Solar panels differ in the amount of volts - the force of the power - and their amps - the flow of charge - that comes from them. Two 10 watt panels may differ by several volts & some amount of amps. This affects how they will charge your battery system.
We're going to keep this simple. There aren't many things worse than math.
When it comes to solar panels, there are watts, amps & volts.
Watts are a measure of the power of a solar panel.
Amps is also referred to as 'current', and is a measure of charging rate.
Volts is also referred to as 'electric potential', and is a measure of the force behind the flow of charge.
Watts is Amps multiplied by Volts (if you remember your high school physics). So a 10 Watt panel is twice as powerful as a 5 Watt panel.
Now to complicate things a bit, you should know that different solar cell chemistry will produce slightly different voltages. For example, a 10 Watt solar panel using the CIGS cell technology (like the panels from Brunton) operate at 16.5V, whereas the PowerFilm amorphous cells create panels that have an open-circuit voltage of 15.6V.
Which is better?
Well, remembering that Watts is Amps multiplied by Volts, it stands to reason that a 10 Watt panel that provides more Amps, will be the solar panel that will charge a battery pack the fastest.
Most solar panels are designed for use in what is termed "12 Volt systems", even though "12V" is not actually an accurate term. 12 Volts is used because this is the nominal voltage of a car battery, and is an easy multiple of 120V AC power that you find in your home. A lead-acid battery like the one in your car, contains 6 cells operating at 2 Volts each. A 12V battery is actually just over 13V when it is completely full, and somewhere around 10.5V when effectively empty.
Lithium batteries have followed this convention as they were the latecomer to the scene.
Charging batteries is somewhat like siphoning water from one bucket to another. As long as the charger has a higher voltage than the battery it is charging, charge will flow between them, and the battery will get charged. This is why solar panels are designed to be 15-18V, in order to properly charge a 12V battery. Charge controllers are used to make sure that the panels do not overcharge the batteries once they are full (in the siphoning example, this would be like closing a valve on the hose once the second bucket is full).
Since lithium batteries are everywhere in small electronics, the personal solar chargers are designed around them. This is why the USB format has become the standard for charging small electronics - the 3.7V lithium cell charges from the 5V USB. Lithium batteries in small electronics also comes with their own built-in controller that stops them from being charged once they are full.
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