A 5 watt solar panel is the minimum practical size.
Depending on the devices to be charged and the strength of the sunlight (geographical latitude, time of year, cloud cover) it just might charge
- handphones
- MP3 players (iPod)
- NiMH AA batteries
A 10 watt panel is a safer bet. 20 watt models are even better (especially for charging notebook and laptop computers) but these are heavy and expensive.
Tricks to Maximize Power
Some models can be joined together to provide more power (depends on manufacturer).
A panel too weak to charge four AA batteries at a time might charge one or two.
If a cellphone or notebook refuses to charge, switching it off might help.
Built-in Battery
Some hybrid chargers have an internal battery.
- The solar panel charges the internal battery
- When charged, the internal battery is then used to charge external devices
A 1 watt panel is not powerful enough to charge most devices but can be enough to charge the internal battery. The internal battery is powerful enough to charge other devices.
Advantages
- Smaller, lighter and cheaper solar panel can be used.
- Once charged, the internal battery can charge an external device relatively quickly.
Disadvantages
- Internal battery will last only a few years. Even new units might have faulty or weak batteries.
- Weak power from panel means long charge time for internal battery.
Choosing a Charger
Solar chargers are purchased for different reasons. These reasons will determine which type of charger is best.
Emergency and survival
- Need reliability: no internal battery
Environmental reasons, green lifestyle
- Minimize waste: no internal battery
Hiking, backpacking
- Weight is important
- Charge on-the-move, clipped to backpack: no hinged or folding panels
Sunlight Conditions
More powerful panels might give usable power on overcast days but small units need direct sunlight.
Placing the panel behind a glass window can substantially reduce the output power.
Accessories and Output Voltage
A 5 volt USB socket output is useful as many of today's devices can be charged from a computer USB port (adaptors might have to be purchased separately).
12 volts output through a car cigarette-lighter socket is also useful as many devices are also designed to be charged from cars.
Some devices may use other voltages and sockets. This should be checked before purchase.
For maximum flexibility, 12 volt inverters can be used to provide mains AC voltage (110 or 230 volts) but this is only practical with 20 watt and larger panels. It also means additional weight, and loss of power due to conversion inefficiencies.
Some panels have a built-in AA and AAA battery charger. Others attach a separate battery charger through the USB port. If not supplied with the panel, a USB battery charger can be purchased separately.
Are Solar Chargers Environmentally Friendly?
The National Renewable Energy Laboratory reports that rooftop photovoltaic solar panels can take 3 to 4 years to recover (payback) the energy used to manufacture the cells (carbon neutral). This is with them sitting under the sun every day.
It is unlikely that a portable solar charger will see this much use. The best "green" result that can be expected is that rechargeable NiMH batteries are used instead of disposable alkalines.
The Best Solar Charger
More so than for other products, solar chargers might not perform as expected. Buying from a retailer with a good return policy is the safe thing to do.
Most problems with chargers are caused by insufficient power. Buying the most powerful panel that is practical, will reduce potential problems.
Resources
- AA and AAA Battery Charger Guide
- Best Emergency Survival Kit
- Solar Powered Garden Lights Guide
- What is the energy payback for PV? National Renewable Energy Laboratory.
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