Solar system losses and the performance ratio (PR)

September 29, 2026 · 5 min read · Danilo Nascimento

Understand what reduces a system’s energy output: temperature, soiling, cables, shading and inverter. See the performance ratio and adjust the losses.

Why delivered energy is below the theoretical figure

A 5 kWp system with 5 peak sun hours “should” generate 25 kWh per day. In practice it delivers less: every stage between the sun and the socket loses a little. These losses are normal and predictable.

25 kWhper day, theoretical generation (5 kWp × 5 h)

≈ 20.6 kWhper day, energy delivered with typical losses

≈ 82%performance ratio (PR)

Where the energy goes

Temperature

≈ 5 to 12%

Hot panels lose efficiency: typically 0.3 to 0.4% of power per °C above 25 °C.

Soiling

≈ 1 to 5%

Dust, leaves and soot block light. Where it rains often, rain helps; in dry places cleaning matters more.

Cables and connections

≈ 1 to 3%

Thin cables and poor connections heat up and waste energy as heat.

Shading and mismatch

≈ 1 to 10%

A shadow on one row cuts the current of the whole string, and different modules limit one another.

Inverter

≈ 2 to 4%

Converting DC to AC is not perfect: current inverters have an efficiency of about 96% to 98.5%, according to the datasheet.

The performance ratio (PR)

The performance ratio (PR) sums up all the losses in a single number: how much of the theoretical energy actually reaches the system’s output. The theoretical reference uses the irradiation on the plane of the modules, measured over the same period as the delivered energy.

20.6 kWhenergy delivered per day÷25 kWhkWp × peak sun hours=82%performance ratio

Adjust the losses

Move each loss and watch the theoretical energy shrink until it reaches what the inverter delivers. Losses multiply: each stage’s loss applies to what is left of the previous one.

What can be reduced

	Typical loss	Can it be reduced?
Temperature	5 to 12%	Partly: with ventilation behind the modules
Soiling	1 to 5%	Yes: by cleaning
Cables and connections	1 to 3%	Yes: correct cable size and connections
Shading and mismatch	1 to 10%	Yes: panel placement, optimisers or microinverters
Inverter	2 to 4%	A little: choose an efficient model

✓ can be reduced · ◐ depends on the case · ✗ point of attention

How to use this when sizing

When you calculate how many panels you need, the 80% overall performance factor (the “system efficiency”) equals the PR, provided the peak sun hours are those on the plane of the modules. If the roof has shade or the climate is very hot, use a lower value; if the system is clean and well ventilated, a little higher.

Start with 80% for a quick estimate
Drop to 70 to 75% with shade, intense heat or an installation without ventilation
Go up to 85% only with a well-ventilated design and a simulation that supports it
After installation, compare with real generation and adjust the value for future projects

Draw it in Editor Solar

This is Editor Solar itself, already loaded with the diagram from this article. Move the equipment, edit labels, connect new parts and export to PNG, SVG or PDF. It is free and runs right in your browser.

A 5 kWp system with typical losses noted: of 25 kWh theoretical per day, about 20.6 kWh remain.

Try it: Note in the diagram text the losses that apply to your roof (shading, heat, long cables) and redo the performance ratio calculation.

Ready-made templates: see all diagram templates

Open this diagram in the full editor ↗

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