own battery.

More:

Why you can trust me

In addition to an electrical engineering degree, I’ve written a book on an engineering-related topic and have spent years reviewing batteries, solar panels, chargers, and related gear, both in a general context and specifically for preparedness.

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Example setups
Clockwise from top left: Pixel 3 phone, Black Diamond ReVolt headlamp, USB cable, portable Anker Li-Ion power pack.

For powering small USB gadgets, like mobile phones and headlamps, compact lithium-ion battery banks can hold most of their power for weeks (especially if kept cool). This is the easiest and lightest way to recharge your handheld devices 1-3 times. The downside is you are limited to the power already stored in the battery pack.

Portable Goal Zero AA (x4) battery charger / power pack, phone, and USB cable.

There are a few products that can both recharge removable AA/AAA batteries via a generator and use those batteries as a power bank to charge external USB devices (as shown). Charging removable batteries is key for devices that don’t have their own internal batteries.

This method works great as a backup — especially since a good prep kit has quality AA-family batteries in it, anyway, and you’re likely to find more in an emergency — but should not be your primary choice because AA-types don’t perform as well as Li-Ion packs for energy density. In practice, that means you get 50% less energy for 50% more weight. That may not be enough to complete a full charge cycle on beefier phones. So you’d rather recharge your devices from a Li-Ion pack, if possible.

Portable Goal Zero solar panel, Anker battery pack, ReVolt headlamp, USB cables.

Broadly speaking, the above popular power setup can keep your core portable devices (phone, light, radio, etc.) charged indefinitely. The portable solar panel can charge removable batteries (like the previous AAx4 pack) or Li-Ion batteries. You can also charge devices with built-in batteries (like this headlamp) directly from the panel. Since you should try to charge external power packs first, then power your devices from that pack, you can get by with one cable if needed and all the devices use the same type (almost everything is USB-B or -C these days).

Clockwise from top: Goal Zero semi-portable solar panel, GZ semi-portable battery, a home blood pressure monitor (plugged into battery socket), headlamp and phone for scale.

This semi-portable setup can be moved by hand over short distances around the house or to/from a car. It can certainly charge the smaller batteries, battery packs, and devices. But it shines for larger batteries used to power medical devices (shown), small heaters and fridges, car jump starters, power tools, ham radio base stations, etc.

It won’t keep your whole household running indefinitely, but can keep the important stuff running when it counts.

Fixed solar installed on our overlanding trailer (left) and truck roof. Not shown: Inverter, charge controller, batteries (inside the vehicles).

Some bug out vehicles, RVs, boats, etc. have fixed installations for constant power, with panels on the roof and charge controllers, inverters, and batteries in a trunk/compartment. One of our personal trucks, shown, uses this setup to power multiple tablets, lights, and a mobile refrigerator to keep medical supplies cool. The gas engine can also support the batteries if there’s a few cloudy days in a row.

Semi-portable gas generator powering a house after a disaster. Not shown: The equipment needed to connect the generator to the home.

Some buildings are hard-wired to automatically switch over to a fixed or semi-portable generator and battery setup when the main grid goes down. The generator power seamlessly flows through the normal building circuits. This is a popular middle-ground choice for people who don’t want full-time solar but need to keep things running in an emergency. Assuming the generator is sized appropriately, your only limit is gas.

Fixed solar panels. Not shown: Lead-acid batteries, charge controller, inverter, and other equipment needed to connect the panels to the home/grid.

Buildings that are primarily or permanently off-grid have a fixed installation of large batteries and generators (solar panels, wind turbines, gas generators) plus extra components like an inverter and charge controller. These setups can get more complex if you wish to sell extra energy back into the grid, have extra backup systems, and so on.

Common terms and AC vs. DC

Batteries store energy. Generators create energy.

Direct current (DC) is a natural, simple flow of electricity in one direction, like water from a hose. Many portable devices are powered directly by DC from a battery or a wall adapter. DC’s main drawback is that it loses power as it’s transmitted over longer cables, which is why charging cables are always shorter than you want them to be.

