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I have recently purchased a watt meter from Adafruit to measure the cumulative watthours of a non-constant (DC) current.

When I use a multimeter to test the voltage and amperage, I get what I expect: 12 V, 1 A. When I use the wattage meter, I get 12 V, 0 A. When I then plug in a load, the current goes up to 1 A and the voltage goes down to 3 V. The load plugged in is 5 V, 1 A. There is also no internal load in the wattage meter.

How do I get the wattage meter to count the cumulative current and voltage correctly, and if that isn't possible, is there an alternative way to do this? I know I can calculate hours x current x voltage to find watthours, but the current and voltage vary a little, and because of this, I can't find the average of the current and volts.

While the multimeter and wattage meter read 12 V without a load, the rated voltage is 10 V. Also, I wouldn't expect it to reach 10 V, more like 7.5 to 8 V, which is essentially what the wattage meter was reading +5 V, which made me wonder if the wattage meter was reading the voltage the load isn't using.

The load is a Firestick TV from 2014. The source of energy is a solar panel.

JRE
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BEn
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    You're plugging a 5v device into a 12v source? – user1850479 Feb 02 '23 at 23:55
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    Solar panels don't react like a voltage source power supply does when you connect a load. The no-load (open) voltage of a solar panel will be different from what you see with a load. What voltage and current does your multimeter show with the load connected, without the watt meter in the circuit? – ocrdu Feb 03 '23 at 00:41
  • @ocrdu I tested the voltage without the watt meter and just the multimeter and got 3 volts and 1 amp. From this, I have a couple of questions. Why do the voltage and amperage change with a load? Does a different load change the voltage and current? What type of load is optimal in a scenario like this? Also, which voltage and current would be more accurate for the solar panel performance? – BEn Feb 03 '23 at 01:23
  • @user1850479 I connected resistors in series to limit the voltage by about 6 volts. The other vault got left out and ran out of resistors. Also, the load is a semi-broken electronic device, so I don't care if I break it more because it has no purpose. The max voltage which is safe to input into it is about 7 volts, and the device also wasn't heating up and doesn't have a battery, so nothing that bad could happen. I know it isn't the optimal load, but I didn't want to buy something else. – BEn Feb 03 '23 at 01:28
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    We need the circuit that you use to measure voltage and current – Roland Feb 03 '23 at 17:38

1 Answers1

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If you are measuring 12V and 1A with the multimeter, then you are probably making the common mistake of trying to measure current like you do voltage.

You measure voltage across two points. You measure current through one point.

Voltage:

schematic

simulate this circuit – Schematic created using CircuitLab

schematic

simulate this circuit

You have to put the meter in series with the load (represented by resistor R1) to measure current.

If you put the ammeter across the solar panel output, then you are measuring the short circuit current of the panel. When you do that, the voltage will be much lower than before.

Your watt meter is doing it correctly. It shows you a lower voltage because the output voltage of the panel drops when you put a load on it. It shows no current when the load is removed because there's no load drawing current.

Trust the watt meter.

Get a second multimeter, and measure voltage from the solar panel and the current through the load at the same time. You will see it does the same thing as the watt meter.

JRE
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