Most home-owners and installers start designing (or defining) their solar PV systems by measuring their roofs and finding out the available space for the solar modules. They've already determined their kW Hour requirements and can therefore move on to figuring out how big a system they can install in order to offset or zero out their consumption.
Picking the system inverter is the next logical step. There are a variety of inverter technologies and sizes which can be leveraged to produce the most efficient system for the lowest long-term cost.
The most important factors in determining what size inverter to choose are:
- The number of strings in the system
- The voltage on a string
- The maximum input current
- The maximum ambient temperature of the location
- The minimum ambient temperature during day light time when the system is supposed to run.
The number of strings are important, because there are typically 2 to 4 string inputs on an inverter. The operating voltage of the system should fall in the operating range of the inverter. Because higher voltages may burn the transformer, lower voltages may not even run the inverter.
Most of the inverter manufacturer's have string sizing tools on their websites. I'd like to list down the available "string sizing tools" from several manufacturers for your convenience, just click on the names below:
- SMA Sunny Design
- PV Powered String Sizing Tool
- Fronius String Sizing Tool
- Solectria String Sizing Tool
- Outback Power String Sizing Tool
- Xantrex GT String Sizing Tool
- Xantrex Three Phase Sizing Tool
- Enphase Micro Inverters Compatibility List (pdf)
- Power-One Aurora String Sizing Tool
Please note that the ambient temperature of the location is an important factor, because it directly affects the voltage of the whole system. You might have to check Weather.com for average temperature details while playing with the string sizing tool. Since the module specifications are measured under standard test conditions 78 F degrees, a higher temperature may reduce the voltage output, whereas a lower temperature may increase the voltage output.
If the operating voltage of the array is just on the edge of the operating voltage of the inverters, you may end up with a solar PV system that is not running on a hot sunny day, or even damage your inverter on a cold day.