The best solar phone charger (solar panel and battery pack tested)

If you want to use solar energy to keep your smartphone or portable electronics charged, you can find hundreds of choices on Amazon. We took a close look at a few of them in an earlier article.

While some of them do a pretty good job, it’s also true that most of them don’t work nearly as well as most people expect. If a product is advertised as a solar phone charger, you generally expect it to actually charge your phone on a sunny day.

Unfortunately, many of the products on the market fail to do just that.

There are a few reasons. One is that these devices have too few solar cells, often advertising 5 watts or less of solar generation. 5 watts is equivalent to the small USB chargers that are included with most smartphones. If they actually achieved that power output, these products would do a decent job.

But solar cells only generate their rated power output under bright, clear skies and cold temperatures, which small solar phone cases will rarely do.

The worst type of solar phone case looks like the one above: a cheap case with just one small layer of solar cells on the rear. You’re meant to place your phone face-down in sunlight to charge your phone. In addition to having too few cells this also means that, unless you’re outdoors at noon, the solar cells won’t be perpendicular to the sun, causing the power output to suffer.

With a product like this, it might take days of bright sun to fully charge a dead phone. Even if you only want a trickle charger, that’s not very useful.

If you want to charge your phone with solar power, get two products instead

There all-in-one products that pair a battery with larger solar arrays that work quite well, but after taking a close look at the available options I think the best solution for most people is to buy two separate products: a good portable solar panel, and a battery that fits your needs.

With this approach, you can select a solar panel large enough to fit your particular power requirements, and also choose a battery that has the features you need. For example, you might want a battery with Qi wireless charging, which is a feature not found in cheaper products.

Why is it a good idea to use a battery?

While you can use a solar panel to charge your phone directly - and you will gain some efficiency by doing this because using a solar panel to charge a battery results in some wasted electricity - it’s often more practical to charge a battery instead. This frees you up to use your phone and lets you use your stored solar electricity when you need it.

You will also discover that your phone has a minimum power requirement before it will start accepting a charge. This means that if your solar panel isn’t outputting enough power, your phone will refuse to charge. A battery usually doesn’t have this limitation and can be charged in lower light conditions that your phone can’t.

Another benefit is that you can find batteries nearly anywhere: gas stations, dollar stores, and pharmacies carry cheap models that can often be had for less than $10. These cheap models don’t carry much power and might only charge your phone once, but you only need to spend a little more money to upgrade to one higher power capacity and more features.

My test setup: 10 watt Renogy and 10,000 mAh battery pack

For this review, I’m testing two products:

• 10 watt Renogy foldable solar panel
• Aukey 10,000 mAh battery with wireless charging

This model of the Aukey battery pack isn’t available anymore, but there are many similar products on the market. I would recommend anything made by Anker, such as their 10,000 mAh battery with wireless charging that is very similar to the one I’m using here.

I’m also using a cheap $8.59 USB multimeter to measure the current generated by the solar panel. This multimeter simply plugins into a USB port and measures the current flowing through it.

Everything here uses USB ports: the solar panel has a USB power outlet into which I can plug the multimeter, and then a USB cable connects to my battery. Here’s what it looks like all connected together:

How much power do you need to charge a phone?

Before I tested the solar panel, I wanted to verify my test setup by using the multimeter to measure the power output of a basic USB charger. If your phone came with a charger, it was probably a cheap 5 watt model. This is also the kind you find at the checkout aisle of any store. You probably have several of these around the house.

Plugging the multimeter into an Apple 5 watt charger, I measured the output as 4.8 watts (0.95 amp times 5.08 volts), which is just a little shy of its rating.

This is really the minimum power output you need to charge a smartphone. With 5 watts, it will take several hours to charge a phone that is completely dead.

For this reason, I’m testing a 10 watt solar panel. I want extra solar cells to make sure I can reach at least 5 watts in good sunlight.

The test setup

This particular Renogy solar panel folds in half for easier storage. When folded, it’s about the size of a paperback book, making it practical to toss into a backpack or carry-on luggage. It comes with a small carabiner that you can clip onto a backpack.

At the corners of the panel are little cutouts into which you can attach four suction cups that are included with the panel. You can attach the suction cups on either the front or rear of the panel, which means you could suction the panel on either the inside or outside of a window, or other smooth surface (such as the side of an RV).

The folding setup is useful if you want to stand your panel up on a surface like this:

While this is useful, having the panel vertical isn’t the ideal direction to face the panel for solar generation except for early in the morning or late in the day. You really want it perpendicular to the sun. This is when the suction cups come in handy, or maybe you could lean the panel against something to keep it at a better angle.

