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Tedx

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Reply with quote  #1 
My charge controller manual says my unit has a "rated charge power" of 1040w on a 24v system.  What could that possibly mean to a layman?  I can't mean "Max PV array power" because that is listed separately at 3120w. 
Does "rated charge power"  mean maximum amount of current the charger can process current from?
Does it mean optimum power for the unit to work efficiently?
Or does it mean something else? 
Or is it just a big secret consumers aren't supposed to know?

stmbtwle

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Reply with quote  #2 
GENERALLY, charge controllers are limited by amps, not watts.  Watts are a function of Amps x Watts.  So your controller is good for about 40 amps. On a 12v system it could handle the same 40 amps but only about 520 watts.  On a 48v system, 2080 watts, etc.  (these numbers are approximate).

This is amps OUT and into the battery. So even if you have a 48v array solar with 2080 watts, your controller can only put 520 watts into a 12v battery bank.  Any excess is wasted as heat, too much heat and your controller goes poof.  On top of that there is a voltage limitation. Too high an input voltage and, again, poof.  

Solar panels are different, as they get hot, voltage drops and they produce LESS power. They're black, so they DO get hot.  On top of that they're "rated" at room temperature, so a panel that's rated at 100w at 70F might only produce 60w at a more normal 130F.  So your 3120w array might only produce 2000 watts in real life.

Configuring your batteries to a higher system voltage is more efficient, but if you already have a 24v inverter, and it can't be changed, you're, well, screwed.  Trying to run a 24v inverter on 48v is not a good idea...  

Confusing, I know.  Maybe Rick can do a better job of explaining it.




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jjackstone

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Reply with quote  #3 
Quote:
Originally Posted by stmbtwle
GENERALLY, charge controllers are limited by amps, not watts.  Watts are a function of Amps x Watts.  



I know this is just an oversight but Watts(power) are a function of Amps(current) times Volts. P=IE

JJ

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Rick H Parker

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Reply with quote  #4 
Rated charge power tells one how fast the controller can transfer energy to a battery.  For the most part this is determined by the current the controller can deliver and the charging voltage.


Power is the rate in which energy is transferred or converted.   Watts is the unit of measurement for electrical power.  Power (watts) is not a quantity it is a rate.
In the  International System of Units (SI) Watts (W) is defined as one joule (J) of energy per second (S) of time, [elec-34-eq-1].

"Rated charge power of 1040w on a 24v system" Means that the controller is rated for a maximum of 1024 Joules of energy per second (1024W) transfer to a 24V battery.
This could be 36 amps @ 28V, 40 Amps at 25.6V or any combination that equals 1024W.


The key words to understanding this are Rate, Rate and Rate.




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Rick H Parker
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stmbtwle

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Reply with quote  #5 
OOPS!

Thanks, JJ, for the correction! It WAS typo.

My basic premise is the same. Increasing the voltage (up to a point) increases the power a given controller can handle. It's usually in the specs.

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Bruce

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Reply with quote  #6 
Rick said: "Power is the rate in which energy is transferred or converted.   Watts is the unit of measurement for electrical power.  Power (watts) is not a quantity it is a rate."

Thank you!  That one sentence cleared up one of life's mysteries for this 62 YO very handy novice who has done all sorts of electrical work, knows a lot about NEC and has been enlightened by that simple explanation!
stmbtwle

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Reply with quote  #7 
You're lucky.

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Rick H Parker

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Reply with quote  #8 
Quote:
Originally Posted by Bruce
Thank you!  That one sentence cleared up one of life's mysteries for this 62 YO very handy novice who has done all sorts of electrical work, knows a lot about NEC and has been enlightened by that simple explanation!


Ever wonder why the power company bills you in Kilowatt-hours (kWh) rather the Kilowatts?

Watts is a rate, the equation for rate is Q/t where Q = Quantity and t = time.
Hours = time.

How watch this:
Q/t * t = Q * t/t = Q * 1 = Q.
Multiplying a rate by time factors out the time component, Quantity is all that remains.

How much energy is in a kWh?

1kW = 1000 Joules/second
1 Hour = 3600 second

1000 J/s * 3600 s = 1000 J * 3600 * (s/s) = 3600000 J * 1 = 3,600,000 Joules of energy.

kWh is a Quantity of energy, because the time component factors out.



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Rick H Parker
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stmbtwle

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Reply with quote  #9 
That's meaningless to me.

In terms even I can understand, 1kwh will heat 3 gallons of water from room temperature to near-boiling.
https://bloglocation.com/art/water-heating-calculator-for-time-energy-power

As Rick says, watts are a rate, so 1kw would do the above in 1 hour. Or 1 gallon in about 20 minutes.

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Solar is like the wind. It may be free, but putting it to work isn't!
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Rick H Parker

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Reply with quote  #10 
Start temperature 70 °F
End temperature 210 °F
Temperature difference 140 °F

Three US gallons of water weights 3 * 8.34 pounds = 25 pounds.

1 Btu is the quantity of energy need raise 1 pound of water 1 °F.

To raise 25 Lbs of water 140 °F you need 25 * 140 * Btu = 3500 Btu of energy.

1 kilowatt-hour of energy = 3412 Btu of energy.

1 kW of power would raise 25 pounds (3 US gallons ) of water from 70 °F to 210 °F in 1 hour.
Because at a transfer rate of 1kW it takes 1 hour to transfer 1 kilowatt-hour of energy.

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Rick H Parker
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