# Measuring joules/watts?

I would like to do a little project with a solar panel, something to measure the actual watts coming out of the thing, and an Arduino to integrate the panel's output over time.

However, I haven't any idea of how I would build a circuit to measure the actual amount of power the panel is producing. Is there an off-the-shelf component that will measure that for me? Or measure some related value(s) through which I can calculate the watts?

Any pointers to circuit diagrams, wikipedia pages, or anything to help me get going would be greatly appreciated.

a voltage divider to scale the voltage so it can be measured, and a current sensor to measure the current. you could use a shunt resistor and measure the voltage drop, or you can use a hall effect sensor to measure it.

Cool. So, measure the volts and amps coming out of the panel, because W = V*A. Makes sense.

Being an electronics NOOB, can anybody recommend some Arduino-friendly parts / circuit diagrams for the ammeter portion?

do you know the power your panels are rated at?

you've got the right answer in #2.

Here's the circuit for current measurement: panel output ----load---1-RRRRR-ground RRRRR = 1Ohm resistor 1 - point to be connected to the analog input of your Arduino. Measure the voltage at this point, the current will be the voltage / 1ohm

1 ohm is kinda high for any significant current though. a smaller shunt and an op-amp would work nicely, or a hall effect.

Sweet! I should have known SparkFun would have something. That looks just about perfect. Thanks so much!

Although its rather expensive - look for other current sensors if you dont require the high-side monitoring feature.

P= (U*U)/R P=(I*I)*R A static known resistance and measure of U or I, willl give a simple equation to calculate P. No need to measure both.

regards

But the requirement is to measure the power delivered into the actual load, not a resistor!

by measuring the voltage drop across a resistor you calculate the current flowing through the circuit, which is equal to the current going into the battery.