Can a 20W solar panel charge a phone as fast as a wall charger?

No. In ideal direct sun, a 20W USB solar panel delivers roughly 10 to 15 watts of actual output, which charges a phone at roughly one-third to one-half the speed of a standard wall adapter. Expect a phone to go from 20% to 80% in about two hours of strong sunlight.

Is 20 watts enough for a USB solar charger, or should I get something bigger?

20 watts is enough for one phone plus a small power bank at a time. If you need to charge multiple devices faster, or if you frequently deal with overcast skies, a 28W or larger panel will give you more headroom. For a single-person kit where weight matters, 20 watts is the practical floor.

Does the FlexSolar 20W work on cloudy days?

It produces some power on cloudy days, but output drops significantly compared to direct sun. Heavy overcast may reduce output to the point where charging is very slow or stalls entirely. This is true of all portable solar panels, not just this one. For the most reliable cloudy-day charging, a larger panel (28W or more) provides more margin.

Home Gear Reviews FlexSolar 20W Solar Charger

Gear Review · Energy

FlexSolar 20W USB Solar Charger.

The lightest USB solar panel worth owning. Under a pound, under $40, and enough output to keep one phone and a power bank running indefinitely in good sunlight.

Rated Power

20 watts

Weight

~0.9 to 1.5 lbs

NWS Verdict

Buy it

View on Amazon

What it is

A compact solar panel that earns its spot in a pack

The FlexSolar 20W is a foldable USB solar panel with two monocrystalline cells that unfold from a compact package roughly the size of a tablet. It has one USB-C port and one USB-A port, each rated at up to 15 watts individually and 10 watts each when both ports are in use at the same time. Like all USB solar panels, it has no internal battery; it converts sunlight to USB power only while deployed and connected.

FlexSolar makes the 20W panel in several variants. The standard version folds to approximately 12 by 10 by 0.6 inches and weighs about 1.5 pounds. An upgraded version trims the weight to under a pound (about 0.86 pounds) and carries an IP67 waterproof rating on the panel surface, a meaningful step above the IPX4 or IP44 ratings on many competing panels. The ETFE film lamination on the cells is more durable and scratch-resistant than the PET film used on cheaper panels.

This is the panel for a go-bag, a daypack, or a vehicle glovebox. It fills the niche where a 28W panel is more than you want to carry but a phone-sized "solar power bank" panel is too small to do anything useful. Pair it with a 10,000 mAh power bank and you have a complete, silent, fuel-free charging system that weighs less than two pounds total.

Why it works

Light enough to actually bring

The weight is the feature

A solar panel you leave at home because it is too heavy does not charge anything. The FlexSolar 20W in its lightest version weighs under a pound. That is light enough to stay in a go-bag permanently, clip to a daypack without noticing it, or slide into a vehicle kit alongside a first-aid pouch and a flashlight. As one experienced backpacking and bikepacking reviewer put it, the FlexSolar 20W sits at a good output-per-gram ratio, between the ultralight 10W options and the heavier 28W panels.

IP67 weathers actual conditions

The upgraded version carries an IP67 rating on the panel surface, meaning the cells themselves can handle brief submersion and sustained rain. Most competing USB solar panels in this price range are rated IPX4 or IP44, which covers splashes but not steady rain. The ETFE film lamination also resists scratches and abrasion better than cheaper PET film panels, which matters when the panel rides strapped to the outside of a pack. The junction box is still the weak point: it is not fully waterproof, so keep the ports covered and dry.

Both port types, no adapter needed

One USB-C port and one USB-A port cover the two connector types you will actually encounter in a kit. Modern phones, newer power banks, and most current headlamps charge over USB-C. Older devices, radios, and legacy power banks use USB-A. Having both means one fewer cable or adapter in the bag.

Entry-level price

At roughly $25 to $40 depending on variant and retailer, the FlexSolar 20W is one of the least expensive USB solar panels from an established manufacturer. That makes it an easy first step for a household that has never owned a solar charger and wants to try the technology before investing in a larger panel.

What owners actually say

The patterns from real buyers

The FlexSolar 20W has accumulated a solid body of owner feedback across Amazon, outdoor forums, and review sites. The patterns are consistent and mostly favorable for what the product is, with one caveat that comes up in nearly every thread.

Owners consistently praise the build quality relative to the price. The panel feels more durable than expected for a sub-$40 product, and the ETFE surface holds up to rough handling in packs and panniers. Several owners note that the compact fold size is the reason they actually carry it rather than leaving it at camp or in the car.

Direct sunlight performance draws steady praise. In hands-on testing, the panel charged a smartphone from 20% to 80% in approximately two hours of good sunlight. Owners pairing it with a 10,000 mAh power bank report the bank reaching a meaningful charge over the course of a sunny day, enough to top off a phone once or twice overnight.

The recurring frustration is the gap between the 20W label and the real-world output. Multiple testers and owners consistently measure actual output in the 10 to 15 watt range even in strong sunlight. This is normal for all portable solar panels (the rated wattage assumes lab conditions), but reviewers note it is more noticeable on a smaller panel because you have less margin. When both ports are in use simultaneously, per-port output drops to around 10 watts, which can feel slow.

A smaller theme: several reviewers wish the panel had some kind of visual charging indicator or ammeter like the BigBlue 28W offers. Without one, you cannot tell whether the panel is producing power, whether a cable is making good contact, or whether your current angle is optimal. You plug in and hope.

