Losses: temperature, soiling, mismatch, wiring, inverter, degradation

Two identical systems on neighbouring roofs can differ in output by 10% or more. The modules are the same; the losses are not. Knowing where energy is lost lets you design it out, explain it to customers, and diagnose a poor performer.

The loss chain

Energy is lost step by step between the sunlight landing on the modules and the units recorded at the meter. Each loss multiplies with the others.

Multiplying the steps: 0.97 × 0.99 × 0.975 × 0.93 × 0.985 × 0.985 × 0.99 × 0.975 × 0.995 × 0.99 ≈ 0.80. That is the performance ratio (PR) of a well-built, well-cleaned system. A neglected one can fall to 0.70 or below.

Each loss in turn

Temperature (the biggest one in India)

Modules are rated at 25 °C, but on an Indian roof cells run at 55–70 °C at midday. Power falls by the Pmax temperature coefficient for every degree above 25 °C.

Worked example: Jaipur, a May afternoon

• Ambient 42 °C, irradiance 900 W/m², NOCT 45 °C
• Cell temperature ≈ 42 + (45 − 20) ÷ 800 × 900 = 42 + 28 = 70 °C
• PERC module (−0.35 %/°C): loss = 0.35 × (70 − 25) = 15.8%
• TOPCon module (−0.29 %/°C): loss = 0.29 × 45 = 13.1%

At 900 W/m² a 5 kWp PERC array would make about 4.5 kW before heat loss, so at that moment it loses about 0.7 kW to heat alone. Averaged over a year, including cool mornings and winters, temperature loss is typically 5–9% in most of India.

What you can control: air gap and airflow under the modules (elevated structures run cooler than flush mounts on hot sheets), light-coloured roofs, and a module with a better temperature coefficient.

Soiling

Dust, pollution, bird droppings and leaves block light. Losses depend heavily on the location and the cleaning routine:

• 2–4% with cleaning every one to two weeks in dry months
• 5–10% or more in dusty Rajasthan and Gujarat sites, near construction, or near highways, if neglected
• Close to zero during the monsoon, when rain does the cleaning

Bird droppings are worse than uniform dust, because they shade single cells. Cleaning is the cheapest loss reduction there is (module m09).

Shading

Covered in module m02. Near-shading from tanks, parapets and masts should be designed out; aim for under 1–2% loss. Remember that in a series string, a small shadow can cost far more than its area suggests.

Reflection (angle of incidence)

When sunlight strikes the glass at a steep angle (morning, evening, winter), more of it reflects away. Anti-reflective coated glass keeps the yearly loss to about 2–3%.

Module quality and light-induced degradation (LID)

Modules lose a little power in their first weeks of sunlight (LID in p-type PERC; much less in n-type TOPCon and HJT). Power tolerance also matters: a 0/+5 W module never starts below nameplate.

Mismatch

No two modules are exactly identical. In a string, the weakest module limits the current. Typical loss is 1–2%. Keep modules from the same production batch together, and never mix wattages or models in one string.

DC and AC wiring

Cables have resistance, and the loss rises with the square of current. Design for about 1–1.5% voltage drop on DC and under 1.5% on AC (module m06). Undersized cables also run hot.

Inverter

Modern string inverters have peak efficiencies of about 97–98.5%. The weighted (European or CEC) efficiency, which reflects real operation, is a little lower. Inverters also derate when hot, so a shaded, ventilated location matters.

Downtime

When the grid trips, an on-grid inverter stops (anti-islanding). In towns with frequent daytime cuts this can cost several percent, which a hybrid system partly recovers. Inverter faults left unattended for weeks also show up here; monitoring catches them.

Degradation: losses over the years

Modules slowly lose output. Typical warranted values (Sunlectric assumptions, October 2026; check each datasheet):

Worked example: a 3 kWp PERC system in Pune

• Year 1: 3 × 1,513 × 0.98 ≈ 4,448 units
• Year 25: 3 × 1,513 × 0.848 ≈ 3,849 units

Average over 25 years is about 91% of the new-module figure. Payback calculations must include this (module m10).

Key points

• Losses multiply: a well-kept Indian rooftop ends up at a PR of about 0.75–0.80.
• Temperature is the biggest loss: 13–16% at a hot midday, 5–9% over a year.
• Soiling is the most controllable loss; regular cleaning is cheap generation.
• Design cables for about 1–1.5% drop, keep inverters cool, and never mix modules in a string.
• Include degradation: PERC is warranted to about 85% at 25 years, TOPCon to about 87% at 30 years.

Check yourself

1. A TOPCon module (−0.30 %/°C) runs at 65 °C. What is its temperature loss?
2. Name two losses an installer can reduce through design and two through maintenance.
3. Why does a flush-mounted array on a tin shed often produce less than an elevated array on an RCC roof in the same city?