Solar Panel String Sizing Calculator

Why connect panels in series and parallel?

Panels in series add their voltages while the current stays that of one panel. A group of series panels is a string. Strings in parallel add their currents while the voltage stays the same. For the inverter to work, string voltage must stay within a certain range and must never exceed the maximum input voltage.

Why temperature decides

Panel voltage rises as temperature falls. Datasheet values apply to a cell temperature of 25 °C. On a cold winter morning the open circuit voltage is clearly above the datasheet value, and at noon in summer the operating voltage is clearly below it.

V(T) = V_STC × (1 + β × (T − 25))

β is the temperature coefficient from the datasheet and carries a minus sign.

Three voltage checks

• Cold open circuit voltage: S × V_oc(T_min) must not exceed the inverter's maximum input voltage.
• Hot operating voltage: S × V_mp(T_max) must stay above the MPPT minimum.
• Cold operating voltage: S × V_mp(T_min) must not exceed the MPPT maximum.

Example

V_oc = 49.5 V, coefficient −0.27 percent per °C, minimum temperature −10 °C. Panel voltage becomes 49.5 × (1 + 0.0027 × 35) = 54.18 V. An inverter with a 1000 V input takes at most 1000 / 54.18 = 18 panels in series. Nineteen panels give 1029 V and exceed the limit. That is why a calculation with the datasheet value (1000 / 49.5 = 20 panels) is misleading.

Current check

The short circuit currents of parallel strings add up and are multiplied by a safety factor. The result must not exceed the short circuit current the inverter allows per input.

Limits

This is a preliminary calculation. The design temperature, safety factor, fusing, cables and protection scheme come from the applicable standard. A fault on the DC side can cause fire and fatal electric shock: the final design must be made by a qualified engineer.