You can see that the battery is attached with a USB cable to the solar panel, which has a single USB-A output port.

For my tests, I completely drained my Aukey battery to see how much charge it would take after putting the solar panel in the sun for a full day. The battery pack doesn’t have an accurate “gas gauge”, so to measure how much solar electricity the battery stored from the sun, I would then use the battery to see how much it could charge a dead smartphone.

Test 1: finding the maximum output of the solar panel

Before doing the charging tests, I wanted to see what the maximum output I could expect from this 10 watt panel. To do that, I simply went outside on a clear cold fall day, held the panel at an angle perpendicular to the sun, and read the output off the multimeter.

The very best power output I could get from this 10 watt Renogy panel was 0.84 amps, which works out to about 4.2 watts. That’s less than a cheap 5 watt USB charger.

That’s okay, but most of the time you won’t have ideal conditions like that. Often it will be cloudy, or you’ll have indirect sunlight. For that reason, I tried testing the solar panel a couple different ways.

Test 2: all-day charging outdoors

For my first charging test, I grabbed some masking tape and some things I could use to keep the solar panel at a decent angle throughout the day. It wasn’t going to be the perfect angle because I didn’t want to futz around all day to keep it pointed toward the sun, but it was better than keep the panel either flat on a table or standing vertical. I ended up using a large 3-ring binder and a paperback:

Not perfect, but pretty good. I did rotate the setup to better face the sun as it moved across the sky during the day, but the angle of the panel to the sky didn’t change at all.

I put the setup on my patio table first thing in the morning and took it back inside after the sun went down, taking a few minutes to rotate the setup to face the sun a few times during the day.

(Test results are at the bottom of this article.)

Test 3: all-day charging through a window

You probably won’t have a perfectly clear day on most days or be able to set up your solar panel at a good angle to capture the most sunlight possible. Instead, you’ll have some clouds or have to position the solar panel anywhere you can.

For this reason, I wanted to do a test where I attach the panel to the inside of a window using the four suction cups and leave it there all day:

This is an east-facing window that gets a few hours of direct sun in the morning, but only indirect light for the rest of the day.

It’s also got a bug screen on it and could use a little cleaning, so that also reduces the amount of light. But I wanted this to be a realistic test of how you might use this setup - say, in a vehicle or maybe you’re at a desk where you don’t have any spare power outlets.

Like I did with Test 2, I set up the panel early in the morning and left it there until the sun went down.

Test results

After both charging tests, I used the Aukey battery to recharge a dead phone until it was exhausted, and then turned on the phone to see how much charge it had.

For reference, my phone has a 1,821 mAh battery, which means that if the external battery was fully charged, it could recharge my phone multiple times. Unfortunately, neither test was able to fully recharge my battery. Here’s the test results:

Test 2: all day charging in full sun

Here’s the result I got after about 9 hours of charging on a fall day with clear skies:

• Best measured output: 0.70 amps
• Phone percentage charge (from dead): 92%

That’s pretty good! If you’re camping or in some other off-grid situation where you need to keep your phone topped up, this setup would work well for you.

Test 3: all day charging through a window

The next day I did the same test through an east-facing window that was less than completely clean, and also slightly obscured by a bug screen:

• Best measured output: 0.24 amps
• Phone percentage charge (from dead): 21%

As you can see, that’s quite a bit worse. Without direct sunlight and a bad angle to the sun for most of the day, I didn’t get very much charge into my phone.

Conclusion

If you need portable solar charging for your electronics, a separate solar panel and battery pack is the way to go. This allows you to buy the right size panel for your needs, and also select the right size battery pack that has the features you might need, such as wireless charging or multiple USB ports.

Another advantage is that you might not always need solar charging, in which case you can take the battery pack and leave the solar panel at home, saving you some weight.

Takeaways:

Buy at least a 10 watt panel, and larger if possible, especially if you want to charge larger electronics such as a tablet or even a small laptop. A good rule of thumb is to find out what wattage you need, and then buy a panel with a rating that is at least double that.

Because their cost is so low, you can always buy multiple battery packs for different situations. If you only need to keep a small phone charged, a small capacity battery pack might suffice, saving you some weight. For times that you need more capacity or more features (such as multiple ports or wireless charging), get a second battery pack. The Anker model listed at the top of this article costs about $20, so it’s not much of an investment.

If you need a portable solar panel, check out my article with some options ranging from 10 watts up to 40 watts.