Solar power banks can be very handy when you are off-grid, away from a mains power source for any length of time. Whether that is on a camping trip, hiking or cycling, using the sun’s energy is an environmentally friendly way to charge your electronic devices. But how long do solar power banks actually take to charge?
Typically in direct, unobstructed sunlight, you should allow up to 50 hours to charge the battery on a standard (25,000mAh) power bank fully.
This is, of course, a very rough estimate based on my personal experience and what manufacturers state. But there are also many other factors to consider too which I will try and cover in detail in this article.
How does a solar power bank work?
Solar energy is one of the most sustainable and environmentally friendly ways to generate electricity.
A solar power bank uses a small built-in solar panel to charge a rechargeable battery (usually a lithium-ion battery).
The panel is a photovoltaic cell which is sandwiched between a semi-conductive material (usually silicon). This silicon has been altered by adding phosphorus to one end which creates a negative charge and boron to the other end which creates a positive charge. This allows an electrical field to be created.
This electric field causes electrons (tiny particles) to move around within the cells. If you imagine sunlight also as a stream of tiny particles (called photons) hitting the solar panel these photons occasionally strike one of the free electrons and knock it out of the circuit where it is then collected by a conductive plate to create electricity and re-charge the battery.
Lithium based batteries are favoured, as lithium is a substance that has a high tendency to lose electrons. Meaning electrons will easily move from one side of the battery to another when prompted, making them great for charging your electronics.
What is the best time of day to charge a solar power bank?
So now you know to re-charge the battery, we need to get as many photons hitting the solar panel as possible to increase the likelihood that an electron will be knocked out of the field to be used in the battery.
The sun’s intensity varies depending on the time of year and the time of day. But remember it is the light not heat that the solar panel uses and so just because it is the hottest part of the day doesn’t mean that is the best time to charge.
In fact, the light energy is often highest earlier in the day before temperatures get to their peak and extreme high temperatures can even affect the performance of the panel, making them less efficient.
Can it charge in indirect sunlight?
The reality with solar power banks is that you will be moving around a lot whilst using them. Many people like to attach them to a backpack for example.
The answer is yes, it will still charge in indirect sunlight but nowhere near as effectively. This probably seems obvious now you know that you need the photon particles in order to generate electricity and charge the batteries. Less direct light hitting the panel means less chance of interaction with the electrons and therefore less charging occurs.
With such a small panel you will need as much direct sunlight as possible and so if you have the option of a shorter time in direct sunlight over a longer time in indirect sunlight I would choose the direct sunlight option every time.
Will it charge in the shade?
A common misconception is that a solar panel will still charge on a hot day, even when in the shade. This comes back to the thought that heat is used to generate power, we now know this is not the case.
You probably guessed it then, that unfortunately, your solar power bank will not charge in the shade as no photons are reaching the photovoltaic cells.
‘Shade’ is a term that will vary greatly though. Most objects causing shade will not be blocking out 100% of the sunlight. A tree, for example, will still let through quite a bit of light and so it will still charge a solar power bank, just much more slowly than in direct sunlight. So obviously if you can avoid shade then you will see much better results.
Will it charge when it’s cloudy?
Clouds have a similar effect to objects causing shading. They prevent as many photons from reaching your panel.
Therefore, the same as in the shade, the solar power bank will still charge but at a much slower speed. Estimates and studies show this to be around 25% of direct sunlight on a moderately cloudy day and as little as 10% on a very cloudy day.
Solar power bank charging time in direct sunlight?
Now you have a better idea of how and when solar power banks work best how long does it actually take to charge one?
As it will start getting too complicated when factoring in shade and cloud I will just assume you have access to constant direct sunlight.
Solar power banks also come in many different shapes and sizes. This will affect the charging time because the size of the battery varies.
The capacity of the battery is measured in milliampere-hours (mAh). You will see this in the description of the product before you buy it. It can vary from a few 2000mAh to 15,000mAh or more.
