DIY Pyranometer for Solar Radiation

A crystalline solar cell's short-circuit current is roughly proportional to the irradiance on that cell. A tiny cell, a meter that can read its current, and a note of how you held it level will compare one hour with another hour in the same plane. That is a useful sun meter for collector tests. It is not a thermopile pyranometer, it does not see the whole solar spectrum the way one of those does, and it cannot support a plus-or-minus two percent claim.

Do not use a house module as the sensor. Short-circuit current on a full-size module is large enough to damage a cheap meter and to start a fire at a careless clip. This instrument is a small cell you could hide in a pocket.

The current it produces becomes a watt-per-square-meter scale only after you compare it, once, with a source you name. A borrowed meter is the clean comparison. A clear midday value you take from the method on the radiation-data guide is a rough scale if you write "approximate" next to it and you do not carry three significant figures into the efficiency test. Kits and experiments in the links below are other people's instruments; the one here is just a small cell and a meter.

Difficulty

Level

Advanced

Time

An hour to mount a cell, and one clear midday to write a scale factor

Skills

Keeping the cell in the collector plane and the house array out of the meter

Cost estimate

$15–40

A small photovoltaic cell, a meter that reads milliamps, and a scrap of board to hold the cell in plane. A used multimeter you already trust may be the whole cost. A laboratory pyranometer is a different purchase and a different claim.

Materials

Item	Qty	Notes
A small solar cell, not a house module	1	Short-circuit current small enough for the meter. If you do not know the current, do not clip it. Find the cell's rating first.
A meter that can measure that current	1	Current mode, leads in the current jacks, range above the cell's short-circuit current.
A flat mount in the collector's plane	1	Same tilt and azimuth as the glass you care about. A horizontal reading is a different instrument.

Build a comparison meter

Mount the cell flat on a board you can set in the same plane as the collector glazing. Shade on the cell from the board's own edge ruins the reading. Keep the face clean. Wire the meter in current mode across the cell so the cell is essentially shorted through the meter, which is what "short-circuit current" means. If your meter is happier with a small resistor and a voltage reading, choose a resistor low enough that the cell voltage stays near zero, and write that choice down. Do not invent a large load and call it short-circuit current. The proportionality is for the short-circuit case.

Read it in the plane of the collector, not flat on the ground, unless the collector is flat. Cosine response, the way a bare cell falls off at a steep angle, is one reason a cheap cell and a real pyranometer disagree in the morning. Temperature changes the current a little. For a same-day comparison of two collectors side by side, those errors mostly cancel. For a claim about a national weather file, they do not.

Write a scale once. On a clear midday, note the milliamps and note a reference irradiance you trust for that hour and that plane. Scale factor is reference divided by milliamps. Date it. Next month the cell may have been scratched or the meter battery may be tired. The factor belongs to that date and that cell.

Use the reading as the sun term in an efficiency test, or simply as "the noon today was 80 percent of the clear noon last week." Both uses are fair if the plane matched. A thermal collector and a photovoltaic cell do not weight the spectrum the same way. Say "cell irradiance" if you are being careful, and do not pretend you measured a thermopile's watts.

Composite: the module as a sensor

Invented and forbidden. Someone clips a multimeter across a rooftop module because the module was handy and the tiny cell was not. The current is far above the meter, the lead heats, and the test ends as a scorch instead of a sun reading. The lesson is plain: use a pocket cell with a known current, and leave the house array alone. There is no real roof and no model number behind it.

1. Use a small cell with a known short-circuit current

   Below what the meter can measure. A house module does not qualify.

2. Mount it in the collector plane

   Clean, unshaded by its own board, same tilt and aim as the glass.

3. Read current with the cell nearly shorted

   Meter in current mode, or a deliberately small resistor. Write the method down.

4. Scale it once, and label the scale as approximate

   Against a better meter or a named clear-sky reference. Date the factor. Do not claim laboratory accuracy.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

• https://www.builditsolar.com/References/Measurements/measurments.htm
• https://www.builditsolar.com/References/Measurements/CollectorPerformance.htm
• https://www.builditsolar.com/References/Measurements/KitPyr/KitPyr.htm
• https://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm
• https://www.builditsolar.com/Experimental/SunSim/SunSim.htm
• https://www.builditsolar.com/Experimental/HeatTransferCoef/HeatTransferCoef.htm