Alternating current (AC) is what comes from your wall outlet. Unlike DC power, AC rapidly alternates back and forth between positive and negative voltage, so a 120 V outlet (the standard in US homes) actually bounces between +120 and -120 volts with a frequency of 60Hz.

Inverters turn simple DC into complex AC. More robust products, like a mini fridge, require the kind of AC current that comes out of your wall socket. A DC battery itself won’t power those products, requiring an inverter in between. Although AC solves the line-length problem with DC, you do lose some power through the conversion.

Charge controllers are like babysitters for a battery. There are lots of different battery chemistries out there, each with their own quirks about how they like to be charged. Yet many generators are “dumb”, so they don’t know when to turn off the supply. The charging controller prevents the battery from overcharging, overheating, etc. Portable devices often have a built-in controller, while larger fixed installs often have an external one.

Volts are a measure of electrical potential, but don’t worry about really grasping this abstract concept. Instead, you should think of a voltage rating for an adapter, battery, or product — 1.5V, 5V, 12V, 120V, etc. — as a sort of “compatibility rating.” You plug 5V devices into 5V outlets, 12V devices into 12V outlets, and so on. But you do not plug a 12V device into a 5V or 120V outlet, because it simply won’t work. So the voltage number is a label that says, “this is what kind of outlet this is,” or “this is what kind of outlet this device expects to be plugged into,” or “this is what kind of battery this gadget takes.” You have to match up the voltages or the device won’t work.

Amps are a measure of current flow, and unlike voltage they can vary quite a bit from what a device, charger, or outlet is actually rated for. If you plug a tablet that wants 3 amps into a 1.5-amp charger, it’ll just draw 1.5 amps because that’s the maximum the charger can deliver. If you plug a phone that wants 1.5 amps into a 3-amp charger, it’ll just draw 1.5 amps because that’s the maximum the phone can ask for. It’s important to know that outlets do not push amps to devices — rather, devices pull amps from outlets.

Watts are a measure of power or energy, and they’re calculated by multiplying volts times amps. Whenever we use electricity to do work, watts are what we’re really using. Voltage and amperage numbers are more about compatibility and capability, while wattage numbers tell you how much real, working energy a charger can put out or a device can consume.

Ways to create electricity (and why solar is king)

Whether it’s solar, wind, crank, gas, water, or nuclear, most of our usable electricity is generated by one of two methods: solar energy or rotational energy.

Nuclear and coal power plants work by heating water into steam, which then pushes the turbines around in an infinite circle. Hydro dams use the natural force of the water current to spin the turbines. Wind turbines use wind. And so on. Even your car engine works the same way, turning the gas-explosion-powered mechanical piston rotation into electricity.

The basic reason: As magnets and wires spin past each other, an electrical current forms in the wires which can be sent off to be used. Every time you use your hand to turn a crank-powered device, you’re spinning magnets and generating a few electrons with each cycle.

These mechanical devices can work fine at large scale, but they become less useful when shrunk down to something for your home or smaller. A hand crank is generally only acceptable for low-power devices in a backup role — namely lights or radios. Gas-powered generators tend to be the only meaningful non-solar option for homes and large devices, although wind is (hopefully) catching up.

Solar doesn’t use rotational energy, instead using a chemical reaction to turn sun beams into a stream of electrons. Solar has become the king of renewable energy because:

  • It’s convenient, can be integrated into other devices, made into flat and foldable panels, etc.
  • Huge amounts of solar radiation hit the ground every day.
  • You don’t need any additional fuel (gas, water, calories spent on cranking).
  • Solar technology has advanced to the point where the value is much greater than the cost.
Batteries are the foundation

The simplest way to power a device without the grid is to have that energy already stored. A small battery rig can recharge simple devices like a phone or headlamp a few times before the energy is drained.

Depending on the device you need to power, that might be in the form of high-quality removable batteries (AA, CR123A, etc.), Li-Ion “battery packs”, or something more robust installed into your home.

