Off-grid power 101

Confused by generating power off the grid? Learn the basics in simple terms, including examples setups with links to gear for your needs.

[See the full post at: Off-grid power 101]

  • Comments (33)

    • 9

      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.

      • 4

        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.

      • 2

        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.

      • 5

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

      • 3

        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?

      • 5

        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.

      • 5

        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.

      • 4

        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.

    • 2

      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?

      • 1

        We just published a big review on portable solar chargers: https://theprepared.com/gea&#8230;

        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.

      • 5

        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.

      • 5

        Thank you. I’ll read the other article.

      • 4

        FYI: Link to portable solar charges broken.

    • 7

      A 120V outlet actually bounces between approximately +170V and -170V.  Or 240V peak to peak.

      120V is an RMS rating. That is very inaccurate info. to be giving people. Otherwise, a great article, and very helpful for beginners.

      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.

      • 4

        Brad, you’re correct that 120V is an RMS rating and it’s not going from -120 to +120. I’ll try to think of a way to reframe that that leaves out the RMS stuff, because that’s too much for an intro article like this. Thanks for the feedback and the correction!

    • 5


      I figured you were aware, but just wanted to make sure. I suggest just saying 120V is a form of arithmetic average. That should be easy enough to understand, no?

      • 5


        Yeah, I’m still pondering how to handle that, though. It will become more of an issue when I do the larger battery packs with AC outputs, at which point I’ll probably have to address it.

    • 7

      Big lesson learned. I could not find any tips on this subject, but this will increase the life of your battery bank! Experience on two banks has shown me what to do to make your battery bank last much longer. Took two banks replaced to figure this one out. Ever notice fluid escaping from a cap? This is your electrolytes being spilled out and gone forever. Just topping the cell off only replaces the water, not the acid. Eventually that cell that is losing fluid goes bad first! Why, because the balance on acid and water becomes displaced. The first clue as to what was happening was pressure being released upon opening the cell. The solution in a second. What I found is the vent on the cap was clogged. Which forced out the valuable contents and deteriorated the cell. Which equals dead cell and weaker bank. The solution, I found removing all caps, and first neutralized the acid on the Caps using baking soda and water. Then washing them with soapy water allowed me to blow through the cap and found clogged vents. These clogged caps were the same cells going bad first, and taking a tole on the bank. Next after neutralizing the acid and cleaning the caps I could blow to verify that the cap was indeed clogged. And lo and behold they were the same caps the created the bad cells. So I found using a tiny screwdriver and forcing a path to the vent solved the problem. There were no more spills coming from that cell! And the bank has lasted much, much longer. Again, I could find no topic on this subject, so here you go, extend the life of your bank……

    • 3

      Thanks for this excellent write up. I’m a little confused, however, after reading its recommendation against NiMH in favor of Li-Ion, “Other chemistry types found in removable batteries, such as NiMH, are not good enough to be considered for these needs.” This seems to be in contrast to the recommendations from the “Best rechargable batteries” and “Beginner’s guide to batteries” posts.

      I understand why Li-Ion is better for solar chargers in a BOB (energy density) and it makes sense, but I’m no longer sure what the advantage is to stocking up on rechargable NiMH is, as recommended by the other articles. If I’m not going to be recharging them in a grid-down emergency, wouldn’t I be better off stocking up on disposable Lithium batteries which are better in every category other than rechargability? Or do the NiMHs still have a role, being recharged by solar for AA-family powered devices, albeit not as effectively? Or do I recharge the NiMHs from the Li-Ion?

      • 9

        Thanks for catching this. I think this was just a bad edit that we didn’t catch. The real issue is that lead-acid and Li-Ion are the two chemistries that you can actually find in the sizes and form factors that work for these setups. I’ve changed this sentence to make it correct.

    • 3

      Do y’all have recommendations for brands/models for large power banks for powering space heaters, fans, and air purifiers?

      Renters like me don’t always have the option for solar or gas power generation, so I’m trying to figure out how myself and folks in my situation can power devices like this if/when the power goes out and weather is severe. 

      • 3

        We are actually are currently working on writing an article about solar generators/larger base station power banks so keep an eye out for that in the near future.

        If you need to get a solution sooner rather than later, look up solar generators. These are going to be the large power banks you are looking for, but if you do not wish to hook them up to solar you don’t have to. You can also charge them off of your normal wall AC outlet or your car’s cigarette lighter port. 

        Two of my favorite models so far are the Ecoflow River Pro and Ecoflow Delta which one you get is going to depend on what you want to power and for how long. Jackery and Goal Zero are also two popular and reputable brands.

        But again, keep an eye out for that article where we will go into greater detail breaking down which will be the best model for your needs and lay out all of the options.

        These solar generators/larger power banks are great solutions for the renter, apartment dweller, or someone in an urban situation that wants to keep things low key. I’m sure am grateful to live in the times we do where the technology is emerging where these exist. Our parents wouldn’t have even dreamed of carrying a small little pack that provides clean quiet energy to power all sorts of devices.

      • 2

        Thank you so much, as always, Gideon!!

      • 1

        Hello again! 

        I looked at this Wirecutter review as well as the options you recommended and drew up this comparison table.

        I’m leaning towards the EcoFlow Delta, which will eat through multiple months’ of my preparedness budget, but it’s seemingly the best option for anyone wishing to keep a fan/purifier going in a power outage, so long as you can afford the cost and also carry a 30 pound object.

      • 2

        I just talked this over with my partner, and they recommended combining multiple options. This would have the benefits of redundancy and portability/flexibility (each station could be in different locations).

