In large C&I and open-access solar projects, EPC contractors sometimes use mixed wattage modules such as Mixing 590W, 600W, and 620W solar panels to match DC capacity. This typically happens due to module availability, cost optimization, or timeline pressure. While this may appear acceptable from a capacity perspective, improper string design can introduce mismatch losses and reduce plant performance over time.

Let’s consider a practical example:

Suppose Project size: 1200 kW DC Inverter configuration: 125 kW × 8 units (1000 kW AC) DC oversizing: ~1.2 Modules used: 590W, 600W and 620W

When modules are connected in series to form a string, the current flowing through the string is limited by the lowest current module. Since 590W, 600W and 620W modules have different operating currents (Imp), mixing them in the same string forces higher wattage modules to operate at the lower current of the smallest module.

This results in mismatch loss.

For example, if one string contains a mix of 590W, 600W and 620W modules, the higher wattage modules (600W and 620W) will not operate at full capacity. The effective power output of the string reduces, even though installed DC capacity remains the same.

Typical impact: • String mismatch loss: ~1.2% to 1.8% • Plant loss (1200 kW): ~14 to 18 kW • Annual generation loss: ~24,000 to 31,000 units • Tariff assumption: ₹4.35 per unit • Annual revenue loss: ~₹1.0 lakh to ₹1.35 lakh • 25-year lifetime loss: ₹25 lakh+

This loss continues for the entire plant life and is usually not visible during commissioning.

However, mixing different wattage modules is not inherently wrong. The issue arises only when modules are mixed inside the same string or same MPPT.

Best engineering practice: • Keep same wattage modules within one string • Allocate 590W, 600W and 620W modules to separate MPPTs • Avoid random mixing inside strings • Maintain uniform string length per wattage • Check MPPT voltage window compatibility • Follow inverter design guidelines

Example of correct configuration:

MPPT 1 → 590W modules only MPPT 2 → 600W modules only MPPT 3 → 620W modules only

With proper grouping, mismatch loss becomes negligible and plant performance remains optimized.

In MW-scale solar projects, small engineering decisions like module grouping can significantly impact long-term energy yield and project IRR. Proper string planning during design stage helps avoid hidden generation losses and improves plant reliability.

Before finalizing module mixing in your next project, evaluate string design — not just total DC capacity.