I'm trying to equip my new trailer (see my other post) with a solar panel to charge phone, camera, sat-nav, etc. After some investigation I realise that not only the suppliers of solar kits are anything but reliable they lie as much as an MP. Some facts: Sun energy at sea level on a bright day at noon will give something around 800W / m2 The best solar panels currently can convert around 20 to 23% energy which will give you no more than 184W . m2. The best way for you to measure how much power you will get from a given solar panel is to multiply the max voltage by the current. (W = V x I Power is the voltage times the current) Depending on the battery condition you will need around 1A to recover a battery with low voltage (around 8 to 10V) for a few hours. Also, it depends on your location. Further north the variation is greater both between summer / winter but also further north you are less sunlight you will get. A trickle charger usually will have a top current of 0.8A (800mA) to 1A as this will mimic the current of the alternator of the bike. Ireland (by your location) will have short days reducing substantially the output of a solar panel. I would assume you may get around 250W / m2 average. Thinking that you will need it more in the winter as it is when the bike will probably be stationary for longer you are looking at 2Kw panels to make sure you have the battery fully charged at all times. With the 2Kw panels you would need to get a controller to make sure it stops charging once the battery is full.