1. Use the Solar Kit Guide to size the modules and inverter for you automatically.
2. Use one of the software calculator tools created by the inverter manufacturer.
3. Do a hand calculation using information from the module and inverter datasheets.
A quick hand calculation that works effectively to size a system is to first determine how many modules you can fit on a single string. Lets call this value the following.
MM : Maximum modules per string.
The information you will need to determine this value is the following.
MSV : Max system voltage of inverter. (Typically 600 V or 1000 V note that the inverter and modules may have separate maximum system voltages. You will want use whichever is lower.)
Voc : Module open circuit voltage. (Typically between 30 and 50 V)
Temp: Minimum ASHRAE temperature of your area in Celsius. (One good source is SolarDesignTemps)
TC : Temperature Coefficient of the Voc. (Usually -0.31 % / degree C)
With this you can use the following Equation.
MSV: 600V for the inverter and 1000V for the modules. Therefore, we will use 600V.
Temp: 0 degree C
TC: -0.29 % / degree C
Plugging these values into the equation above we get the following.
Since the value is 12.78, make sure to round down to the nearest whole number. This leaves you with 12 modules per string.