Reviews: Removable rechargeable batteries and battery chargers

Batteries also form the foundation of any setup more complicated than that simple “plug your phone into a battery” model. Even when you think you’re powering a device directly — like your phone — you’re often powering the internal battery which then powers the device.

The battery is central to all off-grid power because it acts as a reservoir that smooths out the unsteady flow of electricity from constantly shifting natural sources like sun and wind. These fluctuating renewable energy sources fill the battery with charge in fits and starts, and then that charged battery can power its attached devices with a steady, predictable stream of electrical current at a constant voltage.

Charge standalone batteries before charging your devices

Best practice is to use your generator to charge a battery, then use that battery to charge your devices. Besides the smoother energy flow, this means you can charge your device whenever it suits you. That’s strategically better in an emergency so that your gear isn’t out of commission at the worst times, you’re not limited by sun and weather schedules, you can use that power across multiple devices, and so on.

In any case, you want to keep batteries cool and out of the direct sun. Heat is murder on batteries, so if you’re going to wreck or explode a battery by baking it in the sun, then it’s better to ruin an external replaceable battery than one that’s built-in to your e-reader, phone, or tablet.

Only when your standalone batteries are charged should you consider plugging other devices with built-in batteries directly into the generator.

Off-grid battery chemistries: Li-Ion and lead-acid

There are two basic rechargeable battery chemistries in off-grid power: lithium-ion and lead-acid.

Choosing between the two depends on your budget, the amount of electricity you need to store, and the level of portability you’re looking for. Li-Ion tends to be come in smaller sizes, whereas lead-acid is always on the larger side and is a lot cheaper for storing larger amounts of electricity.

Other chemistry types found in removable batteries, such as NiMH, are not good enough to be considered for these needs.

More: The differences between common battery chemistries

Lead-acid is the type of chemistry found in traditional car batteries. It’s relatively cheap and can be used and abused in ways the more delicate Li-Ion chemistry cannot. But lead-acid batteries are a lot heavier per Watt-hour of capacity, so this chemistry is mostly used for fixed or vehicle-based setups.

This isn’t the place to go into the different lead-acid battery options, but it’s worth knowing a few related terms and concepts so that you can better make sense of online discussions of solar installations:

Deep-cycle batteries: Most batteries, and this includes the lead-acid batteries used in car starters, are not meant to be fully drained and then refilled. But large solar installations — where the battery bank may be drained at night and then refilled during the day — need batteries that can support this kind of “deep cycling,” as it’s called. Hence the market for deep-cycle batteries that are appropriate for use with solar.

Forklift and marine batteries: The batteries commonly used in forklifts or boating applications are also popular for off-grid use, because they are relatively inexpensive deep-cycle batteries.

AGM: This acronym comes up a lot in solar battery talk. It stands for “absorbent glass mat.” It’s a type of lead-acid battery with better weight, reliability, and charging efficiency than other types of lead-acid batteries. Not all AGM batteries are deep-cycle, but deep-cycle AGMs are popular for home solar installations.

Making the most of your off-grid battery capacity

Remember:

  • The faster you drain a battery from 100% to 0%, the less of its rated capacity you’ll get and the more of its stored electricity you’ll waste. You only get the rated storage capacity out of a battery if you drain it slowly enough.
  • Which means it’s easier to get the rated capacity out of a bigger battery because you can drain it more slowly at a fixed power level.
  • When comparing different battery capacity ratings, strongly prefer Watt-hours (Wh) to milliamp-hours (mAh) to keep things apples-to-apples.

Battery capacities are usually described in either milliamp-hours (mAh) or watt-hours (Wh). For example, many smaller devices, including phones and Li-Ion power packs, are usually labeled in the 2,000-4,000 mAh range.

mAh is not a great comparison number because it depends on device voltage. It’s a lot like when you’re shopping for frozen food and you see that a meal takes “10 minutes to cook in the oven” — that’s 10 minutes at a specific temperature. Comparing the marketed cook times of two meals doesn’t matter if you don’t know the stated temps.

In contrast, a watt-hour rating is exactly what it sounds like: it tells you how big of an electrical load, measured in watts, a battery can supply for one hour before running out of charge. So a 100 Wh battery can (in theory, but not in practice!) power a 100-watt light bulb for exactly one hour before it runs totally dry.

The reality of battery chemistry, though, is that the faster you drain a battery the less of its stored energy you can actually get out of it. For example, if you drain a 500 watt-hour battery by plugging a 500-watt desktop computer into it, you won’t actually get a full hour of 500-watt output from it — you’ll get a fraction of that amount of time. But if you drain it over the course of 10 hours by plugging a 50-watt laptop into it, you’ll get most of that 10 hours out of it.

What all of this means in the real world, is that if you want to power a 500W computer for one hour, then you had better buy a 1000-Wh or larger battery so that the battery drains more slowly relative to its size.

Comparing milliamp-hours vs. watt-hours

Fortunately, a basic, usable understanding of the relationship between watt-hours and milliamp-hours only requires a tiny bit of math — it’s just multiplication, and you don’t even have to truly understand the underlying terms.

When you’re looking at a battery pack and you see a mAh rating, what you really need to know is what voltage the internal battery operates at. You plug that voltage into the following formula to get the watt-hours:

(milliamp-hours x volts) / 1000 = watt-hours

So to make the numbers easy, if a 5,000 mAh battery operates at 5V, then you can calculate the approximate watt-hours as follows:

(5,000 mAh x 5 volts) / 1000 = 25 watt-hours

In general, though, it’s best to get a Wh rating from the manufacturer, so that you don’t have to untangle which voltage they were working with when they assigned a mAh rating.

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  • Avatar
    int19h

    The vast majority of USB power banks on the market give mAh ratings that have nothing to do with the voltage of either input or output ports. Instead, they use 3.7 V – i.e. the nominal voltage of the Li-Ion battery inside, before it gets converted to 5V for USB etc. The reason is simply that it allows them to use larger numbers.

    So, a 5,000 mAh battery with a USB port on it is not 25 Wh, but rather 18.5 Wh.

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    • Jon Stokes
      Jon StokesThe Prepared int19h

      Thanks for the feedback. That section was just supposed to help people get a ballpark number without having to know such details like internal battery voltages, so I tweaked it to make it a little more clear what’s going on.

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    • Avatar
      int19h int19h

      The point here is that the notion that voltage of output ports is what matters is fundamentally wrong – nobody measures it like that, so it’s not good even as a ballpark measure. If you go with this method for USB power banks, you are overestimating the ideal capacity by about 30% – and that’s before you account for conversion losses etc.

      And then you have the ones that can produce very different output voltages – especially common in larger power banks and the so-called “solar generators”, but increasingly common in general with the adoption of USB QC and USB-C with its variable voltage. These days, you can see power banks that can do 5V, 9V, 12V, 16V or 20V on a single port. And bigger ones sometimes also have an AC socket at 110V. How would you even apply that method to such a thing?

      The nominal battery voltage for Li-Ion cells is always 3.7 V, and minor variations of that is what’s used for pretty much all consumer battery packs, so it should just be assumed when it’s not specified. Anyone using anything different will specify it (and usually also give Wh numbers). So the ballpark for mAh -> Wh conversion is to multiply by 3.7.

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    • Jon Stokes
      Jon StokesThe Prepared int19h

      The way you compute Wh from mAh with an AC socket is to multiple the RMS voltage by the amps.

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    • Avatar
      int19h int19h

      Also, in the updated verbiage, you cite Goal Zero Sherpa as an example. If Sherpa actually outputted 3.89 V, as you claim, it couldn’t charge anything, because most USB devices won’t charge below 4.5 V in practice. The reason is that a fully charged Li-Ion battery goes to ~4.2 V (and a fully discharged is ~3.2 V – 3.7 V nominal is the average between these two), so you need voltage higher than that – after accounting for losses in the cable and the connector – to charge it.

