# Measuring Solar Radiation, put resistor in Serial or Parallel?

I want to measure solar radiation for a home project (just looking for ballpark measurements). I’m using a simple wallet sized solar panel and temperature gauge. No problem with temperature sensor (I didn’t include it in the picture) but when hooking up the solar panel I’ve come across two websites that say this is how to do it… but one the resistor is in serial the other it’s in parallel and I’m not knowledgeable enough to understand which is right or what does one mean and the other. Which way makes sense please? I included a diagram of the two ways shown, and it boils down to which way is right Serial or Parallel on the resistor? Thank you

Both are wrong...

You want to measure the short-circuit current of the panel, as thats linear with light flux and not very temperature-dependent. The voltage of the panel is very sensitive to temperature and very non-linear in light flux.

This means using a low value shunt resistor and an opamp to amplify the small voltage across the shunt to the 5V range.

I suspect that size of panel will have output in 50 to 500mA range, so something like a 0.1 ohm shunt might be right - but without details of the panel its a complete guess.

Oh... thank you! Would you say the easiest way then is to use a lux sensor instead such as Adafruit Lux Sensor?

I made some DIY solar heating panels and trying to ballpark their output/performance and want to measure how much sunlight can transfer into heat and wasn't sure what sensor is best at that. I'm trying to create a pyranometer basically, and the two sites I saw both used a little solar panel and resistor and temp probe.

I'm not sure if you're implying I measure lux (that's what I interpreted), when researching a lux probe I figured it measures light levels based on the human eye range but may not measure how much energy is available in the light (as in cloudy days are bright but my solar panel blowers don't turn on so is low in solar radiation whereas fall clear days I get a ton of heat).

Thanks!

MarkMello:
I made some DIY solar heating panels and trying to ballpark their output/performance and want to measure how much sunlight can transfer into heat and wasn't sure what sensor is best at that.

Using a PV panel may not be a good way of doing that. They are only 20~30% efficient at turning sunlight in electricity. Thermal panels can be ~80% efficient at turning sunlight into heat. And they may have different absorption profiles.

MarkMello:
a lux probe I figured it measures light levels based on the human eye range

You're right, that is probably a different profile to either a PV or a thermal panel.

On a cloudless day and on a given date, the total energy ("solar constant") of the sunlight falling on a surface at noon can be looked up. On the equator, a commonly used figure is 1388 Watt/m^2.

Correct for latitude, or look for charts specific to your part of the world.

MarkT:
This means using a low value shunt resistor and an opamp to amplify the small voltage across the shunt to the 5V range.

Or a transimpedance amplifier.

Transimpedance amplifiers make sure the voltage across the solar panel stays zero volt.
You measure the effort of the amplifier doing that.
Leo..

PaulRB:
Using a PV panel may not be a good way of doing that. They are only 20~30% efficient at turning sunlight in electricity. Thermal panels can be ~80% efficient at turning sunlight into heat. And they may have different absorption profiles.
You're right, that is probably a different profile to either a PV or a thermal panel.

The current depends on the number of photons landing on the panel, really very well. Its just a super large photodiode. The voltage is a completely different story...

Or a transimpedance amplifier.

Transimpedance amplifiers make sure the voltage across the solar panel stays zero volt.
You measure the effort of the amplifier doing that.

No, that works for a normal photodiode which produces a few mA at most, but a PV panel pushes large currents around, use a shunt. Perhaps losing the PV panel and using a photodiode is a better plan, I did wonder...

"I'm using a simple wallet sized solar panel"
Don't know about OP's wallet, but mine is pretty small.
Leo..