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Power from a solar panel

Electricity and magnetism · level 4, lesson 5 of 6

A question to start

A solar panel's label says "18 V, 5.5 A". What power does it give, and what changes when a cloud passes over?

The idea

A solar panel is made of many solar cells. Each silicon cell gives only about half a volt, so cells are joined in a line (in series) to add their voltages. A panel of 36 cells gives about 18 V.

The current depends on how much sunlight falls on the panel. Halve the sunlight and the current roughly halves, while the voltage stays almost the same. The power is

\[P = VI.\]

To get more current, panels are joined side by side (in parallel). To get more voltage, they are joined in a line.

full sunlight: 5.5 Ahalf the sunlight: about 2.75 Avoltage stays near 18 V36 cells in a line

Worked examples

1. The label. \(P = 18 \times 5.5 = 99\) W in full sunlight.

2. Under a thin cloud. Half the sunlight: \(I \approx 2.75\) A, so \(P \approx 18 \times 2.75 = 50\) W.

3. Two panels. Two such panels in parallel give 18 V and 11 A, 198 W. In series, 36 V and 5.5 A: the same 198 W.

Try it

  1. How many half-volt cells in series give 12 V?
  2. A panel gives 20 V and 4 A. What is its power?
  3. Dust blocks a quarter of the light on that panel. Roughly what power does it give now?
  4. You need 36 V from 18 V panels. How do you join them?
Answers
  1. \(12/0.5 = 24\) cells.
  2. \(20 \times 4 = 80\) W.
  3. Current about \(0.75 \times 4 = 3\) A, so about \(20 \times 3 = 60\) W.
  4. Two in series.

Common mistake

"Weaker sunlight lowers the voltage." It mostly lowers the current. A panel in dim light still shows nearly its full voltage on a meter, but it can deliver far less power.

Where it leads