What it shows
The electrical output of a solar cell falls as the light strikes it at a shallower angle, because the same beam is spread over a bigger area. Measuring voltage and current at known tilt angles shows why rooftop panels in India are tilted roughly at the latitude angle.
The science
A beam of sunlight carries a fixed power per square metre. When the cell faces the sun squarely, all that power lands on its surface. Tilted at an angle, the cell catches only the component of the beam that is perpendicular to it, which falls as the cosine of the tilt angle. The current from the cell is proportional to the light collected, so current drops sharply with angle while the open-circuit voltage drops only a little. The LED with its 220 Ω resistor shows the effect visually while the multimeter gives numbers.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A small solar cell rated about 6 V
- A multimeter
- One red LED and a 220 Ω resistor
- A protractor and a cardboard stand with a hinge of tape
- A notebook and a sunny morning
Safety
For this project: Do not look at the sun. Work in the shade of a hat and take breaks; test in the morning or late afternoon rather than at midday in summer.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Tape the cell to the cardboard stand and mark angles of 0, 30, 60 and 90 degrees from pointing straight at the sun using the protractor.
Connect the multimeter on the 20 V DC range across the cell with nothing else attached and point the cell straight at the sun; record the open-circuit voltage.
Repeat at 30, 60 and 90 degrees tilt, keeping each reading within a minute of the others so the sun does not move.
Switch the meter to the 200 mA DC range and connect it in series with the cell shorted through the meter; record the short-circuit current at each angle.
Now connect the LED through the 220 Ω resistor across the cell and watch its brightness change at each angle.
Plot current against angle and compare the shape with a cosine curve.
Repeat the whole set an hour later and note how the sun’s movement changed the readings.
What you should see
A table and graph showing voltage falling slightly and current falling steeply as the tilt angle increases, with the LED dimming to match.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the current fall faster than the voltage with angle?
Current depends on the light collected; voltage depends mainly on the cell material and changes little.
2What angle gives the most output?
Zero degrees, when the cell faces the sun directly.
3Why are rooftop panels in Delhi tilted at about 28 degrees?
That is the latitude, so the panel faces the sun squarely on average through the year.
4Why measure quickly at each angle?
The sun moves and clouds change the light, which would spoil the comparison.
Take it further
Build a two-position tracker that tilts the cell by hand each hour and compare total output with a fixed cell.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely