Calculating the amount of watts being generated

[spendlove]

I have a 12 volt system and can see the amount of volts and amps that the panels on our solar tracker are generating. Because it's a 12 volt system, I've always been calculating the watts by multiplying the amps by 12. But the thought occurred to me that maybe I should be multiplying the actual volts and amps, considering the volts are always higher than 12. What is the most accurate way of making the measurement? Thanks!

 

[SCClockDr]

Your second thought is the way to go.

 

[SCClockDr]

What is the most accurate way of making the measurement? Thanks!

Install either a hall effect or shunt based wattmeter.

 

[Rider]

Many of the solar chargers have a display with KWH. That's an accurate representation of power generated.

 

[spendlove]

Install either a hall effect or shunt based wattmeter.

Can you provide a URL of a particular brand you recommend? Thanks!

 

[SCClockDr]

Here are 3

1. DROK DC Voltmeter Ammeter, 0-90V 100A Digital Voltage Current Capacity AH WH Power Time Multimeter, 12V 24V 36V 48V 60V 80V LCD Battery Charge Discharge Monitor Volt Amp Watt Tester with Hall Sensor
   1. Has a clock function
   2. Will not track negative parameters below zero.
2. DROK DC 0-300V 200A STN LCD Display Digital Multimeter Voltage Ampere Power Energy Ammeter Voltmeter Battery Volt Amp Meter AH Monitor Panel with Hall Sensor ... DC Multifunction Battery Monitor Meter,0-200V,0-100A (Widely Applied to 12V/24V/48V RV/Car Battery) LCD Display Digital Current Voltage Solar Power Meter Multimeter Ammeter Voltmeter
   1. No clock display
   2. Will track data in either direction to the unit's maximum limits +.
3. DC Multifunction Battery Monitor Meter,0-200V,0-100A (Widely Applied to 12V/24V/48V RV/Car Battery) LCD Display Digital Current Voltage Solar Power Meter Multimeter Ammeter Voltmeter
   1. I've not used this one yet.

 

[offgriddle]

Yes, with a multimeter measure your actual volts at the charge controller input, then, measure the actual amps at the charge controller input, then P = I x E it, and Viola, you will know what is coming in from the panels.

 

[SolarRat]

I *think* it will depend if your charge controller is PWM or MPPT. With PWM you'd use the charge voltage as the multiplier (~13.x V). With MPPT, you'd use the panel voltage as the multiplier (~17v+).

PWMs can't make use of the extra voltage, MPPTs can.... But in either case using the panel voltage as the multiplier will give you the actual wattage the panels are producing...not what's going into the battery.

 

[SCClockDr]

I *think* it will depend if your charge controller is PWM or MPPT. With PWM you'd use the charge voltage as the multiplier (~13.x V). With MPPT, you'd use the panel voltage as the multiplier (~17v+).

PWMs can't make use of the extra voltage, MPPTs can.... But in either case using the panel voltage as the multiplier will give you the actual wattage the panels are producing...not what's going into the battery.

In either case the voltage must be paired with the current flowing in the same circuit to achieve accurate results.

1. PV voltage & current pre SCC
2. SCC voltage & current post SCC prior to any other connections.

The difference in these results would reflect the SCC's efficiency.

 

[ScoTTyBEEE]

Before the controller the volts and amps should equal the same watts as after the controller, the only difference is after the controller is lower volts but higher amps, which is what their function is.

Before 50v x 10a = 500W
After 13.8v x 36a = 497W

Something like that.

 

[SolarRat]

Before the controller the volts and amps should equal the same watts as after the controller, the only difference is after the controller is lower volts but higher amps, which is what their function is.

Before 50v x 10a = 500W
After 13.8v x 36a = 497W

Something like that.

You are assuming 100% efficiency which won't happen. And if using a PWM controller the "after" in your example above would be only 13.8v x 10a because it can't make use of the higher input voltage. Again, I *think*.

 

[SCClockDr]

You are assuming 100% efficiency which won't happen. And if using a PWM controller the "after" in your example above would be only 13.8v x 10a because it can't make use of the higher input voltage. Again, I *think*.

The difference would still be the effective real world efficiency of the installation.

 

[efficientPV]

Far more interesting would be to have a small independent pilot panel that tells you how much power you could actually be using. I have one with a load resistor sized to prevent it from getting near power point. Charge controllers have a habit of sitting around not doing too much. In a battery based system you have to have excessive capability in order to be successful. I try to run opportunity loads when there is more power than the batteries can use. It will drive you crazy if you actually see how much power is wasted each day.

 

[erik.calco]

@spendlove I haven't done it for DC, yet. But, look what $13 gets you in the AC world!

When I get a DC one, I'll just create an inline one, putting plugs on both sides, like XT-90s, which should work with my 10 AWG PV cable and can safely handle 90 or so amps.

In DC world, you use a shunt to handle high amperage connections, as it measures a portion of the current in a fixed ratio. In AC world, you tunnel the positive lead through clamp or ring. In both cases, your meter can measure accurately without directly running the high current through it.