        So for example, 1x EcoFlow River with 2x Jackery Explorer 160 would yield 1,054 Wh at $929 ($0.86/Wh overall).

        Getting just the EcoFlow Delta would be 1,260 Wh at $1,199 ($0.95/Wh).

        The combo has the aforementioned redundancy/flexibility benefits over the Delta alone, and seems to be a better bang for buck. The most striking tradeoff is in max output wattage (Delta at 1,800 W and “Combo” at “only” 600 W), but the only thing I am currently planning to power is a fan or air purifier, so I think I’m OK here.

        Do you have any concerns with this Combo approach?

      • 2

        That definitely is a good idea and strategy to have multiple smaller units which can be spread out throughout the house instead of running extension cords everywhere. 

        You do mention one of the drawbacks to this strategy with having a lower max wattage available. You will not be able to run a space heater off of either of these units. One of my main usages of a generator during an emergency would be to keep my fridge/freezer running so I don’t spoil thousands of dollars of frozen food storage. Not sure what the wattage requirements of your fridge would be, but that is something to consider.

        If you really only wanted to power a fan or that air purifier, your combo approach could possibly work. That Jackery 160 though is seriously so small that I think you will regret buying it pretty quickly beyond charging your phone from. You will only get 2 hours of runtime of your air purifier off of it. I would only get 300wh+ units, preferably 500+ and a good sweet spot would be that 700wh of the River Pro. Especially if you don’t have a way to recharge the unit by solar.

        What if the power goes out for 2 days, you have 2 hours of runtime of your air purifier with each of those Jackerys and a bit over 9 hours with the River Pro. So you are at 13 hours of running that purifier and nothing else. That could last you during the night, and if you had the solar panels to charge them then you could charge during the day and be ready for another night. But with three units will come three sets of panels required. 

        There definitely are pros and cons to your combo approach and pros and cons to a single unit approach. That’s why the article we are writing is not just going to have one answer for everyone but teach them the principles and what to look out for so they can apply it to their particular situation and energy requirements.

        We did review the Jackery 240 last year, so give that a read and see what you think of that unit. The 240 is going to be an additional 80wh than the 160.

      • 1

        That’s interesting about the freezer / food storage! I personally definitely have less than thousands of dollars of frozen food though, so this isn’t as much of a priority for me. I’ve got 2 weeks of freeze dried food though, so I think I’ll be OK for that length of time.

        I’m also now considering a more energy efficient (35W) air purifier, which will more than double the runtime I get out of any power station I choose (e.g., 4.8 hrs with the Jackery Explorer 160).

        That’s real solid thinking about the longer term angle though. With the EcoFlow River, I’d be *much* better set up: only need one solar panel, and there’s enough capacity in the power station to last me the night! Now leaning in this direction!

      • 3

        How about adding some discussion of the new, big kid on the block-the Inergy line. High capacity, with the ability to expand, by adding more batteries, as you need more power.  It look pretty tempting. Reviews have been generally positive.


      • 2

        Thank you for the suggestion! We will add it to our list of solar generators to research and look into for the review.

    • 2

      Hey Jon! Will you clue us in on these neat vehicles you’ve got? What’ve they got going for them other than the solar panels?


      • 3

        Hi Lowell,

        Thanks for showing interest in these two vehicles, I personally am intrigued myself and wonder what prepping and bug out surprises lie under the hood. Even though Jon wrote the article on Off-grid power, these vehicles belong to another member of the team here named Tom. He is in the middle of working on finishing up our new first aid course right now but I’ll check in with him after he wraps things up with that and see if we can get some spoilers on everything he has going on in this picture.

        The vehicle on the left (according to the name in the corner) is an Extreme UEV 490. Doing a search led me to this neat off road camper. This link will show you some of the cool features it has. 

        If you are interested in these 4×4 trailers, check out John’s review on the Overland Turtleback Expedition. That’s another really neat trailer with lots of compartments and features. It is a bugging out dream as well.

    • 2

      Battery costs have been falling fast, storage is what has been holding me back from PV. I finally decided to jump in and purchased a 30kWh battery bank in LiFePo4 chemistry. I thought this was a good time because the price might not continue to fall with demand ramping up for lithium and such things. I paid 30¢ w/h or $10k for 6 48v 100ah rack mount batteries.

      I had thought to get the batteries and use as a UPS while the surplus of silicone builds and panel prices fall. Read today that LG is quitting the business because of high material cost. LG was the number one rated panel mfg in many reviews I’ve read.

      I think I’m moving ahead on the panels and all-in-one charger/inverter/transfer switch asap. Even if we move soon, I’ll take it with me. My impression is fracking is getting a little long in the tooth already and we’ll likely see it begin to decline in a handful of years—it is all that’s between us and a global supply shortage. The current mess in Europe could well be Russia’s last gasp before her oil production goes into decline.

      Regardless of that, expect lots of short term pain everywhere and even here in the US. Not only do we import from Russia but we now export to the EU as well. We’re working overtime to export the last bit of oil/gas we can scrape up. Expect high fuel and electricity price. Expect shortages in parts and materials even if we don’t get sideways with China too. Demand for renewables and BEV is going to skyrocket behind high fuel price.

      Well, that’s my little bit of doom-mongering for today.

      • 2

        Those LiFePo4’s are very impressive batteries, and getting them at 30 cents w/h is a great deal! I hope to see some pictures in the future as you get things installed and running.

        Also, I want to say good job for analyzing the market and buying when you think is the best time!