Voltage, also known as electromotive force can be compared to pressure in a water pipe. The more pressure in a water pipe, the more water flows per unit of time. The same situation exists when using a solar module to charge a battery. The greater the voltage, the greater the flow of available electrical current.
This is especially important in solar modules because contrary to popular belief, the hotter it is, the poorer a solar module will perform. In fact, solar modules perform best in in the dead of winter in full sun, when it's nice and cold. Ironically, heat is actually a solar module's worst enemy. Solar modules operate from light not heat.
As the temperature rises, solar cell efficiency and voltage begins to drop. The lower the voltage, the less current is able to flow in a circuit. If a solar module is not rated at a high enough voltage, elevated temperatures could cause a state of equilibrium between the battery voltage and the solar module voltage and no current would flow at all. So a high current rating on a solar module is only half the story, there must also be a high enough Voltage rating or the Amperage rating is meaningless.
Imagine two tanks of water connected to each other through a pipe, with one tank filled to a higher level than the other tank.
As long as the pressure or voltage is higher in the solar module than in the battery, current will flow into the battery.
As the voltage or pressure in the solar module begins to fall due to increased air temperatures as illustrated in the above example, less current flows into the battery. For example, a typical 75 watt panel can drop to as low as 55 watts at 80 degree centigrade.
This is why it's important to start with a solar module that has a high enough voltage to compensate for the voltage drop that occurs when air temperatures rise. This is especially important if the solar system is being used in hot climates.
To further complicate matters, solar modules are rated to output a specific number of amps at a specific voltage. When charging a discharged battery using a conventional non-MPPT charge controller, the charge controller simply connects the solar module directly to the battery. This direct connections pulls the solar modules operating voltage down to a level that is near the voltage level of the battery. For example if a solar module is rated at 4.4 amps at 17.1 Volts, using Ohms Law 4.4 Amps X 17.1 Volts = 75.24 Watts. By pulling the solar module's voltage down to the battery's 12 level we now get approximately 53 Watts. 4.4 Amps X 12 Volts = 52.8 Watts.
Using a new MPPT or Maximum Power Point Tracking charge controller changes the equation dramatically. An MPPT charge controller constantly calculates the specific voltage at which the solar module is able to produce maximum power and allows the solar module to operate at that voltage before this maximum power level is fed to the battery. This is achieved by using a high efficiency DC to DC converter, which effectively isolates the solar module's voltage from the battery's voltage. It's sort of analogous to a car's automatic transmission which constantly changes gear in order to transfer more power to the wheels.
So by starting with a higher voltage panel such as the ReliaGen TM ST1250 or the BP 585 and coupling either of these two modules with a MPPT charge controller, you are able to produce up to 30% more power when compared to conventional solar module and charge controller technologies.