      Of course, this has nothing to do with voltage output. The internal nominal battery voltage of Sherpa is 3.79 V, if you divide their mAh rating by Wh, but like any other cell-powered USB power bank, it then upconverts that voltage to ~5 V that USB requires.

      And yet you specifically claim that “most of the USB battery packs and solar panels we’ve tested in the lab also meter at just under 4V at their USB ports”. Would you care to share more details about said testing – which USB battery packs and solar panels (even just a few more names), what voltage meters have you used etc?

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    • Jon Stokes
      Jon StokesThe Prepared int19h

      I used the AVHzY CT-2 multimeter, recording a sample every 10s, and very few of the USB solar chargers we’ve tested can even get up to 4V in full sunlight, much less sustain it. If it were as you say, and USB devices won’t charge below 4.5V in practice, then very few of the panels we tested would actually charge anything at all. So I don’t believe this is correct.

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    • Avatar
      int19h int19h

      Even just going by the USB specification, the lowest that it allows is 4.0, and that’s in USB 3.0 – earlier versions have higher requirements. If I remember correctly, 4.5 V is USB 2.0 – and that’s the requirement for the device to operate, not charge! Charging is fundamentally limited by battery voltage, unless the device itself upconverts the voltage it receives – but why would it do that, if the spec is intentionally designed to give it voltage high enough to charge?

      Now, with cheap USB portable solar chargers specifically, I can believe that they are undervolted – this would mirror my personal experience with them being basically useless in anything other than direct sunlight, and weak even then. I have one sitting outside right now, actually, for almost a week straight – and it can’t get past 40% charge on the indicator. If a fully discharged Li-Ion is 3 V, and a fully charged one is 4.2 V, then 40% would be at about 3.48 V. A charger that’s outputting around 3.8 V could realistically get it to that point, accounting for cable and connector losses.

      You should try this experiment with those chargers that you’ve measured – take a few fully discharged devices, and see how much they can get charged before they flatline. If a charger can only get something to 30-40% even in the best conditions, that’s an important thing to know, IMO.

      However, this is for solar chargers, and I presume the reason is because they don’t bother with a stepping circuit. USB power banks should never do this. What you should see when measuring voltage is mostly steady, gradually decreasing output within the allowed limits, until the very end, when the internal battery voltage is so low that the stepping circuit can’t do anything about it – then the output starts falling very suddenly and rapidly.

      This might also depend on the current. I know that cheaper ones are often undervolted under load – so they advertise 5V/2A, but in reality it’s more like 5V or 2A. This is why some are unable to charge powerful smartphones at all – the smartphone negotiates the highest current that it can get, and then draws that, but the voltage isn’t high enough for it to use that.

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    • Avatar
      Nic Bryan int19h

      I’ve actually run a few of these tests myself. And yes, I get <4VDC out of a usb port on the charger. But not from the 5V pin (Which actually outputs ~7VDC) the <4VDC is the signal pin, which tells the phone it is plugged into an authorized charger. I’ve got 5 different portable battery packs for my phone. 1 is solar, but the panel on it is maybe 1W output.

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  • Avatar
    Sandra

    We are looking for solar power we can take with us if needed. There is so much information out there so we are a little confused. Have you ever heard of (Smart Solar Box)? I’m a little skeptical but intrigued after watching the video. Does anyone else have this power source? It seems easy and that’s what makes me skeptical. Any advice?

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    • John Ramey
      John RameyThe Prepared Sandra

      We just published a big review on portable solar chargers: https://theprepared.com/gea

      The box you linked (I removed the link) is not really portable. A few people have asked us about that box since posting this review, but it smells to us like there are some affiliate spam games going on. When combined with the weirdness around the product/company, we’re not likely to recommend the product, but will consider it when we look at semi-portable generators.

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    • Avatar
      Sandra Sandra

      Thanks. If you ever do review it, please respond to me. We are in an urban environment so bugging out is less likely and we may have to stay put but want power. Thanks again.

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    • Avatar
      Sandra Sandra

      Thank you. I’ll read the other article.

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