So I could give you lots of equations here allowing you to calculate the exact time it would take to fully charge your solar power bank, factoring in shade and sunlight intensity…… but I don’t think that would be very useful.
The reality is that most manufacturers actually advise against trying to fully charge your solar power bank using the sun alone. The small panel can only generate so much power, even when in full sunlight and most power banks now have a very high battery capacity.
Below are a few examples of solar power banks and charging times in the sun estimated from manufacturers and user feedback:
- PEALIKER (10,000mAh) – 30 hours to fully charge in the sun
- OUTXE (20,000mAh) – 50 – 70 hours full charge (recommend emergency use only)
- ADDTOP Solar Charger (25000mAh) 4 Panels – much quicker some claim 4 hours in direct sunlight but the manufacturer says 25-50 hours
So what’s the point then?
You can see that it is slow to charge these banks in just the sun, around 50 hours! This compares to around 5 or 6 hours from mains electricity.
You may be asking now, what is the point in even trying to charge the power bank using the sun?
Most manufacturers therefore only really recommend charging from the sun to ‘top-up’ the battery or for use in emergencies.
This is how I use mine. I’ll charge it from the mains before I go on a trip and then I’ll leave it attached to the outside of my bag or on the ground when I’m camping to get some extra charge.
I still find this very useful, if you are out camping for a long time it can get you some vital battery which you might not have been able to get otherwise.
It’s a balance between practicality/ size and performance. With current solar technology, you would need several panels to provide the charge required to match that of your household plug socket. That wouldn’t exactly be the most practical solution for carrying around.
The nature of charging batteries too also means it will charge much more rapidly for the first 50% and then slow down thereafter. You may never use the sun to fully charge the power bank but might just use it to get a little extra ‘juice’ for those emergency situations when you are far away from the nearest power source.
Can you speed up the charging?
Add more panels
As you might have noticed with the examples. The ADDTOP which has 4 panels rather than the standard 1, charges much more rapidly.
More panels give you more surface area for photons to strike and therefore electricity to be generated.
Taking this into account the best way to speed up solar charging is to add some more panels. This obviously requires you to have more space but there are some great options out there now.
The Nektek Solar Charger is a solar charger designed for the outdoors that has 3 decent size panels that fold up. There is no battery included in the unit but with USB outputs this will allow you to recharge your solar power bank more rapidly.
And because it has 2 USB charging points you can be recharging your device and recharging the power bank at the same time, making the best use of any available sunlight!
Using this you should be able to fully charge a large solar power bank in just one day.
Optimize conditions
This one is a bit more obvious, but make sure you optimize the chances of getting as much energy as possible to the panel. Keep it in direct sunlight and avoid shade.
If you have the option of staying still for a few hours then that will be better as you can move the panel and ensure it is always getting direct sunlight. If you are on the move and have it attached to a backpack for instance then a lot of the time you will be facing away from the sun or standing in the shade.
How do I tell when it’s fully charged?
Pretty much all solar power banks I have used or seen have a series of LED lights on the side to indicate the level of charge. When all the LED lights are on the device is fully charged.
However, the device will still work fine when only half charged so don’t worry about waiting for full charge before using it!
How many times will it charge my phone when fully charged?
This will again vary depending on the size of your power bank and the type of phone you are trying to charge. But from my experience with a 25,000mAh power bank, fully charged you can expect about 3-4 full charges of an iPhone.
Conclusion
As you now know it takes a long time to fully charge a solar power bank using just the sun’s energy. But I don’t think that really matters and it isn’t how these devices are designed to be used.
Charge them up fully before you go (of course make sure your home electricity is renewable too 😉) and then use the suns energy to ‘top-up’ the power bank to extend the use further than it would normally go. Think of the solar panel as more of a bonus feature.
If you are going ‘off grid’ for a while the reality is you will need to get yourself some more panels or you will be sat around for weeks! There are lots of great folding, portable solar panels sets out there now.