Series or Parallel

One question we are constantly getting from beginners is about series and parallel connections of both batteries and solar modules. 

Basically, parallel is when all of the positives are connected together and all of the negative terminals are connected together.  In this case, whether a battery bank or solar array, the voltage always stays the same.  So, if you hook three solar modules in parallel or whether you hook three batteries in parallel, the voltage will be the same as if you only had one battery or only one solar panel.

parallel connection

Series, on the other hand, adds the voltage of each individual unit in the string.  A series connection is obtained by connecting the negative terminal of the first unit (battery or module), to the positive terminal of a second unit.  When you do this, whether you connect 2 batteries or twelve trillion, there is always only one negative terminal and one positive terminal when you are done that is unused.  When connecting a volt meter to the two terminals that are not connected to anything, you will always get a voltage reading that equals each of the individual voltages added together.

Series connection 

When connecting solar modules, it is important to keep the amps less than the capacity of the charge controller.  Lets say you have a 24 volt inverter.  This means you have a 24 VDC system.  If each solar panel produces 36 VDC open circuit, that is without a load, the charge controller will automatically adjust the charging voltage to meet the batteries current state of charge.  The formula is similar to Ohms Law.  Ohms Law states that E = I * R.  We aren't as concerned with resistance in our solar module installation, so the rule we use is P = E * I.  P or Power means watts.  E or electromotive force means volts.  I means current or amps.  Therefore, if a 225 watt solar panel (P) is connected to a 24 volt system (E), then it produces electricity at the rate of 225 = 24 * 9.375 amps.  You can divide the watts by the volts to get amps.  So, if you have a 30 amp charge controller, you better not connect any more than 3 solar modules to it or it will fry.  On larger solar arrays, you can either use a 60 amp charge controller and thereby connect up to 6 modules safely, or you can simply use as many charge controllers as it takes for how ever many solar panels you have.  Many charge controllers can be daisy chained to handle an array of any size.  Grid-tie systems do not have this consideration as they may or may not even have batteries or a controller to charge them.