The one thing to know

20 watts is the floor, not the ceiling

A 20-watt USB solar panel is enough to keep one person's phone and a small power bank charged in good sunlight. It is the practical minimum for a solar charging system that produces useful results in a reasonable time. Below 20 watts, charging slows to the point where it barely keeps pace with a phone's idle battery drain, and a single cloud can stall charging entirely.

If your household has more than one person's devices to charge, or if your area is frequently overcast, a 28W panel like the BigBlue 28W gives you meaningfully more headroom. The FlexSolar 20W is the right choice when weight and pack space are the limiting factors, not charging speed.

How it fails, and what to do then

What breaks and when

The failure modes are the same as any foldable USB solar panel. Solar cells degrade slowly from UV exposure over years. The ETFE film on the FlexSolar is more durable than cheaper PET film, but it still ages. A panel stored indoors between uses will last longer than one left permanently deployed in the sun.

The junction box and USB ports are the most likely failure point. Despite the IP67 panel rating, the port area is not fully waterproof. Water intrusion around the ports after rain exposure, or sand and debris working into the connectors, can degrade or kill the charging electronics. Keep the port covers closed when not in use and dry the junction box after any wet-weather exposure.

The folding seams and attachment points are the physical wear points. Frequent folding and unfolding, combined with the stress of hanging from a pack or clothesline by the carabiner loops, can weaken stitching over time. Inspect the loops and seams before relying on it for a trip.

FlexSolar, founded in 2009, offers customer service for defective units. Owners report reasonable responsiveness to claims. If a panel stops charging within the first few months, contact them before discarding it.

What it costs over time

The cheapest energy system you can own

The FlexSolar 20W runs $25 to $40 for the panel. Add a power bank ($15 to $30) and the total system cost is $40 to $70 for indefinite phone and small-device charging with no fuel, no noise, and no recurring cost. Replace USB cables as they wear. That is the entire ownership expense.

If the panel dies after several years, replace it. At this price point, a USB solar panel is closer to a consumable than a capital investment. Budget to replace it every 3 to 5 years of regular outdoor use, though a panel stored properly between uses may last considerably longer.

Who should skip this

When 20 watts is not enough

Households with more than one or two phones to charge. The FlexSolar 20W takes most of a sunny day to fill a 10,000 mAh power bank. If multiple people need their phones and devices kept alive simultaneously, the BigBlue 28W or a larger panel is a better fit.

Anyone who values real-time feedback. Without an ammeter or charging indicator, you cannot tell whether the panel is actually delivering power, whether you have the best angle, or whether a cable connection is solid. If you want to know what is happening, the BigBlue 28W with its built-in ammeter solves that.

Anyone in a consistently overcast climate. A 20W panel in heavy cloud cover may produce 3 to 5 watts, which barely charges a phone at all. A 28W or larger panel gives you more margin to still produce a useful charge even under gray skies.

Where it fits in your readiness

The solar charger that fits anywhere

The FlexSolar 20W belongs in the Sustain tier (2-week readiness) and works well into Endure (long-term self-reliance) for single-person or lightweight kits. Its best role is as the solar component in a go-bag, vehicle kit, or evacuation pack where weight and size are limited. It also works as a household backup panel stored in a closet for power outages.

It matters most for hurricanes (where power may be out for days after the storm passes), winter storms, extended power outages, and evacuation scenarios where you leave home with whatever fits in one bag.

It contributes to the solar-charging line on the Energy Preparedness checklist and the communications-power portion of several hazard-specific checklists.

Alternatives worth knowing

Other paths to the same goal

BigBlue 28W (~$50 to $70) - more power, slightly heavier

The BigBlue 28W is the NWS top pick in the USB solar charger category. It charges faster, handles cloudy conditions better, and includes a built-in ammeter that the FlexSolar lacks. The tradeoff is about half a pound more weight and roughly double the price. If weight is not the primary constraint, the BigBlue is the stronger choice.

Nekteck 28W (~$40 to $55) - more power, USB-A only

A well-reviewed 28W panel at a price between the FlexSolar and the BigBlue, but with dual USB-A ports only (no USB-C). Heavier than the FlexSolar but delivers more output. A reasonable middle ground if you do not need USB-C.

FlexSolar E40 (~$60 to $80) - the step up from the same brand

FlexSolar's 40W panel adds USB-C, USB-A, and a DC output for small power stations. OutdoorGearLab's testing showed it nearly doubled the energy collection of the 28W class in direct sun. If you want to stay with the FlexSolar brand but need more power, this is the natural upgrade.

Maintenance

Keeping it ready

Store the panel folded and indoors when not in use. A go-bag, glove box, or closet shelf works. Avoid long-term storage in a hot car trunk during summer months; heat accelerates degradation of the cells and adhesives.

Wipe the panel face with a soft, dry cloth before each use. Dust, pollen, and fingerprints reduce output. Do not use solvents or abrasive cloths on the ETFE surface.

After rain or moisture exposure, unfold the panel fully and dry it before storing. Close the port covers whenever you are not actively charging. Inspect the carabiner attachment loops and fold seams before any trip where you will depend on it.

Test it once or twice a year by plugging in a phone and confirming it charges in direct sun. A panel that charges noticeably slower than when it was new, under comparable sunlight, is aging. At this price point, replace it rather than try to troubleshoot degraded cells.

NWS recommendation

If you want the lightest possible solar charging setup for a go-bag, vehicle kit, or solo pack, buy the FlexSolar 20W and a 10,000 mAh power bank. That combination weighs under two pounds, costs under $70, and keeps a phone charged indefinitely in sunny conditions. If charging speed and cloudy-weather performance matter more than weight, get the BigBlue 28W instead.