Quick Answer
To determine the optimal string size for specific solar panels, calculate the maximum power output of each panel and divide it by the maximum allowed current of the wiring and inverter. Typically, 60-80% of the panel's maximum power output is used to avoid excessive wiring and inverter heat. Use the smaller of the two calculations to determine the optimal string size.
Calculating String Size
When calculating the optimal string size, start by finding the short circuit current (ISC) of each solar panel. This value is usually found on the manufacturer’s datasheet or label. For example, a 280 watt solar panel with an ISC of 7.5 amps. Next, divide the panel’s maximum power output by the maximum allowed current of the wiring and inverter, typically 15-20 amps. This will give you the optimal string size in terms of power output. For the same example, 280 watts divided by 15 amps gives you 18.67 watts per amp, or a string size of 19 panels in series.
Considerations for Series-Parallel Configuration
When deciding on the optimal string size, consider the series-parallel configuration of your solar array. If you have multiple strings in parallel, you can increase the total power output while keeping the voltage within the allowed range. For example, if you have 3 strings of 19 panels in series, each string can be 240 volts, and the total power output is 3 x 280 watts, or 840 watts. This configuration is useful for larger solar arrays or when using multiple inverters.
Example Calculation for a Specific Solar Panel
For a 375-watt solar panel with an ISC of 8.75 amps, the maximum allowed current of the wiring and inverter is 15 amps. Divide the panel’s maximum power output by the maximum allowed current: 375 watts / 15 amps = 25 watts per amp. This gives you an optimal string size of 25 panels in series.
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